Re: [Elecraft] dynamic range questions
Inline On Jun 1, 2007, at 10:56 AM, [EMAIL PROTECTED] wrote: I assume that receiver circuit characteristics before the roofing filters (or IF bandpass filter) essentially determine the dynamic range. However, dynamic range figures "always" seem to degrade for small separations of fc and fc+x. Is this because fc and/or fc+x are falling within the IF passband and are producing spurious signals in the stages following the roofing filters? You have to consider which stages set the dynamic range of the receiver. In a well designed receiver it should be the first mixer that sets the dynamic range (not the RF amp, if one is used, the filter itself, the post-mixer amp or any of the IF or AF stages. Yes (it's fc) that does the "damage" if it gets by a wide roofing filter to the post-filter amp. An in-band measurement will give you some info on the first mixer and the crystal filter (the later starts to become an issue for mixers with a very high intercept). See Sherwoods measurements going down to 2kHz seperation (though for CW signals this should still be outside the first filter). It's a good idea to do the measurement at multiple seperation and to ALWAYS quote the separation with the dynamic range. For the case of strong signals in the IF passband, are the dynamic range tests run with the AGC disabled? In actual operation, wouldn't the AGC reduce signal levels below the point where distortion products were being generated (in the stages following the roofing filters)? You normally have no AGC elements before the first mixer. It's usually all after the crystal filter as the filter a delay in system that will cause problems in getting the AGC loop well controlled. Usually the AGC if off to make the measurement though. The two tone test is only a proxy for the "real world". In actual operation, if any signal (or signals) in the RF passband or mixer passband exceeds the receiver dynamic range, will ALL of the signals in the passband begin contributing third order products? Yes, but only the strongest signals really make an impact. The spurious signals are third order products so they vary as the cube of the input power. Or their slope when you plot a graph in dB is three times steeper than the input power. The spurious signals increase twice as fast (on a logarithmic plot) as the input power increases. The intercept point is the power when the spurious signal is the same power as a single test tone. You can't actually get to that point -- gain compression takes over 10 or 15dB before you hit that point. This is a more serious issue for CW with discrete, narrow (50Hz), tones. The spurious signals often sound like "bad" morse code when two CW signals intermod together -- they're the product (the AND in a logical sense) of one or more CW transmissions. In other cases they can sound like a good CW signal (say a strong CW signal intermodulatinging with a SW broadcast carrier) on the wrong frequency. For SSB there is less of an issue (in general). The power is spread over a 3kHz so the components are typically -18dB down on the same power CW signal (3kHz/50Hz) so you are less likely to hear the problem in the same RF environment. When it does manifest it's more like the noise floor increasing in stregth than discrete signals though in the case of an SSB signal intermodulating with a strong carrier you would hear a distorted SSB-like signal. You can do the calculation to determine what a passband looks (sounds) like when using "non-equal power" tones for the DR test (or as you know them, the ham bands). Wes Hayward does it in his "Introduction to RF Design" book. Is the magnitude of the third order distortion products a function of the "degree" of the nonlinearity? Yes. This non-linearity is what controls the intercept points -- the measure usually quoted for amplifiers, mixers and other devices (the 3rd order intercept point is the default but of course there is a second order intercept point that is important too :-) Can different receiver models with the same dynamic range numbers (operating under the same conditions) differ considerably in the "signal strength" of junk signals? By "junk signals" you mean spurious signals from (2nd or) 3rd order IMD so the answer is no. Assuming they measure the same at the But of course there are other differences (phase noise which might mask these effects) and other sources of spurious signals. I'd recommend reading Peter E. Chadwick, G3RZP, QEX article "HF Receiver dynamic Range: How Much Do We Need?" (QEX May/June 2002, p36-41). A good receiver needs a dynamic range of about 100dB according to a study by Peter E. Chadwick, G3RZP HF Receiver dynamic Range: How Much Do We Need? which appeared in the May/June 2002 issue of QEX. The number 100dB for phase noise limited dynamic range given by G3RZP refers to "SSB bandwidth" which means that the noise floor
Re: [Elecraft] dynamic range questions
