Re: Topband: SDR Mythbusters - ADC Overload myths debunked...
Steve Hicks, N5AC and the VP of Engineering at FlexRadio has posted an excellent explanation and bust of the ADC overload myth on the FlexRadio community. You don’t need to be registered on the community to read this excellent write up: https://community.flexradio.com/flexradio/topics/adc-overload-myths-debunked?utm_source=notification&utm_medium=email&utm_campaign=new_topic&utm_content=topic_link I have no experience with Flex Radio equipment, (it might be great stuff for all I know), so I will confine my comments to the theory discussed in the "ADC overload myths debunked" paper. A lot of what I read didn't make a lot of sense to me, or seemed irrelevant. To begin with, I'm not sure as to the exact nature of the "myth". Initally, the myth is supposed to be that hams think average power of an ensemble of uncorrelated signals is the sum of the power of the components. This is not a myth, it is true. Then it is suggested that hams believe peak voltages add up, as in a 6 dB increase for two signals. Supposedly, hams don't realize that the high peaks only occur rarely. I'm not aware of any ham lore exhibiting this misunderstanding. The discussion of crest factor obscures the fact that average power still adds. 100 signals at S9 still has a power of 20 dB over S9, on the average. Once in a while it looks like 40 dB over S9. The rest of the time, the combined power of all the signals still tests the dynamic range of the receiver. It's not like a bunch of S9 signals is no worse than a single S9 signal. Then there is this statement: "The individual data points that make up a signal you are listening to are almost never going to fall in the same time as the overload, statistically." I have no idea what this means in terms of Nyquist sampling theory. The paper goes on to say: "With a noise blanker, we remove thousands of samples with no negative effects to the signal being monitored and a momentary overload from the addition of many signals summing up will have a much lower effect" I don't know whether this means Flex (IE "we") has invented some sort of magic digital noise blanker that removes samples corrupted by overload (I'm skeptical) or whether it means that a noise blanking effect just happens as part of the sampling process (in which case, I'm still skeptical). Then the subject shifts to decimation and "processing gain", which are simply references to digital filters. These techniques are all based on linearity. Adding digital filtering after a nonlinear front end cannot repair the damage caused by nonlinearity. Just like adding crystal filters to the IF in an analog receiver won't overcome front end overload caused by enabling the receiver's built in preamp. There is an assertion that the large amount of "noise" added by hundreds of signals results in "linearization", which I believe is referring to what is usually called "dithering". This is a complete misunderstanding of dithering, which uses small amounts of noise and does not involve clipping in the ADC. High quality ADC's have dithering and similar randomization processes built in and don't need help from external noise anyway. The paper then changes the subject to phase noise. This has nothing to do with ADC overload. I will note that digital radios are much more sensitive to clock jitter (IE phase noise) than analog radios. If anything, the phase noise issue is an argument against digital. There are various distractions such as the Central Limit Theorem and the Jupiter effect that don't add much to the discussion. The dubious argument is made that the existence of 1000's of receivers in the field without complaints from their owners "proves" that overload problems do not exist. Until last month, we could make a similar statement about the millions of satisfied Diesel Volkswagen owners. The concluding statement is quite a stretch: " it is simply mathematically true. FlexRadio Systems makes the best amateur transceivers available." Mathematically true? Maybe it's that new Common Core math. Rick N6RK _ Topband Reflector Archives - http://www.contesting.com/_topband
Re: Topband: SDR Mythbusters - ADC Overload myths debunked...
I have no experience with Flex Radio equipment, (it might be great stuff for all I know), so I will confine my comments to the theory discussed in the "ADC overload myths debunked" paper. A lot of what I read didn't make a lot of sense to me, or seemed irrelevant. Same here. Didn't make sense to me. Sounded like a combination of marketing hyperbole and magic. _ Topband Reflector Archives - http://www.contesting.com/_topband
Re: Topband: SDR Mythbusters - ADC Overload myths debunked...
