Re: [Elecraft] K3 sensitivity + preamp: looking for wisdom
Ron, Your detailed explanation is one of the best that I have ever read. Although I understand the reasoning for and the performance of preamps on given HF bands, and the conditions of when to use and when to not use them, your verbiage does extreme justice to ansering the question AND describing in words most will understand the full sequence of events surrounding use or non-use of preamps at HF. Thanks for your time and effort to put forth this layman's explanation. 73, de Milt, N5IA - Original Message - From: "Ron D'Eau Claire" To: "'DOUGLAS ZWIEBEL'" ; "'Elecraft Reflector'" Sent: Monday, December 15, 2008 11:14 AM Subject: RE: [Elecraft] K3 sensitivity + preamp: looking for wisdom Keith is quite right. All receivers generate internal noise in their amplifiers. The goal is to make sure this internal noise does not compete with the received signal so the only thing that limits the ability to hear weak signals is the unavoidable atmospheric or "antenna noise". Of course, the amount of "antenna noise" you hear depends upon the filter bandwidth used. Narrower bandwidths admit less noise. That's why narrow band modes (e.g. CW, PSK31, etc.) get through when wider bandwidth modes won't. In order to keep very strong off-frequency signals from overloading the amplifier stages in the receiver, the ideal receiver has as little gain as possible before the filters that set the selectivity. But any noise generated in the very first stages of a receiver (mixer, etc.) gets amplified along with the signal and appears in the speaker or phones. On lower frequencies (up to about 10 MHz) the "antenna noise" in the quietest locations is still relatively high. That allows a receiver for those frequencies to be designed for best dynamic range (minimum gain ahead of the filters) and still the antenna noise will dominate, even at the narrowest filter setting. So the ability to hear weak signals it limited solely by the "antenna noise" and not by the receiver's internal noise. On the higher frequencies, the atmospheric (antenna) noise drops dramatically. Depending upon the noise in one's location, somewhere between 10 and 20 MHz the noise generated internally in most receivers designed for optimum dynamic range on the lower frequencies will start to compete with the antenna noise. The K3 (and K2) are no exceptions. The answer is to add more gain near the antenna using a low-noise "preamplifier" that boosts the signal and antenna noise levels up above the receiver's internal noise. That's why the K3 (and K2) have built-in "preamplifiers" that can be switched in on the higher frequencies. "Low noise" refers to the fact that the preamplifier has low enough internal noise that it won't compete with signals down at the level of the "antenna noise". In general, the "quieter" (lower noise) the preamplifier is, the more susceptible to overload it becomes. And you don't want more gain than necessary ahead of the receiver's filters. Every dB of gain added reduces the dynamic range by bringing all signals and noise up nearer the clipping point of the subsequent amplifier stages in the receiver. That's why the preamplifiers are switchable. Smart ops turn them off on the lower frequencies where they aren't needed. Indeed, that's why there's an attenuator. Under extreme conditions, attenuating all the signals - wanted and unwanted alike - provides greater dynamic range and better performance, as long as the antenna noise is still greater than that generated internally in the receiver. That's a very common situation on the 160, 80 and sometimes the 40 meter Ham bands for many Hams. It depends upon their antenna and the level of the "antenna noise". The built in preamplifier in the K3 does a great job on frequencies up to 30 MHz or so but it's marginal for best weak-signal reception in many locations on 6 meters. That's why the PR6 was added. It's optimized for best 6 meter weak-signal reception under most circumstances by providing enough gain to bring the very low "antenna noise" and signals on 6 meters up above the internal receiver noise. Ron AC7AC -Original Message- Hi... The following request for information is not specific to the K3 (or even to Elecraft). Please craft your response to be generic. Despite being an active ham/contester for over 40 continuous years (but not being an ee), I just do not understand how a "preamp" works. It is not a "smart" device, so how does it "know" to amplify the signal and not the noise? I first used a preamp way back when (1960's) when I was a teenager...it was an Ameco job. It was more "fun" than useful, because I could give out "b
RE: [Elecraft] K3 sensitivity + preamp: looking for wisdom