My pleasure Mike, my 3am "Autopilot" appeared to be working, maybe! One final comment. The two in passband signal tests will tell you how a receiver will perform in use for example in a pile up situation where the DX station is not working split, whereas the wide spacing tests will only give you a hint. But you need very good test equipment. As Bill W5WVO said in his post "at the end of the day it's dynamic range". Once a large dynamic range has been obtained, then in CW receivers zero beat QRM stripping circuitry can be introduced - but that is another subject. 73, Geoff GM4ESD Corboy-Poteet <[EMAIL PROTECTED]> wrote: Geoff, thank you very much for the response. I am posting to the list as others may find your comments helpful. Mike W5FTD Mike, A couple of comments before I crash - it is past 3 am here!! IMD for the K3 (Orion, FTxxx, etc) refers to spurious signals produced by operation of a stage (or stages) in the nonlinear portion of its transfer curve (or whatever it's called)? ___ 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] dynamic range questions
Geoff, thank you very much for the response. I am posting to the list as others may find your comments helpful. Mike W5FTD Mike, A couple of comments before I crash - it is past 3 am here!! > IMD for the K3 (Orion, FTxxx, etc) refers to spurious signals produced > by operation of a stage (or stages) in the nonlinear portion of its > transfer curve (or whatever it's called)? Yes, and the test result should specify which spurious products e.g. 3rd Order. > The measurement "ground rules" imply two signals: one at fc and the > other at fc+x, where "x" is 20khz, 10khz, ... ? This is where the "numbers game" starts! A proper test has both signals in the passband along with the products, but the great majority of commercial amateur receivers would not do too well. So with great reluctance on the part of ARRL the spacing "x" has been gradually decreased with both signals outside of the passband so that the product being measured is within the passband. > The parameter usually being determined is referred to as the two tone > dynamic range and indicates the point at which third order signals > resulting from 2fc-(fc+x) and 2(fc+x)-fc begin to appear in the signal > path? This would be the 3rd Order dynamic range. The dynamic range is sometimes measured from the noise floor but not always. This is because some amplifiers / mixers do not "behave" properly because the level of the 3rd Order products over some part of the allowed input level range does not increase at the rate of 3db for every 1db increase in the level of the input signals. > fc and fc+x are chosen to put (at least one of) the third order > signals in the IF passband? Yes. > I assume that receiver circuit characteristics before the roofing > filters (or IF bandpass filter) essentially determine the dynamic > range. However, dynamic range figures "always" seem to degrade for > small separations of fc and fc+x. Is this because fc and/or fc+x are > falling within the IF passband and are producing spurious signals in > the stages following the roofing filters? Yes, and this is something that many designers seem to forget. > For the case of strong signals in the IF passband, are the dynamic > range tests run with the AGC disabled? In actual operation, wouldn't > the AGC reduce signal levels below the point where distortion products > were being generated (in the stages following the roofing filters)? I believe that most if not all 3rd Order dynamic range tests on amateur receivers are run with the AGC disabled, but don't quote me! In actual operation the IMD generated by some types of AGC controlled amplifier can get worse as AGC is applied. It is up to the designer to do his job properly! > The two tone test is only a proxy for the "real world". In actual > operation, if any signal (or signals) in the RF passband or mixer > passband exceeds the receiver dynamic range, will ALL of the signals > in the passband begin contributing third order products? Yes, how much will depend on individual levels and of course the receiver design. > Is the magnitude of the third order distortion products a function of > the "degree" of the nonlinearity? Can different receiver models with > the same dynamic range numbers (operating under the same conditions) > differ considerably in the "signal strength" of junk signals? Yes is the short answer. This is one good reason why 3rd Order dynamic range tests should be done with both signals in the passband which can expose problems which might exist after the IF filter in model A but not in model B. IMD generated by IF filters can be a problem which might not be fully exposed if the test signals are widely spaced. Two major issues are of concern here (1) the effect of the filter on the dynamic range of the driving stage and (2) the generation of IMD products within the filter which are passed on to the following IF cascade. > When test results are "phase noise limited", what