I'm not an RF engineer so any "direction" or "stay inside these boundaries, Dood" assistance is appreciated! However, as a Systems Engineer with a background in statistics, I missed the "what?" factor when he mentioned Central Limit TheoremI shoulda put on my "Huh?" hat so I'm almost embarrassedhihi I re-read the "explanation," Rick and Tom, and walked away with a clearer - "clearer" = more questions - view of its content. The "Jupiter effect" is another one I shoulda looked for that hat as well anf headed to Google to see how I could link that statement/phrase to ADC overload! Thanks to you and Tom for your comments that caused me to re-read it and end up at a point I shoulda arrived at earlier Sherwood Engineering's rcvr measurements are not there to debunk myths.they support real comparisons/analysis. 72 de Jim R. K9JWV > Subject: Re: Topband: SDR Mythbusters - ADC Overload myths debunked... > To: rodenkirch_...@msn.com; topband@contesting.com > From: rich...@karlquist.com > Date: Sun, 11 Oct 2015 00:10:56 -0700 > > > Steve Hicks, N5AC and the VP of Engineering at FlexRadio > > has posted an excellent explanation and bust of the > > ADC overload myth on the FlexRadio community. You don’t > > need to be registered on the community to read this excellent write up: > > > > > > https://community.flexradio.com/flexradio/topics/adc-overload-myths-debunked?utm_source=notification&utm_medium=email&utm_campaign=new_topic&utm_content=topic_link > > > > I have no experience with Flex Radio equipment, > (it might be great stuff for all I know), > so I will confine my comments to the theory > discussed in the "ADC overload myths debunked" > paper. A lot of what I read didn't make a > lot of sense to me, or seemed irrelevant. > > To begin with, I'm not sure as to the exact > nature of the "myth". Initally, the myth is > supposed to be that hams think average power > of an ensemble of uncorrelated signals is > the sum of the power of the components. This > is not a myth, it is true. Then it is suggested > that hams believe peak voltages add up, as > in a 6 dB increase for two signals. Supposedly, > hams don't realize that the high peaks only > occur rarely. I'm not aware of any ham lore > exhibiting this misunderstanding. > > The discussion of crest factor obscures the > fact that average power still adds. 100 signals > at S9 still has a power of 20 dB over S9, on > the average. Once in a while it looks like 40 > dB over S9. The rest of the time, the combined > power of all the signals still tests the > dynamic range of the receiver. It's not like > a bunch of S9 signals is no worse than a single > S9 signal. > > Then there is this statement: > > "The individual data points that make up a signal > you are listening to are almost never going > to fall in the same time as the overload, statistically." > > I have no idea what this means in terms of > Nyquist sampling theory. The paper goes on to > say: > > "With a noise blanker, we remove thousands of samples > with no negative effects to the signal being > monitored and a momentary overload from the > addition of many signals summing up will have a > much lower effect" > > I don't know whether this means Flex (IE "we") has invented > some sort of magic digital noise blanker that removes > samples corrupted by overload (I'm skeptical) or > whether it means that a noise blanking effect > just happens as part of the sampling process > (in which case, I'm still skeptical). > > Then the subject shifts to decimation and "processing > gain", which are simply references to digital filters. > These techniques are all based on linearity. Adding > digital filtering after a nonlinear front end cannot > repair the damage caused by nonlinearity. Just > like adding crystal filters to the IF in an analog > receiver won't overcome front end overload caused > by enabling the receiver's built in preamp. > > There is an assertion that the large amount of > "noise" added by hundreds of signals results in > "linearization", which I believe is referring to > what is usually called "dithering". This is a > complete misunderstanding of dithering, which uses > small amounts of noise and does not involve clipping > in the ADC. High quality ADC's have dithering > and similar randomization processes built in and > don't need help from external noise anyway. > > The paper then changes the subject to phase noise. > This has nothing to do with ADC overload. I will > note that digital radios are much more sensitive > to clock jitter (IE phase noise) than analog radios. > If anything, the phase noise issue is an argument > against digital. > > There are various distractions such as the Central > Limit Theorem and the Jupiter effect that don't > add much to the discussion. > > The dubious argument is made that the > existence of 1000's of receivers in the field > without complaints from their owners "proves" that >
Re: Topband: SDR Mythbusters - ADC Overload myths debunked...