Hello Doug, Here some basics about preamps. I will try to make it short, but I could expand to pages A preamp only makes sense if the noise-figure (NF) of the preamp is better than the NF of the normal input stage (usually the normal frontend input). Each stage in a receiver (frontend, mixer, pads, amplifiers and buffers) adds his internal noise to the combination of signal and noise injected into that stage. If the noise-figure (NF) of the first stage is better then the NF of the following stage, the amplification factor of the first stage is determining if, and how much, the S/N ratio becomes better (or not). So, to stay with your words: The preamp is not some kind of intelligent amplifier selecting signal out of the noise but it should present a better signal to noise ratio to the next stage because it adds less internal (thermal) noise to the signal than the following stage. The best result in signal to noise ratio is coming out of your antenna connector (at the antenna) everything done after, is worsening the situation. The first decrease in S/N ration is due to attenuation of the coaxial cable. So the best place to put the preamp is at the antenna so the signal + antenna noise is amplified to a "maximum" level. The amount of noise added by the preamp and coaxial cable is only relative small compared to the amplified antenna signal. A preamp has got 1 big disadvantage: it decreases the dynamic range of the original receiver setup (with about the same number of dB's as the gain of the pre-amplifier). The best solution whatever is a frontend that has to little gain of it's own but a relative low NF (let's say 3 dB) and capable of handling BIG signals. A separate preamp that has a NF of 0,5 dB and a gain of 13 dB would be good enough to have a good overall performance (low NF and not too much gain before the first mixer stage). Most preamps in HF equipment are TOTALLY useless and only worsening the situation (but looks good on your S-meter). The discussion about the need for a low NF preamp on HF is easy for me: The less noise the preamp adds from itself (and so worsening the S/N ratio) the better it is. The effect of a low noise preamp is lessening when the atmospheric noise is higher, but the effect will never be zero! On low band the atmospheric noise is high and local signals very strong and so stressing the dynamic range of the receiver. On these bands it's sometimes better to switch the preamp off. 73's Evert PA2KW -Original Message- From: elecraft-boun...@mailman.qth.net [mailto:elecraft-boun...@mailman.qth.net] On Behalf Of DOUGLAS ZWIEBEL Sent: Monday, December 15, 2008 17:15 To: Elecraft Reflector Subject: [Elecraft] K3 sensitivity + preamp: looking for wisdom Hi... The following request for information is not specific to the K3 (or even to Elecraft). Please craft your response to be generic. Despite being an active ham/contester for over 40 continuous years (but not being an ee), I just do not understand how a "preamp" works. It is not a "smart" device, so how does it "know" to amplify the signal and not the noise? I first used a preamp way back when (1960's) when I was a teenager...it was an Ameco job. It was more "fun" than useful, because I could give out "big" S meter reports. Later, when listening in the noise became more important to my style of operation, I never could notice a difference between putting the preamp "on" and just turning up the volume (many rigs with built-in preamps too). Sometimes, turning up the AF gain worked better. So...HOW does a preamp help to "bring a signal out of the noise" without the use of some fancy computer algorithms backed by lots of computing power? Further, does a preamp really "improve" sensitivity? I have yet to find a signal that I could not copy (or hear) with any preamp "on" vs increasing the AF gain. PS...I am talking about HF. I know that lots of guys use a preamp on 160m with their beverages, so the effect must be real...right? HOW does a "dumb" device do it? de Doug KR2Q ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
RE: [Elecraft] K3 sensitivity + preamp: looking for wisdom
Keith is quite right. All receivers generate internal noise in their amplifiers. The goal is to make sure this internal noise does not compete with the received signal so the only thing that limits the ability to hear weak signals is the unavoidable atmospheric or "antenna noise". Of course, the amount of "antenna noise" you hear depends upon the filter bandwidth used. Narrower bandwidths admit less noise. That's why narrow band modes (e.g. CW, PSK31, etc.) get through when wider bandwidth modes won't. In order to keep very strong off-frequency signals from overloading the amplifier stages in the receiver, the ideal receiver has as little gain as possible before the filters that set the selectivity. But any noise generated in the very first stages of a receiver (mixer, etc.) gets amplified along with the signal and appears in the speaker or phones. On lower frequencies (up to about 10 MHz) the "antenna noise" in the quietest locations is still relatively high. That allows a receiver for those frequencies to be designed for best dynamic range (minimum gain ahead of the filters) and still the antenna noise will dominate, even at the narrowest filter setting. So the ability to hear weak signals it limited solely by the "antenna noise" and not by the receiver's internal noise. On the higher frequencies, the atmospheric (antenna) noise drops dramatically. Depending upon the noise in one's location, somewhere between 10 and 20 MHz the noise generated internally in most receivers designed for optimum dynamic range on the lower frequencies will start to compete with the antenna noise. The K3 (and K2) are no exceptions. The answer is to add more gain near the antenna using a low-noise "preamplifier" that boosts the signal and antenna noise levels up above the receiver's internal noise. That's why the K3 (and K2) have built-in "preamplifiers" that can be switched in on the higher frequencies. "Low noise" refers to the fact that the preamplifier has