is going on? Is the > phase noise acting simply as RF noise that masks the spurious signals > being observed? Or is it acting as a "third signal" that mixes with fc > and fc+x and causes the production of third order products at a lower > signal level than fc and fc+x would alone? Phase noise from a noisy LO has the effect of lifting the noise floor and is involved in reciprocal mixing. "Phase Noise Limited" means that the useful performance of the receiver has been compromised by LO noise, for example the 3rd Order dynamic range is reduced or limited by phase noise because there is less "headroom" between the increased noise floor and the point where the receiver runs out of steam. Reciprocal mixing takes place when the LO phase noise mixes with an unwanted signal to produce a "noise" IF signal which could mask weak real signals - an unwanted LO if you like. I hope that this helps a little. 73, Geoff GM4ESD ___ Elecraft mailing list Post to: Elecra
Re: [Elecraft] dynamic range questions
Don, Your suggestion that both applied signals should be in the receiver's passband in order to obtain a 'figure of merit' agrees with the argument for a more useful 3rd Order dynamic range test on amateur receivers, a discussion which has been taking place for several years as far as I know. Of course the phase noise of each test signal source has to be suitably low in level at a very small offset to avoid corruption of the test results, or if you like 'Test equipment phase noise limited' results. Assuming that the receiver's 3rd Order dynamic range is not limited by its own LO's phase noise, this requirement placed on the test equipment could be one reason for resisting any change to the current test parameters however unsatisfactory they might be. Arguably one could move up from the noise floor to determine the 3rd Order dynamic range. With regard to the protection provided by roofing filters, one approach used during the design of specialized high performance receivers whose end cost was not, within reason, a controlling factor was to assume that the filter would not provide protection, although filter generated IMD was taken into account. This meant that the IMD performance of the entire receiver was maintained at a high level, and any protection provided by the filter was 'icing on the cake'. From that point on in the design process one could make adjustments aimed at reducing power consumption and cost provided that the receiver's IMD performance within the filter's passband was not compromised. MDS as you appreciate means 'Minimum Discernible Signal' not a signal at the noise floor. IIRC the term was first intended to quantify the signal which could be discerned by a human or device connected to a receiver's output, which certainly in the human case introduces a variable. Some people can hear a signal well below the noise floor thanks to the filter between the ears, others with impaired hearing may not hear the signal until it rises well above the noise floor. You might be interested to read SM5BSZ's comments on the misuse of the term MDS in QEX March / April 2006 page 36, others have commented before. Thank you for your comment. 73, Geoff GM4ESD Don Wilhelm <[EMAIL PROTECTED]> wrote: Geoff, While all the situations that you pose are valid measurements, I believe that the 'figure of merit' that should be applied is with both signals within the receiver passband. That alone will indicate the ability to copy a weak signal in the presence of a strong one - that is basic BDR (IMHO). Third order blocking IMD is quite another thing - there are many variables to consider. The best that we can hope for is that the 'standardized' tests will provide a good indicator of the performance on the real world - a receiver with narrow 'roofing filters' should excel in that test - although the test reports should specify the filter widths. The close spacing tests go a long way toward simulating the 'real world', but are really only a set of conditions that are defined for lab tests - the real world operating conditions may present an entirely different set of parameters. Current tests do consider MDS to be the minimum signal that can be demodulated with no other considerations. Right or wrong, that is the way it is for now - something must be defined for lab measurements to be valid - we just trust that they are adequate to be useful in actual operating conditions. 73, Don W3FPR ___ 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] dynamic range questions