The opinion expressed in the Flexradio editorial about noise blankers being completely harmless, is not an opinion, it is even false. What Flexradio says: ""With a noise blanker, we remove thousands of samples with no negative effects to the signal being monitored and a momentary overload from the addition of many signals summing up will have a much lower effect". What I have found: I have often had to resort to noise blankers for reasons varying from the Russian Woodpecker to local 120Hz utility RFI. Noise blankers can be incredibly effective when there are only very weak signals on the band. But if there are other loud signals on the band (aka any contest), mixing from simple blanking action starts mixing all the other loud signals together and smooshing them everywhere across the band. Looked at from a "reciprocal mixing" standpoint, think of blanking (either because of noise blanker or ADC overload) as chopping your LO signal. This causes wide wide noise sidebands in your effective LO. When there are only weak signals on the band you probably do not notice the reciprocal mixing. But the reciprocal mixing quickly becomes a limiting factor in contest conditions. I think some of the other Flexradio opinions are similar - when there is only a single weak signal they may have some validity. But the instant you start "removing samples" when there are loud signals on the band, you cause mixing and intermod amongst all the loud signals. There have some improvements in noise blanking in recent years. The best DSP noise blankers - the K3 and Orion in particular comes to mind - seem to make some effort to reduce reciprocal mixing by (I think) shaping the blanking pulse to reduce the bandwidth of resulting LO sidebands and limiting the range over which the reciprocal mixing action happens. Again, this DSP noise blanking only works if your DSP has the dynamic range to begin with. Tim N3QE On Tue, Oct 6, 2015 at 7:56 AM, James Rodenkirch wrote: > > > > > > From Stu, K6TU > > As with any technological change, there are many myths, past truths or > part truths that get repeated endlessly and out of context by those > frightened or challenged by the change. > > Software defined radios are no exception to this in the world of Ham > Radio. > > Steve Hicks, N5AC and the VP of Engineering at FlexRadio has posted an > excellent explanation and bust of the ADC overload myth on the FlexRadio > community. You don’t need to be registered on the community to read this > excellent write up: > > > https://community.flexradio.com/flexradio/topics/adc-overload-myths-debunked?utm_source=notification&utm_medium=email&utm_campaign=new_topic&utm_content=topic_link > > > Here is an extract of the first few paragraphs to whet your appetite - > well worth a read as extending education on the world of Software Defined > Radios… > > Stu K6TU > > ADC overload myths debunked > > I've received some feedback that there is some confusion circulating on > other ham radio reflectors regarding how analog to digital converters > (ADCs) work in radio applications. Specifically, some of the comments tend > to say that direct sampling ADCs just won't work in strong signal > environments so I'd like to explain why this is not factual for those who > are interested. I have a few points to illustrate this. > > As hams we tend to think of strong signals in terms of their total power, > how many total Watts they are. When you think of signals in this way, you > can add their power in your head and think: two -10dBm signals add to -7dBm > total power (3dB increase). In fact, you can take multiple signals and add > them together in a power meter and the power meter will show the total > power of all signals. But this is the average and not instantaneous power. > > An ADC, on the other hand, is really a discrete signal device. All of > the signals get chopped into samples and so the real question is: how do > the signals add together in the discrete time domain? To answer this, we > have to look at the signals and how they interact. An RF carrier is like > any AC signal -- it is a sine wave that varies from negative to positive > voltage along the curve of a sine wave. If we add two sine waves of > exactly the same amplitude, frequency and phase, the peak voltage will be > doubled (6 dB). > > But two signals of the same amplitude and phase on the same frequency > isn't reality. Reality is signals all across the bands that are totally > unrelated (uncorrelated)... > ___ > > CQ-Contest mailing list > > cq-cont...@contesting.com > > http://lists.contesting.com/mailman/listinfo/cq-contest > > > > _ > Topband Reflector Archives - http://www.contesting.com/_topband > _ Topband Reflector Archives - http://www.contesting.com/_topband
Re: Topband: SDR Mythbusters - ADC Overload myths debunked...
Hi guys I would like to add my 2 cents on this matter. If you not good, or a very good, RF engineer it is hard to understand what DDU/DUC radio do for you. The most important thing, DDC/DUC radios are not the same just because of the DDC. Number of bits, jitter on the clock works like phase noise from LO. What's in front of the ADC sets the performance as well the software and the ADC ADC overload only can be the limit when everything before the DDC can handle more power than the ADC itself. For the band you are listen. Most SDR radios are using preamps to cover 1 to 50 MHz and guess what? Poor performance on 160m. Example a common used T50-2 toroid barely can handle 20db IP3!!! Most of DDC uses the Linear Technologies LTC2208 16 bit. Or similar DDC with 16 bit. http://cds.linear.com/docs/en/datasheet/2208fc.pdf These ADC devices overload near +10dBm , that's very high, its over +50db IP3. However everything before that should be +50db better, and that is not the case. I like the QS1R because does not have preamp and the LPF/BALUN is quite good to handle strong signals. Another issue with DDC radios available nowadays is the lack of shield, they looks like a computer board and that's it, performs like a computer board. No good RF practice to protect the noise floor or common mode noise. Really not ready for field use. With all the new software development, construction and good RF design is way behind. Lab tests on lab environment also does not tell the true value of a radio. It gives you specific results for specific test conditions. I believe DDC is the future, but we are not there yet. Regards JC N4IS _ Topband Reflector Archives - http://www.contesting.com/_topband
Re: Topband: SDR Mythbusters - ADC Overload myths debunked...