low enough internal noise that it won't compete with signals down at the level of the "antenna noise". In general, the "quieter" (lower noise) the preamplifier is, the more susceptible to overload it becomes. And you don't want more gain than necessary ahead of the receiver's filters. Every dB of gain added reduces the dynamic range by bringing all signals and noise up nearer the clipping point of the subsequent amplifier stages in the receiver. That's why the preamplifiers are switchable. Smart ops turn them off on the lower frequencies where they aren't needed. Indeed, that's why there's an attenuator. Under extreme conditions, attenuating all the signals - wanted and unwanted alike - provides greater dynamic range and better performance, as long as the antenna noise is still greater than that generated internally in the receiver. That's a very common situation on the 160, 80 and sometimes the 40 meter Ham bands for many Hams. It depends upon their antenna and the level of the "antenna noise". The built in preamplifier in the K3 does a great job on frequencies up to 30 MHz or so but it's marginal for best weak-signal reception in many locations on 6 meters. That's why the PR6 was added. It's optimized for best 6 meter weak-signal reception under most circumstances by providing enough gain to bring the very low "antenna noise" and signals on 6 meters up above the internal receiver noise. Ron AC7AC -Original Message- Hi... The following request for information is not specific to the K3 (or even to Elecraft). Please craft your response to be generic. Despite being an active ham/contester for over 40 continuous years (but not being an ee), I just do not understand how a "preamp" works. It is not a "smart" device, so how does it "know" to amplify the signal and not the noise? I first used a preamp way back when (1960's) when I was a teenager...it was an Ameco job. It was more "fun" than useful, because I could give out "big" S meter reports. Later, when listening in the noise became more important to my style of operation, I never could notice a difference between putting the preamp "on" and just turning up the volume (many rigs with built-in preamps too). Sometimes, turning up the AF gain worked better. So...HOW does a preamp help to "bring a signal out of the noise" without the use of some fancy computer algorithms backed by lots of computing power? Further, does a preamp really "improve" sensitivity? I have yet to find a signal that I could not copy (or hear) with any preamp "on" vs increasing the AF gain. PS...I am talking about HF. I know that lots of guys use a preamp on 160m with their beverages, so the effect must be real...right? HOW does a "dumb" device do it? de Doug KR2Q ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help
Re: [Elecraft] K3 sensitivity + preamp: looking for wisdom
DOUGLAS ZWIEBEL wrote: > > Hi... > > The following request for information is not specific to the K3 (or > even to Elecraft). Please craft your response to be generic. > > Despite being an active ham/contester for over 40 continuous years > (but not being an ee), I just do not understand how a "preamp" works. > It is not a "smart" device, so how does it "know" to amplify the > signal and not the noise? I first used a preamp way back when > (1960's) when I was a teenager...it was an Ameco job. It was more > "fun" than useful, because I could give out "big" S meter reports. > > Later, when listening in the noise became more important to my style > of operation, I never could notice a difference between putting the > preamp "on" and just turning up the volume (many rigs with built-in > preamps too). Sometimes, turning up the AF gain worked better. > > So...HOW does a preamp help to "bring a signal out of the noise" > without the use of some fancy computer algorithms backed by lots of > computing power? Further, does a preamp really "improve" sensitivity? > I have yet to find a signal that I could not copy (or hear) with any > preamp "on" vs increasing the AF gain. PS...I am talking about HF. I > know that lots of guys use a preamp on 160m with their beverages, so > the effect must be real...right? HOW does a "dumb" device do it? > > de Doug KR2Q > > There is residual noise within the receiver. By amplifying the signal, a preamp can improve the signal to noise ratio, if the noise being received by the antenna is close to the amount of noise that is generated in the receiver. Therefore preamps are most often needed on VHF and up where the atmospheric noise level is low and greater gain is needed to raise it to the point where it swamps internally generated noise. If, as I do, you live in an electrically noisy location, then a preamp may seem to make no difference at all. I do not find the K3 preamp has any benefit at all, since the S meter is always reading a couple of S points (often a lot more!) even on a dead band. Perhaps when the higher frequency bands become workable again the preamp might show some benefits. It is probably also useful on 6m. - Julian, G4ILO. K2 #392 K3 #222. http://www.g4ilo.com/ G4ILO's Shack http://www.ham-directory.com/ Ham Directoryhttp://www.g4ilo.com/kcomm.html KComm for Elecraft K2 and K3 -- View this message in context: http://n2.nabble.com/K3-sensitivity-%2B-preamp%3A-looking-for-wisdom-tp1658815p1658925.html Sent from the Elecraft mailing list archive at Nabble.com. ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