Don Wilhelm wrote: Third order blocking IMD is quite another thing - there are many variables to consider. The best that we can hope for is that the 'standardized' tests will provide a good indicator of the performance on the real world - a receiver with narrow 'roofing filters' should excel in that test - although the test reports should specify the filter widths. Rob Sherwood always specifies rig settings/options -- don't know about ARRL. Not so much, I don't think. This should be done universally, and in detail. The devil is definitely in the details here. That said, I believe real-world performance can be expected to pretty much follow the bench metrics, based on my own limited experience (especially on the negative end). To wit: Some years ago, before I was dry behind the ears in terms of understanding receiver design concepts and metrics, I acquired a Kenwood TS-2000, seduced by its sexy looks, multi-band multi-mode capabilitities, and so forth. Having used this rig on 6M under difficult contest conditions (extraordinarily strong sporadic-E openings with many S9+40 signals) over several years, I can tell you for sure that this rig absolutely falls on its derrière in the presence of nearby strong signals. It is the next thing to useless as a contest radio, especially on SSB. (I don't think much of it on CW, either.) Subsequent to my acquiring it, Rob Sherwood came out with his 2 kHz dynamic range metrics, and these showed the TS-2000 to be almost at the bottom of the pack in terms of 3rd-order IMD dynamic range (57 dB). ARRL's numbers pointed in the same direction, though they only measured to 5 kHz separation. This experience made a believer out of me, as the test data exactly illustrated my own on-air experiences with this whoreson dog of a radio. ;-) On the other end of things, the TenTec Orion and Orion II continue to be the contesters' radios of choice, despite the plethora of problems both these radios have had with design, manufacturing, and firmware. I doubt that steely-eyed contesters are going to stick with a problematical radio unless there is really something going there in terms of basic performance that they can't get anywhere else. At the end of the day, it's dyanamic range, dynamic range, and dynamic range. So yes, I do believe in receiver bench metrics! Bill / W5WVO ___ 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] dynamic range questions
Geoff, While all the situations that you pose are valid measurements, I believe that the 'figure of merit' that should be applied is with both signals within the receiver passband. That alone will indicate the ability to copy a weak signal in the presence of a strong one - that is basic BDR (IMHO). Third order blocking IMD is quite another thing - there are many variables to consider. The best that we can hope for is that the 'standardized' tests will provide a good indicator of the performance on the real world - a receiver with narrow 'roofing filters' should excel in that test - although the test reports should specify the filter widths. The close spacing tests go a long way toward simulating the 'real world', but are really only a set of conditions that are defined for lab tests - the real world operating conditions may present an entirely different set of parameters. Current tests do consider MDS to be the minimum signal that can be demodulated with no other considerations. Right or wrong, that is the way it is for now - something must be defined for lab measurements to be valid - we just trust that they are adequate to be useful in actual operating conditions. 73, Don W3FPR Geoffrey Mackenzie-Kennedy wrote: Curt, Would you not agree that a 3rd Order dynamic range test performed with both test tones placed outside of the IF filter's passband ,with a product in the passband, does not provide a complete measure of a receiver's odd order IMD performance but only that of its front end? A receiver with a "weak" cascade following the IF filter might appear to be be a good performer based on tests done this way, but will probably fall apart when two or more strong signals enter the IF filter's passband. During the development of high performance receivers common practice in my experience was to employ multiple signal tests, two tones in the IF passband and one or two placed on the skirts. I am aware of suggestions that have been made during the past years regarding these test procedures used to evaluate amateur receivers, likewise to restore proper useage of the term MDS which is another story :-) ___ 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] dynamic range questions
Yes I agree, but not all up-conversion radios suffer from this problem. It is a great pity that the design of amateur up-conversion radios do not seem to include very strong IF amplifiers IIP3 in the +45dbm region, which allows for some relaxation in roofing filter requirements , cost I suspect. Far fewer close in spurious responses to deal with. Geoff GM4ESD Bill W5WVO <[EMAIL PROTECTED]> wrote: Well... yeah,sure. That's why the K3 offers a selection of appropriately narrow roofing filters at the 1st IF frequency. If you're trying to copy CW with the 6 kHz AM filter inline, of course you're going to have problems, just as you certainly do with all the up-conversion radios (like the TS-2000 I own) with broad-as-a-barn-door roofing filters in the 1st IF. Strong off-frequency signals within that wide passband will cause later stages to fall apart, no question. That's why the K3 (and a few other radios like the Orions) are designed the way they are -- to do everything possible to mitigate against that possibility. Bill / W5WVO ___ 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] dynamic range questions
