|Sep 6th 2011, 00:23|
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I've received a few interesting and useful replies by email to the posts I made earlier. The discussion looks useful to the topic, so I am reposting them anonymously. Posts are listed on the ARRL forums with the newest posts appearing first, so it may be useful to look at the earlier posts on two-tone, third-order IMD, reciprocal mixing and blocking gain compression first.
Ed Hare, W1RFI
> How can measurement of a third-order product buried in noise,
> that one cannot hear, have any significance to the amateur?
If one makes a measurement of two-tone, third-order dynamic range and measures noise instead of the intermodulation procucts, a measurment of dynamic range or third-order intercept point (IP3) was not made; a measument of the noise was made instead. A measurement made of TTTODR, a 3rd-order phenomenon, is NOT made when the measurement is noise limited. The resultant value CANNOT be used to calculate intermodulation dynamic range or IP3.
In many cases, for a variety of reasons, the intermodulation characteristics of many receivers do not follow the classic response of having the first-order (on channel) signals increase in a linear fashion and having third-order intermodulation products increase at a 3:1 rate, , and we often see an IP3 that is significantly different at the noise floor than it is for an -103 dBm response (S5) and at 0 dBm input signal levels. A meaurement of IP3 at the noise floor, even if masked by noise, is a valid measurement of IP3 that allows extrapolation of that IP3 to the IP3 measured at -103 dBm, IF the actual intermodulation signal is dug out of the noise.
The result would also have some direct validity to the Amateur that is doing some of the very narrow bandwidth work, although that is a small subset, to be sure. The measurement of IP3 under those circumstances does have enough value to warrant doing.
> It might be 100% noise, or it might be a combination of noise and
> the third-order product. The net result was the radio had
> degraded to the point that the undesired product(s) plus the
> receiver noise had increased 3 dB. That was meaningful and
> represented a "real-world" performance limit.
I agree, but a noise-limited measured value is a poor, inaccurate and somewhat relative measurement of the noise performance of the receiver. It is a poor substitute at best, and a meaningless substitute at worse, for the acccurate measurement of receiver reciprocal mixing that was added to the ARRL test battery. This results in one test that measures intermodulation, another that measures gain compression, independtly of noise, and a third test that accurately measures noise.
> That gave what was known as the spurious-free dynamic range.
Not at all, as there are many spurious responses that are not third-order in nature. The older method gave only either an accurate measurement of IP3 and dynamic range, or a poor measurement of noise performance, depending on whether the measurement were reported as being noise limited or not..
> It would be helpful for a design engineer to know where the weak
> link in the chain is for a given radio. If he measures the
> third-order product 10 dB below the phase noise, then he clearly
> knows the next thing to try to improve would be the synthesizer.
> What is of interest to the amateur is when a given radio has
> reached its dynamic range limit. He can then decide
> which model and how much money he wants to spend for
> whatever level of performance he believes he needs. The needs
> of an SSB rag chewer is vastly different from a serious CW
> contester / DXpile-up operator.
You have made my point well. The older ARRL test method did not measure the third-order product relative to the phase noise at all; if noise limited, it crudely measured one impact of that noise on a receiver. ALL one knew was that the measurement of the third-order product could not be made at all; noise limited the ability to make the measurement. The new methodology accurately measures the third order product (or gain compression) that the good engineer CAN compare it to the separate -- and accurate -- measurement of receiver noise.
I agree also that the needs of the SSB ragchewer is differnent from the big gun contester. For some use of a receiver, receiver noise will be the limiting factor, but for stronger signals, that 3 dB increase in noise at the noise floor of the receiver will not matter. In fact, for most Amateur use of a receiver, the noise floor of the receiver is meaningless, as external noise is much higher.
I think that all of the points you made are best addressed by an accurate measurement of the third-order product and an accurate measurement of receiver noise. That is exactly what you get with the separate noise and IMD tests that ARRL has done since 2007. To try to claim that the less-accurate measurement of noise that comes from a "noise limited test" is a way to know that the third-order product is 10 dB below the phase noise is, to use the word you have thrown about wrt ARRL's testing, bogus.
> If you compare a K3 to a Flex 3000, the phase noise
> performance differs by almost 20 dB. Yet with the League's new
> method, the 3000 (measured 2007 or later) looks as good as a
> K3 (measured 2006 or earlier) when looking at the dynamic
> range figures.