The opinion expressed in the Flexradio editorial about noise blankers being completely harmless, is not an opinion, it is even false. It really isn't an accurate or factual technical article. It is a sales brochure meant to enforce the love each of us have for a particular system, in this case the magic of SDR. You might as well read an MFJ advertisement for technical accuracy, watch a year old VW diesel commercial, or listen to a politician of any party telling us how they will fix everything. It is all sales and stoking people's emotions or existing opinions, with just enough truth woven in to avoid prison. Oddly, the K3 Elecraft is largely SDR. It is an SDR receiver with a standard analog front end as a frequency converter with roofing filter. IMO, the worse part of the K3 is the SDR part. Try this test. Poll users who work very weak CW signals routinely in all sorts of station environments, just the general population of weak signal CW DXers. Ask them if analog detection is better for pulling a weak signal below noise floor out of noise, or if a DSP detection system is better. The results are always that DSP detection systems are viewed unfavorably over good analog systems by significant number of people. The limits of this case are digging weak signals, that are at or below noise, out of the noise. There is always a certain loss of dynamics to me, when I listen to any SDR. It has been this way for me with any SDR or DSP detection radio. Something my ear depends on to know the difference between a signal below noise and just noise without signal is lost in every single DSP detection system I listen to. It melts the signal into the noise with a distortion. We did a blind A-B test (I have audio lines that bring every receiver to a jack ). It is easy to tune two receivers to the same signal, and have someone else switch the lines. People who cannot copy below noise signals seem to not notice this effect. 73 Tom _ Topband Reflector Archives - http://www.contesting.com/_topband
Topband: ADC Overload
Rick, I hope it's not an issue for me to post here directly. I am posting here because I believe that amateur radio has a huge educational component and ultimately incorrect information services no one. I got started when I was 12 and really knew very little about the hobby. My journey, like most hams, has been a long, exciting educational process. Still, there are so many that know so much more than I do. I am constantly amazed at the breadth and depth of the hobby and the people in it. My points below are to clarify what I have observed, calculated and believe to be true and are presented in the interest of mutual education: On Sun, Oct 11, 2015 at 6:17 AM, wrote: > > I have no experience with Flex Radio equipment, (it might be great stuff > for > all I know), so I will confine my comments to the theory discussed in the > "ADC overload myths debunked" > paper. A lot of what I read didn't make a lot of sense to me, or seemed > irrelevant. > > To begin with, I'm not sure as to the exact nature of the "myth". Recently, a post was made to a reflector that definitively stated that direct sampling receivers simply did not function -- that they would overload with a minimal number of signals and/or signals of relatively small magnitude. > Initally, > the myth is supposed to be that hams think average power of an ensemble of > uncorrelated signals is the sum of the power of the components. This is > not > a myth, it is true. Then it is suggested that hams believe peak voltages > add up, as in a 6 dB increase for two signals. Supposedly, hams don't > realize that the high peaks only occur rarely. I'm not aware of any ham > lore exhibiting this misunderstanding. > The discussion of crest factor obscures the fact that average power still > adds. 100 signals at S9 still has a power of 20 dB over S9, on the > average. > Once in a while it looks like 40 dB over S9. The rest of the time, the > combined power of all the signals still tests the dynamic range of the > receiver. It's not like a bunch of S9 signals is no worse than a single > S9 signal. > The misunderstanding centers around a belief that an ADC reacts negatively to a large average power. There are two primary beliefs rolled into this one: (1) That by taking the sum of any number of known signals in the power domain we can reach a total, that when compared with the overload point of the ADC, will definitively predict an overload of the ADC, and (2) That the overload of an ADC is a singular and complete event -- when it occurs the ADC no longer functions. Addressing each of these individually and getting more specific, the first (1) belief is that if we have an ADC that overloads at +10dBm, that I can take 100 -10dBm signals and completely overload the ADC to a point of non-functioning. This really seems like common sense to most. We all fully expect to be able to take 100 disparate signal generators, feed them through a lossless combiner, read a power meter and see +10dBm and then stick that in the ADC and overload it. But this is not how it works. To understand what actually happens, we have to look at how a discrete sampled system works. The ADCs we use are oversampled and run at somewhere between 100-300MHz. Each sample period, the ADC essentially takes a voltage reading on the antenna and records this value, transmitting it to to the computing element in an SDR. The instantaneous voltage of any RF signal varies with the sine wave that defines the RF carrier so it varies from the bias point of the ADC to the bias plus the voltage amplitude of the signal, back through the bias point, down to the the bias minus the amplitude of the signal and back to the bias point each cycle of the RF signal. It is a sine wave of given voltage amplitude centered around the bias point of the ADC. If I add a second signal of equal amplitude, the second signal will add to the first and I will get an instantaneous voltage that is the sum of the two signals. But the voltage is not simply 2x the voltage of the first -- this is only the case if the two signals are on exactly the same frequency, phase and amplitude. What actually happens is a beat-note between the two signals who's envelope oscillates in time at the frequency of the beat note (difference in frequency of the two signals). Periodically, the peaks of the two signals will be exactly aligned and we will get a doubling of the voltage. For two signals this happens fairly frequently. Similar to the two signals adding, they will also subtract to result in a voltage magnitude (absolute value) lower than either of the two signals would have by themselves. For example, one signal might be at +1V while the other is at -0.66V. The resulting voltage measured in the converter, due to linear superposition, is +0.33V. Assuming for a moment that the two signals are large compared to the overload of the ADC, say they are at +7dBm compared to the +10dBm overload of the converter, they will ov
Topband: Virtually All Modern Ham Gear is Un-shielded!