Geoffrey Mackenzie-Kennedy wrote: A receiver with a "weak" cascade following the IF filter might appear to be be a good performer based on tests done this way, but will probably fall apart when two or more strong signals enter the IF filter's passband. Well... yeah,sure. That's why the K3 offers a selection of appropriately narrow roofing filters at the 1st IF frequency. If you're trying to copy CW with the 6 kHz AM filter inline, of course you're going to have problems, just as you certainly do with all the up-conversion radios (like the TS-2000 I own) with broad-as-a-barn-door roofing filters in the 1st IF. Strong off-frequency signals within that wide passband will cause later stages to fall apart, no question. That's why the K3 (and a few other radios like the Orions) are designed the way they are -- to do everything possible to mitigate against that possibility. Bill / W5WVO During the development of high performance receivers common practice in my experience was to employ multiple signal tests, two tones in the IF passband and one or two placed on the skirts. I am aware of suggestions that have been made during the past years regarding these test procedures used to evaluate amateur receivers, likewise to restore proper useage of the term MDS which is another story :-) 73, Geoff GM4ESD Curt Milton <[EMAIL PROTECTED]> wrote: Mike for two tone dynamic range testing, both signals are located intentionally outside the detection passband (narrowest filter) of the receiver, and the receiver is tuned so that one of the third order intermods is within the passband. you may find that this is a useful reference: http://www.arrl.org/tis/info/pdf/109435.pdf when the measurement is bounded by phase noise, it means that instead of introducing a distortion product or reducing the level of a weak signal, instead the phase noise introduced by the receiver itself (generally the local oscillators) has degraded detection of the weak signal. otherwise, you seem to be on the right track with your understanding. 73, curt ___ 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] dynamic range questions
Curt, Would you not agree that a 3rd Order dynamic range test performed with both test tones placed outside of the IF filter's passband ,with a product in the passband, does not provide a complete measure of a receiver's odd order IMD performance but only that of its front end? A receiver with a "weak" cascade following the IF filter might appear to be be a good performer based on tests done this way, but will probably fall apart when two or more strong signals enter the IF filter's passband. During the development of high performance receivers common practice in my experience was to employ multiple signal tests, two tones in the IF passband and one or two placed on the skirts. I am aware of suggestions that have been made during the past years regarding these test procedures used to evaluate amateur receivers, likewise to restore proper useage of the term MDS which is another story :-) 73, Geoff GM4ESD Curt Milton <[EMAIL PROTECTED]> wrote: Mike for two tone dynamic range testing, both signals are located intentionally outside the detection passband (narrowest filter) of the receiver, and the receiver is tuned so that one of the third order intermods is within the passband. you may find that this is a useful reference: http://www.arrl.org/tis/info/pdf/109435.pdf when the measurement is bounded by phase noise, it means that instead of introducing a distortion product or reducing the level of a weak signal, instead the phase noise introduced by the receiver itself (generally the local oscillators) has degraded detection of the weak signal. otherwise, you seem to be on the right track with your understanding. 73, curt ___ 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] dynamic range questions
Mike for two tone dynamic range testing, both signals are located intentionally outside the detection passband (narrowest filter) of the receiver, and the receiver is tuned so that one of the third order intermods is within the passband. you may find that this is a useful reference: http://www.arrl.org/tis/info/pdf/109435.pdf when the measurement is bounded by phase noise, it means that instead of introducing a distortion product or reducing the level of a weak signal, instead the phase noise introduced by the receiver itself (generally the local oscillators) has degraded detection of the weak signal. otherwise, you seem to be on the right track with your understanding. 