But if you look at the measurement of noise, the Flex 3000 does not look better than the K3, and that noise was reported in the Product Review as well. It is less valid to try to estimate the noise performance of a receiver by looking at the non-measurement of a test designed to measure something else than it would be to look at the actual measurement of the noise performance of the receiver.
> With today's synthesized products, the practical dynamic range of
> a radio is likely to be the reciprocal mixing value published in
> QST. In the case of the IC-7410, the reciprocal mixing limit is 78
> dB (@ 2kHz), but the numbers hams have been looking at for
> over decades is listed as 88 dB. How is that helpful to anyone
> except the OEMs who are selling radios? It certainly doesn't help
> the ham who isn't savvy to these fine details.
The practical dynamic range of the receiver at weak signal levels is likely to be the reciprocal mixing value publshed in QST. At stronger signal levels, the limiting factor is apt to be intermodulation. For that reason, it is important that both the noise and intermodulation performance of the receiver be measured and reported. It is not, as you have said in public presentations, bogus; in fact, if I were to use that word, it would apply more to a test purported to be a measurement of intermodulation dynamic range that is a measurement of reciprocal-mixing instead.
The results of all 3 measurements are published -- intermodulation dynamic range (and intercept point at 3 different levels), blocking gain compression dynamic range and what is best described as reciprocal-mixing-noise dynamic range.
If there is a shortcoming, it is found in the people that could explain to hams the real import of those numbers at different levels, but choose instead to focus on the intentional misinterpretation of those numbers that could mistakenly leave someone to believe that the 78 dB level is not the limiting factor, using the example you provided.. That is the most unfortunate outcome of all of this, IMHO.
The ARRL two-tone, third-order IMD dynamic range test is a valid measurement of dynamic range and IP3, and the gain-compression dynamic range test is a valid measurement of gain compression, and the useful number reciprocal-mixing noise data is also presented. Instead of persuading people that a less useful way of making that measurement is what is needed, or claiming that ARRL does it to make adversers look good (even though the reciprocal mixing test it introduced at the same time makes them look worse...), I offer that is is far more useful to explain to hams why they need to pay more attention to the reciprocal mxing number, because if its worse than the TTTODR number, then the test is not only noise limited, but one can, with a grammar-school subtraction, determine the precise degree to which is it noise limited. (Keeping in mind that the noise-limited aspect of the measurement is relative to the receiver's noise floor, which is many tens of dB lower than the signal levels that most hams use in practice.)
This is why the ITU-R Recommendation recommends that the actual IMD product or actual gain compression be dug out of noise. When it comes to standardization, ITU-R "trumps" all other and if ARRL want to have its test methods be standards based, there is no better standards source.
> I certainly agree that IP3 values were invalid if they were
> published from the old method. One can also increase the IP3 by
> punching in an attenuator. What is really important is the
> dynamic range, which can then be moved around as needed for
> the given conditions by judicious use of an attenuator or a
> preamp. The new method cannot be justified just to fix a problem
> in the past with improper IP3 values.
There were only one or two IP3 numbers erroneously reported, I believe, as at some point, the realization that noise is not a substitute for intermodulation won the day. The new method was not developed to resolve a problem that was resolved in the last 1980s; that's pretty far fetched. It was developed to allow an accurate measurement of intermodulation, an accurate measurement of gain compression, simultanously coupled with a publication of an accurate measurement of receiver noise, instead of the crude "measurement" that resulted from a "noise limited" measurement.
Having said that, the most valid point in all of this is that there are hams that don't understand that reciprocal mixing is the limiting factor, so it is my expectation that we can start listing that as "reciprocal-mixing noise dynamic range" as a way of better flagging the import. I thinik that this is much more accurate, and more supported by international standards, than going backwards to use the TTTODR and gain-compression tests as a crude substitute for noise testing.
I would much rather have this discussion on the new forum I created on the ARRL web site, because I think that more hams need to understand the reasons that the testing is done the way it is, and more need to understand the significance of the reciprocal-mixing test.