On Sun,10/11/2015 6:09 AM, JC wrote: Another issue with DDC radios available nowadays is the lack of shield, they looks like a computer board and that's it, performs like a computer board. No good RF practice to protect the noise floor or common mode noise. Really not ready for field use. ALL modern ham gear is either unshielded or poorly shielded. Anyone who thinks ham gear is shielded either hasn't looked at I/O wiring or doesn't understand the fundamentals of shielding! In all the gear I looked at in Dayton last year, only the RF output coax connector is screwed down to the chassis. Virtually all other cable shields go THROUGH the chassis to a circuit board, as do all other signal and control cables. They are not even bypassed to a chassis. When a cable goes THROUGH the chassis without making contact, it CONDUCTS RF into the box, so what's inside the box is not shielded. This is in addition to "the Pin One Problem," recognized by an RF guy working in pro audio in 1994. MANY pieces of ham gear have metal enclosures that fail to make contact at junctions between parts of the enclosure, usually because they are insulated by paint. Such an enclosure FAILS as a shield. One of my Astron power supplies has this problem, and EVERY Astron I've opened fails to connect the power line green wire to the chassis, as required by virtually all Electrical Codes. Again, the problem is paint between the log where the green wire is soldered and the chassis. Not only that, but even worse, Astron bonds V- to that same lug! Talk about a recipe for RFI! 73, Jim K9YC _ Topband Reflector Archives - http://www.contesting.com/_topband
Re: Topband: ADC Overload
I hope it's not, too. :-) SDR fascinates me, and I intend to experiment with the technology on 160 meters starting with a Softrock Xtall Lite 9.0 and the matching switchable BPF. 73, Mike www.w0btu.com On Sun, Oct 11, 2015 at 10:23 AM, Stephen Hicks, N5AC wrote: > > I hope it's not an issue for me to post here directly. ... _ Topband Reflector Archives - http://www.contesting.com/_topband
Re: Topband: Bandpass filters for receive antennas
Where can I find a schematic for a good high pass filter that rolls off at 1700 kHz? Yesterday, I installed a preamp --that I made over 20 years ago-- right at one of my 2-wire Beverage antennas. Even in the reverse direction (NW) where losses are the greatest, it easily overloads from out-of-band signals. Today, I even noticed it oscillating. It uses an MRF5812 NPN bipolar. I built it over 20 years ago, before I knew some things I've learned since then (such as being more careful about separating the input and output). I just took it out of line and brought it back to the house. It's the one on the right near the bottom of www.w0btu.com/W0BTU-broadband-preamps.html on the Radio Shack proto board. Not shown in the photo is an RF choke between the input and output that passes the 12 VDC through the coax, so it can power the relay that switches from SE to NW. Also, the back-to-back diodes and the input circuit are slightly different. There are some other things I want to do to it, but I think a HP filter should be near the top of the agenda. Any suggestions would be appreciated. 73, Mike www.w0btu.com On Mon, Aug 31, 2015 at 1:43 PM, Tom W8JI wrote: > Bandpass filters do nothing for in-band signals -- their only function is >> to reject OUT OF BAND signals. The primary reason for using bandpass >> filters IS for RX. A good 160M bandpass filter would be effective at >> reducing overload from AM broadcast stations. So would a high pass filter. >> There is a survey of bandpass filters for contesting at this link. >> http://k9yc.com/BandpassFilterSurvey.pdf >> > > I use a high pass filter for rejection of the AM BCB band. Between 70 and > 80% of the net power (or voltage) into my RX system comes from distant AM > BCB signals. > > Without a small 5-pole highpass that starts to roll off at 1700 kHz, I can > connect a miniature 12V 50 mA incandescent lamp (like the MFJ 1025 uses as > a fuse) and it illuminates a dull red. > > This is with no attempt at matching power to the filament cold resistance. > > My system can be bothered by the sum of all those thousands of signals, I > add a BCB high pass, and then I can run 1500 watts and not bother my own RX > when transmitting on 80 or 40 while receiving on 160. Of course I have > 500-2000 ft separation on antennas, but this still shows how a bunch of > small signals can add up to disaster if they hit something non-linear > before being filtered. > > Always remember there are two problems. One is the absolute limit of > in-band signal a receiver system can take. The other is the absolute limit > of the sum of all the signals entering an overload sensitive point in the > system. > > Less than one volt peak line voltage is not enough headroom to prevent IM > products in a reasonably good system. Back-to-back parallel diodes are fine > for Sky Buddy receivers and FT101's. A single diode opposing another diode > in parallel will clamp at about 6 dBm if your receiver looks like 75 ohms. > Almost all receivers will conservatively take 15-20 dBm, or 2-4 volts peak, > at the antenna port in band. > > If you have a good system, you'll want something other than back-to-back > diodes. > _ Topband Reflector Archives - http://www.contesting.com/_topband