73, curt --- [EMAIL PROTECTED] wrote: > I've got some questions for the receiver meisters. > These are not K3 > questions but will affect my understanding of the K3 > specs when they > appear. > > IMD for the K3 (Orion, FTxxx, etc) refers to > spurious signals produced > by operation of a stage (or stages) in the nonlinear > portion of its > transfer curve (or whatever it's called)? > > The measurement "ground rules" imply two signals: > one at fc and the > other at fc+x, where "x" is 20khz, 10khz, ... ? > > The parameter usually being determined is referred > to as the two tone > dynamic range and indicates the point at which third > order signals > resulting from 2fc-(fc+x) and 2(fc+x)-fc begin to > appear in the signal > path? > > fc and fc+x are chosen to put (at least one of) the > third order > signals in the IF passband? > > I assume that receiver circuit characteristics > before the roofing > filters (or IF bandpass filter) essentially > determine the dynamic > range. However, dynamic range figures "always" seem > to degrade for > small separations of fc and fc+x. Is this because fc > and/or fc+x are > falling within the IF passband and are producing > spurious signals in > the stages following the roofing filters? > > For the case of strong signals in the IF passband, > are the dynamic > range tests run with the AGC disabled? In actual > operation, wouldn't > the AGC reduce signal levels below the point where > distortion products > were being generated (in the stages following the > roofing filters)? > > The two tone test is only a proxy for the "real > world". In actual > operation, if any signal (or signals) in the RF > passband or mixer > passband exceeds the receiver dynamic range, will > ALL of the signals > in the passband begin contributing third order > products? > > Is the magnitude of the third order distortion > products a function of > the "degree" of the nonlinearity? Can different > receiver models with > the same dynamic range numbers (operating under the > same conditions) > differ considerably in the "signal strength" of junk > signals? > > When test results are "phase noise limited", what is > going on? Is the > phase noise acting simply as RF noise that masks the > spurious signals > being observed? Or is it acting as a "third signal" > that mixes with fc > and fc+x and causes the production of third order > products at a lower > signal level than fc and fc+x would alone? > > > Mike W5FTD > > ___ > 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 > Got a little couch potato? Check out fun summer activities for kids. http://search.yahoo.com/search?fr=oni_on_mail&p=summer+activities+for+kids&cs=bz ___ 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] dynamic range questions
I've got some questions for the receiver meisters. These are not K3 questions but will affect my understanding of the K3 specs when they appear. IMD for the K3 (Orion, FTxxx, etc) refers to spurious signals produced by operation of a stage (or stages) in the nonlinear portion of its transfer curve (or whatever it's called)? The measurement "ground rules" imply two signals: one at fc and the other at fc+x, where "x" is 20khz, 10khz, ... ? The parameter usually being determined is referred to as the two tone dynamic range and indicates the point at which third order signals resulting from 2fc-(fc+x) and 2(fc+x)-fc begin to appear in the signal path? fc and fc+x are chosen to put (at least one of) the third order signals in the IF passband? I assume that receiver circuit characteristics before the roofing filters (or IF bandpass filter) essentially determine the dynamic range. However, dynamic range figures "always" seem to degrade for small separations of fc and fc+x. Is this because fc and/or fc+x are falling within the IF passband and are producing spurious signals in the stages following the roofing filters? For the case of strong signals in the IF passband, are the dynamic range tests run with the AGC disabled? In actual operation, wouldn't the AGC reduce signal levels below the point where distortion products were being generated (in the stages following the roofing filters)? The two tone test is only a proxy for the "real world". In actual operation, if any signal (or signals) in the RF passband or mixer passband exceeds the receiver dynamic range, will ALL of the signals in the passband begin contributing third order products? Is the magnitude of the third order distortion products a function of the "degree" of the nonlinearity? Can different receiver models with the same dynamic range numbers (operating under the same conditions) differ considerably in the "signal strength" of junk signals? When test results are "phase noise limited", what is going on? Is the phase noise acting simply as RF noise that masks the spurious signals being observed? Or is it acting as a "third signal" that mixes with fc and fc+x and causes the production of third order products at a lower signal level than fc and fc+x would alone? Mike W5FTD ___ 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