Ed Hare, W1RFI
ARRL - The national association for Amateur Radio
ARRL Laboratory Manager
225 Main St
Newington, CT 06111
Member: IEEE, Standards Association, Electromagnetic Compatibility Society
Secretary: IEEE, Connecticut Chapter
Secretary: IEEE EMC Society Standards Development Committee
Member: ASC C63 EMC Committee, Chairman: Subcommittee 5, Immunity
Vice Chair: IEEE P1775 BPL EMC Committee
Member: ICES SCC-28 RF Safety
Member: Society of Automotive Engineers EMC/EMR Committee
Board of Directors: QRP Amateur Radio Club International
|Sep 6th 2011, 00:39|
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> IP3 is a completely different subject. I don't understand what it
> has to do with the subject of measuring third-order products in
One determines IP3 by measuring third-order products. That is a fundamental concept. The only way to measure IP3 for IMD products at the noise floor of a receiver is to dig that product out of the noise if there is also reciprocal-mixing noise. Otherwise, the resultant measurement is not an IP3 measurement, it is a noise measuement.
> Absolutely. That is why I don't publish IP3. It is a calculated
> number that moves all over the map, depending on at what level
> the measurements are made. The IP3 of a radio can vary as
> much as 40 dB, depending on the settings of preamp(s) /
> attenuators. On the other hand, the DR3 values are virtually
> constant. IP3 is a useful concept for a mixer or an amplifier, but
> not very useful when applied to a radio.
It varies with levels for some radios. Others are more constant. I disagree that the number is not useful. Although dependent on the sensitivity of a receiver, or external attenuation, IP3 is a number that is a single figure that combines sensitivity and dynamic range into a single number. It matters little to the competent engineer whether dynamic range and sensitivity are specified, or IP3 and sensitivity. Both have told you exactly the same thing about the radio. It is much like saying that a resistor of a value of 1 ohm has 1 volt across it, a resistor with a value of 1 ohm has 1 A of current flowing through it or a resisitor of a value of 1 ohm is dissipating 1 watt. Both say exactly the same thing. Whether one prefers IP3 and sensitivity or dynamic range and sensitivity is simply a matter of how one's mind visualizes receivers.
IP3 has usefulness that extends beyond a simple dynamic-range and sensitivity comparison, though. Dynamic range has a very specific meaning that is defined at the noise floor of a receiver. One cannot measure "dynamic range" based on input levels of -50 dBm, for example, because the definition pegs that at the noise floor. One CAN, however, measure IP3 directly bases on input levels of -50 dBm, so IP3 has more value than simple dynamic range.
While IP3 does vary across the operating range of some receivers, I disagree that it is not useful to know that. Knowing that a receiver is more (or less) susceptible to overload at stronger signal levels than it is at weaker signal levels is important and useful. For some applications, an IMD product at the noise floor of a receiver could have an impact on the ability to use a receiver. A 28 MHz receiver with a poor antenna in a very quiet location, for example, may not pick up enough external noise to have external noise be a limiting factor. OTOH, for an HF receiver at 3.5 MHz with a good antenna, the intermodulation product at the noise floor of the receiver would be entirely moot, so if the IP3 were different for that receiver at higher levels, that is more useful to know than knowing the IP3 based on the noise floor alone.
And, of course, if one takes your premise that intermodulation signals should not be dug out of the noise, and does not actually measure IP3 with intermod products at the natural receiver noise floor, one is not measuring IP3 at all, so one would have no idea of what the intermod performance of that receiver is. That is not useful at all, and knowing only the approximate level of noise that prevented one from making the measurement using an audio voltmeter is not the same thing at all.
I think that it is very useful to measure and publish IP3 across a wide range of input signals simply because it *does* vary in some receivers over that range and one can use it to determine the intermodulation performance of a receiver at different levels. I can't imagine why any engineer would not want to know that a receiver's intermodulation performance was or was not following the "classic" curves and why they would not want to know that the performance was different at different levels of received signals.
> Considering my noise limited DR3 of the IC-7410 is exactly the
> same value as your reciprocal mixing value, I don't see it as
> inaccurate. It is really the same measurement in most radios
> today. Just like when one looks at the third-order products
> (assuming not noise limited) on both the high and the low side,
> we are looking for worst case.
Perhaps so, but even if accurate, if a dynamic range measurement is the same as the reciprocal-mixing measurement (in some cases?), that again gets back to the premise that if a measurement of TTTODR is noise limited, one has NOT made a measurement of the TTTODR of that receiver; one has made a measurement of reciprocal mixing. It is entirely incorrect to publish a noise test as the TTTODR of the receiver, as some of the tables I have seen on line attempt to do. There is no valid technical reason to muddy those waters that way, and making a measurement of the actual intermodulation is more useful than using a measurement of reciprocal-mixing in a table of dynamic range. One should make and report the measurements separately.