Re: Topband: Bandpass filters for receive antennas
Hi Mike, Here is a link to a simple website I created that describes a high pass filter I built earlier this year that rejects both the LW & MW band. It was originally described in a Feb 1978 QST article. I use it in front of an SDR receiver, and really happy with its performance. Don (wd8dsb) On Sun, Oct 11, 2015 at 8:35 PM, Mike Waters wrote: > Where can I find a schematic for a good high pass filter that rolls off at > 1700 kHz? > > Yesterday, I installed a preamp --that I made over 20 years ago-- right at > one of my 2-wire Beverage antennas. Even in the reverse direction (NW) > where losses are the greatest, it easily overloads from out-of-band > signals. Today, I even noticed it oscillating. > > It uses an MRF5812 NPN bipolar. I built it over 20 years ago, before I knew > some things I've learned since then (such as being more careful about > separating the input and output). > > I just took it out of line and brought it back to the house. It's the one > on the right near the bottom of > www.w0btu.com/W0BTU-broadband-preamps.html on the Radio Shack proto board. > Not shown in the photo is an RF choke between the input and output that > passes the 12 VDC through the coax, so it can power the relay that switches > from SE to NW. Also, the back-to-back diodes and the input circuit are > slightly different. > > There are some other things I want to do to it, but I think a HP filter > should be near the top of the agenda. Any suggestions would be appreciated. > > 73, Mike > www.w0btu.com > > > On Mon, Aug 31, 2015 at 1:43 PM, Tom W8JI wrote: > > > Bandpass filters do nothing for in-band signals -- their only function is > >> to reject OUT OF BAND signals. The primary reason for using bandpass > >> filters IS for RX. A good 160M bandpass filter would be effective at > >> reducing overload from AM broadcast stations. So would a high pass > filter. > >> There is a survey of bandpass filters for contesting at this link. > >> http://k9yc.com/BandpassFilterSurvey.pdf > >> > > > > I use a high pass filter for rejection of the AM BCB band. Between 70 and > > 80% of the net power (or voltage) into my RX system comes from distant AM > > BCB signals. > > > > Without a small 5-pole highpass that starts to roll off at 1700 kHz, I > can > > connect a miniature 12V 50 mA incandescent lamp (like the MFJ 1025 uses > as > > a fuse) and it illuminates a dull red. > > > > This is with no attempt at matching power to the filament cold > resistance. > > > > My system can be bothered by the sum of all those thousands of signals, I > > add a BCB high pass, and then I can run 1500 watts and not bother my own > RX > > when transmitting on 80 or 40 while receiving on 160. Of course I have > > 500-2000 ft separation on antennas, but this still shows how a bunch of > > small signals can add up to disaster if they hit something non-linear > > before being filtered. > > > > Always remember there are two problems. One is the absolute limit of > > in-band signal a receiver system can take. The other is the absolute > limit > > of the sum of all the signals entering an overload sensitive point in the > > system. > > > > Less than one volt peak line voltage is not enough headroom to prevent IM > > products in a reasonably good system. Back-to-back parallel diodes are > fine > > for Sky Buddy receivers and FT101's. A single diode opposing another > diode > > in parallel will clamp at about 6 dBm if your receiver looks like 75 > ohms. > > Almost all receivers will conservatively take 15-20 dBm, or 2-4 volts > peak, > > at the antenna port in band. > > > > If you have a good system, you'll want something other than back-to-back > > diodes. > > > _ > Topband Reflector Archives - http://www.contesting.com/_topband > _ Topband Reflector Archives - http://www.contesting.com/_topband
Re: Topband: Bandpass filters for receive antennas