Re: [Elecraft] K3 sensitivity + preamp: looking for wisdom
Doug, the preamp does not discriminate between signal and noise. It does not bring the signal "out of the noise". It will raise the level of the signal and the noise if the noise level is very low such as happens frequently on VHF, sometimes on ten meters and occasionally on 15 meters. Preamps seldom help on the lower bands. The ten meter contest this weekend was a time where a preamp helped. I used the one in my K3. For me, most of the time the noise level was low enough that the preamp was of benefit. It raised the signal about an S unit which was enough. Part of the time the local power line noise was enough that I turned the preamp off because the noise level was moving the S meter. The best place for a preamp is at the antenna before the coax. Needless to say, this presents a lot of complications for transmitting antennas so hams usually use a preamp at the receiver. Preamps are great for receive only antennas that are far away from the shack, particularly if they are not very sensitive (such as a Beverage on 160 meters) As you observed, preamps are not very smart. They are just a transistor or two and some filtering. They are useful sometimes, harmful at others. I hope this helps. Willis 'Cookie' Cooke K5EWJ --- On Mon, 12/15/08, DOUGLAS ZWIEBEL wrote: > From: DOUGLAS ZWIEBEL > Subject: [Elecraft] K3 sensitivity + preamp: looking for wisdom > To: "Elecraft Reflector" > Date: Monday, December 15, 2008, 8:14 AM > Hi... > > The following request for information is not specific to > the K3 (or > even to Elecraft). Please craft your response to be > generic. > > Despite being an active ham/contester for over 40 > continuous years > (but not being an ee), I just do not understand how a > "preamp" works. > It is not a "smart" device, so how does it > "know" to amplify the > signal and not the noise? I first used a preamp way back > when > (1960's) when I was a teenager...it was an Ameco job. > It was more > "fun" than useful, because I could give out > "big" S meter reports. > > Later, when listening in the noise became more important to > my style > of operation, I never could notice a difference between > putting the > preamp "on" and just turning up the volume (many > rigs with built-in > preamps too). Sometimes, turning up the AF gain worked > better. > > So...HOW does a preamp help to "bring a signal out of > the noise" > without the use of some fancy computer algorithms backed by > lots of > computing power? Further, does a preamp really > "improve" sensitivity? > I have yet to find a signal that I could not copy (or > hear) with any > preamp "on" vs increasing the AF gain. PS...I am > talking about HF. I > know that lots of guys use a preamp on 160m with their > beverages, so > the effect must be real...right? HOW does a > "dumb" device do it? > > de Doug KR2Q > ___ > Elecraft mailing list > Post to: Elecraft@mailman.qth.net > You must be a subscriber to post to the list. > Subscriber Info (Addr. Change, sub, unsub etc.): > http://mailman.qth.net/mailman/listinfo/elecraft > > Help: http://mailman.qth.net/subscribers.htm > Elecraft web page: http://www.elecraft.com ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
RE: [Elecraft] K3 sensitivity + preamp: looking for wisdom
I think the theory is that the atmospheric noise is below the receiver's detection level so the noise you hear is generated inside the rig. Add a preamp and both the antenna noise and signal come up by the same amount bringing the antenna noise up to the detection floor and boosting the signal at the same time. The overall noise floor doesn't change much if any but switched from internal noise to antenna noise. The signal, however has come up and is easier to copy. - Keith N1AS - - K3 711 - -Original Message- From: elecraft-boun...@mailman.qth.net [mailto:elecraft-boun...@mailman.qth.net] On Behalf Of DOUGLAS ZWIEBEL Sent: Monday, December 15, 2008 11:15 AM To: Elecraft Reflector Subject: [Elecraft] K3 sensitivity + preamp: looking for wisdom Hi... The following request for information is not specific to the K3 (or even to Elecraft). Please craft your response to be generic. Despite being an active ham/contester for over 40 continuous years (but not being an ee), I just do not understand how a "preamp" works. It is not a "smart" device, so how does it "know" to amplify the signal and not the noise? I first used a preamp way back when (1960's) when I was a teenager...it was an Ameco job. It was more "fun" than useful, because I could give out "big" S meter reports. Later, when listening in the noise became more important to my style of operation, I never could notice a difference between putting the preamp "on" and just turning up the volume (many rigs with built-in preamps too). Sometimes, turning up the AF gain worked better. So...HOW does a preamp help to "bring a signal out of the noise" without the use of some fancy computer algorithms backed by lots of computing power? Further, does a preamp really "improve" sensitivity? I have yet to find a signal that I could not copy (or hear) with any preamp "on" vs increasing the AF gain. PS...I am talking about HF. I know that lots of guys use a preamp on 160m with their beverages, so the effect must be real...right? HOW does a "dumb" device do it? de Doug KR2Q ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