Oops. Forgot to include the link. Here is the link. http://sites.google.com/site/lwmwbandrejectfilter/ 73, Don (wd8dsb) On Sun, Oct 11, 2015 at 9:03 PM, Don Kirk wrote: > Hi Mike, > > > Here is a link to a simple website I created that describes a high pass > filter I built earlier this year that rejects both the LW & MW band. It > was originally described in a Feb 1978 QST article. I use it in front of an > SDR receiver, and really happy with its performance. > > Don (wd8dsb) > > On Sun, Oct 11, 2015 at 8:35 PM, Mike Waters wrote: > >> Where can I find a schematic for a good high pass filter that rolls off at >> 1700 kHz? >> >> Yesterday, I installed a preamp --that I made over 20 years ago-- right >> at >> one of my 2-wire Beverage antennas. Even in the reverse direction (NW) >> where losses are the greatest, it easily overloads from out-of-band >> signals. Today, I even noticed it oscillating. >> >> It uses an MRF5812 NPN bipolar. I built it over 20 years ago, before I >> knew >> some things I've learned since then (such as being more careful about >> separating the input and output). >> >> I just took it out of line and brought it back to the house. It's the one >> on the right near the bottom of >> www.w0btu.com/W0BTU-broadband-preamps.html on the Radio Shack proto >> board. >> Not shown in the photo is an RF choke between the input and output that >> passes the 12 VDC through the coax, so it can power the relay that >> switches >> from SE to NW. Also, the back-to-back diodes and the input circuit are >> slightly different. >> >> There are some other things I want to do to it, but I think a HP filter >> should be near the top of the agenda. Any suggestions would be >> appreciated. >> >> 73, Mike >> www.w0btu.com >> >> >> On Mon, Aug 31, 2015 at 1:43 PM, Tom W8JI wrote: >> >> > Bandpass filters do nothing for in-band signals -- their only function >> is >> >> to reject OUT OF BAND signals. The primary reason for using bandpass >> >> filters IS for RX. A good 160M bandpass filter would be effective at >> >> reducing overload from AM broadcast stations. So would a high pass >> filter. >> >> There is a survey of bandpass filters for contesting at this link. >> >> http://k9yc.com/BandpassFilterSurvey.pdf >> >> >> > >> > I use a high pass filter for rejection of the AM BCB band. Between 70 >> and >> > 80% of the net power (or voltage) into my RX system comes from distant >> AM >> > BCB signals. >> > >> > Without a small 5-pole highpass that starts to roll off at 1700 kHz, I >> can >> > connect a miniature 12V 50 mA incandescent lamp (like the MFJ 1025 >> uses as >> > a fuse) and it illuminates a dull red. >> > >> > This is with no attempt at matching power to the filament cold >> resistance. >> > >> > My system can be bothered by the sum of all those thousands of signals, >> I >> > add a BCB high pass, and then I can run 1500 watts and not bother my >> own RX >> > when transmitting on 80 or 40 while receiving on 160. Of course I have >> > 500-2000 ft separation on antennas, but this still shows how a bunch of >> > small signals can add up to disaster if they hit something non-linear >> > before being filtered. >> > >> > Always remember there are two problems. One is the absolute limit of >> > in-band signal a receiver system can take. The other is the absolute >> limit >> > of the sum of all the signals entering an overload sensitive point in >> the >> > system. >> > >> > Less than one volt peak line voltage is not enough headroom to prevent >> IM >> > products in a reasonably good system. Back-to-back parallel diodes are >> fine >> > for Sky Buddy receivers and FT101's. A single diode opposing another >> diode >> > in parallel will clamp at about 6 dBm if your receiver looks like 75 >> ohms. >> > Almost all receivers will conservatively take 15-20 dBm, or 2-4 volts >> peak, >> > at the antenna port in band. >> > >> > If you have a good system, you'll want something other than back-to-back >> > diodes. >> > >> _ >> Topband Reflector Archives - http://www.contesting.com/_topband >> > > _ Topband Reflector Archives - http://www.contesting.com/_topband
Re: Topband: Bandpass filters for receive antennas
Hi Mike! I use filters made by Jack Smith at http://www.cliftonlaboratories.com/current_products.htm Great filters ( and expertly pre-tuned ) at a reasonable price - especially so if you purchase just the bare filter board ( w/o enclosure ). 73 Lloyd - N9LB -Original Message- From: Topband [mailto:topband-boun...@contesting.com] On Behalf Of Mike Waters Sent: Sunday, October 11, 2015 7:35 PM To: topband Subject: Re: Topband: Bandpass filters for receive antennas Where can I find a schematic for a good high pass filter that rolls off at 1700 kHz? Yesterday, I installed a preamp --that I made over 20 years ago-- right at one of my 2-wire Beverage antennas. Even in the reverse direction (NW) where losses are the greatest, it easily overloads from out-of-band signals. Today, I even noticed it oscillating. It uses an MRF5812 NPN bipolar. I built it over 20 years ago, before I knew some things I've learned since then (such as being more careful about separating the input and output). I just took it out of line and brought it back to the house. It's the one on the right near the bottom of www.w0btu.com/W0BTU-broadband-preamps.html on the Radio Shack proto board. Not shown in the photo is an RF choke between the input and output that passes the 12 VDC through the coax, so it can power the relay that switches from SE to NW. Also, the back-to-back diodes and the input circuit are slightly different. There are some other things I want to do to it, but I think a HP filter should be near the top of the agenda. Any suggestions would be appreciated. 73, Mike www.w0btu.com On Mon, Aug 31, 2015 at 1:43 PM, Tom W8JI wrote: > Bandpass filters do nothing for in-band signals -- their only function > is >> to reject OUT OF BAND signals. The primary reason for using bandpass >> filters IS for RX. A good 160M bandpass filter would be effective at >> reducing overload from AM broadcast stations. So would a high pass filter. >> There is a survey of bandpass filters for contesting at this link. >> http://k9yc.com/BandpassFilterSurvey.pdf >> > > I use a high pass filter for rejection of the AM BCB band. Between 70 > and 80% of the net power (or voltage) into my RX system comes from > distant AM BCB signals. > > Without a small 5-pole highpass that starts to roll off at 1700 kHz, I > can connect a miniature 12V 50 mA incandescent lamp (like the MFJ > 1025 uses as a fuse) and it illuminates a dull red. > > This is with no attempt at matching power to the filament cold resistance. > > My system can be bothered by the sum of all those thousands of > signals, I add a BCB high pass, and then I can run 1500 watts and not > bother my own RX when transmitting on 80 or 40 while receiving on 160. > Of course I have > 500-2000 ft separation on antennas, but this still shows how a bunch > of small signals can add up to disaster if they hit something > non-linear before being filtered. > > Always remember there are two problems. One is the absolute limit of > in-band signal a receiver system can take. The other is the absolute > limit of the sum of all the signals entering an overload sensitive > point in the system. > > Less than one volt peak line voltage is not enough headroom to prevent > IM products in a reasonably good system. Back-to-back parallel diodes > are fine for Sky Buddy receivers and FT101's. A single diode opposing > another diode in parallel will clamp at about 6 dBm if your receiver looks like 75 ohms. > Almost all receivers will conservatively take 15-20 dBm, or 2-4 volts > peak, at the antenna port in band. > > If you have a good system, you'll want something other than > back-to-back diodes. > _ Topband Reflector Archives - http://www.contesting.com/_topband _ Topband Reflector Archives - http://www.contesting.com/_topband
Re: Topband: Bandpass filters for receive antennas
Where can I find a schematic for a good high pass filter that rolls off at 1700 kHz? I can't even have back to back diodes here on Beverages without amplifiers, because the accumulation of hundreds and hundreds of BCB stations sum to more than two volts peak. With a 10dB gain amp, I can light 12 volt lamps. I cure this with a very simple basic 5 pole filter that has 3dB roll off at 1600 kHz. Unless you have something on top of you at upper AM BCB, you don't have to worry about 1700 kHz plus. You need two inductors and three silver micas, and get a good preamp. Do not put it at the antenna. It just adds to overload issues. My amps are all in the shack, and some of the cables are 3000 foot long mixes of F6, F11, and only a few have significant hardline runs. _ Topband Reflector Archives - http://www.contesting.com/_topband
Re: Topband: Bandpass filters for receive antennas
Thanks for all the replies. Tom, I put that preamp at the antenna to hear a specific station on 20 meters to the NW. On 20, I cannot always hear the background noise in the reverse direction. The Beverage is too long for that band (580'), not to mention it's made from electric fence wire. My main preamp is in the shack. Always has been, and that location works on 40 meters and below. 73, Mike www.w0btu.com On Sun, Oct 11, 2015 at 8:48 PM, Tom W8JI wrote: ... Do not put it at the antenna. It just adds to overload issues. My amps > are all in the shack, ... > _ Topband Reflector Archives - http://www.contesting.com/_topband