And in the case of gain-compression dynamic range, a measuremnt more likely to be noise limited than TTTODR, the "noise limited" value is NOT the same as reciprocal mixing noise at the noise floor because the signal levels are higher. It is very useful, however, to dig that compressed signal out of the noise because that dynamic range value will also apply to signals that are much higher in level, also blocked to the same degree (approximately) by the strong off-channel signal, even with AGC active. For the gain compression test, the level at which "noise limiting" will occur is linearly dependent on the level of the signal being compressed, so reporting a "noise limited" value is an even worse substitute for a real reciprocal mixing test, and doing so prevents one from knowing the level of off-channel signal that will cause gain compression at higher signal levels where the receiver will still compress, but the noise that "limited" the test would not be a factor.
> Certainly there are all sorts of spurious, some of which are
>birdies, but if you go back to the concepts of Hayward & DeMaw,
> they were defining dynamic range as the point at which internal
> generated third-order products were just poking their head up out
> of the noise.
That specification was referenced to the noise floor of the front end of the receiver. The very concept of dynamic range is tied to the sensitivity of the receiver, so again, a "noise limited" measurement of TTTODR or GCDR is NOT a measurment of either phenomenon and it is incorrect and inaccurate to report it as such. If dynamic range is referenced to the noise floor of the receiver, that is the reference that should be used to calculate it, not the additional noise of the local-oscillator. That must be measured and reported separately.
> Forgetting IP3, which I think is spurious to this disagreement, if
> we are going to publish a single number for DR3, it needs to be
> worst case. When does the radio degrade a specified amount, in
> this case equal to the noise floor.
I agree that one should know the level at which a radio degrades by a specified amount. I disagree that misnaming a noise measurement as DR3 is the way to do it. Because the impact of intermodulation is a third-order effect that varies differently than the impact of reciprocal-mixing noise, which is (approximately) a first-order effect, a "single number" is much less useful than measuring and reporting those parameters separately. One cannot use a first-order result to estimate the effect of intermodulation that may occur at a different and unknown level, which is exactly what you get when you stop testing intermodulation and report the result as noise limited because you think that any degradation is all that one needs to know about a receiver. I disagree, as if one has an indication of the intermodulation at the noise floor, one can use that information to estimate the intermodulation performance at higher levels, where reciprocal mixing is not going to have much of an impact at using that receiver. On HF receivers especially, external noise is the limiting factor almost all of time, so knowing something about intermodulation at higher levels is of use.
If one really wants the oversimplification of having a single figure because he or she is unable to consider the ways that noise, intermod and gain compression may have different effects under different circumstances of using that receiver, all one needs to do is to look at the TTTODR, the GCDR and the reciprocal-mixing levels and take the worst case. That is grammar-school math and anyone who would have the foggiest idea of what the numbers mean in the first place can easily obtain that figure of merit at a glance.
> You are accurately measuring the third-order product, but that
> value does not correlate to the performance of most radios today.
Sure it does. That third-order product may be at the noise floor, a handful of dB below it, or tens of dB below it. If you don't make the measurement, you don't know whether the IMD is one dB or twenty one dB below the noise. But if you simply report "noise limited," that is exactly what you don't know. That intermodulation, following a third-order response, will rise very quickly above the noise floor or the reciprocal-mixing noise, so by knowing the level of intermodulation referenced to the receiver's front-end noise, a competent engineer can easily estimate the intermodulation performance of the receiver at the higher levels where is it likely that the receiver will be used anyway. Having the additional data points of IMD measured at an intermod level of -103 dBm (S5) and at 0 dBm (strong-signal peformance), a competent engineer can have a mighty fine idea of what the receiver does across a wide range.
The less experienced evaluators of radios can, if they wish, take the worst of the two numbers, TTODR or reciprocal-mixing, and be no worse off than having a simple "noise limited" measurement as you believe ARRL should continue doing.
> It comes back to what information is useful to the ham. IP3 is a
> theoretical number, and rather meaningless for reasons you and I
> both listed above.
IP3 is NOT a theoretical number. It is a calcuated number, based on the slope of the first order and third order responses. Although a radio will compress long before the IP3 level of input signals, it is not intended that IP3 be used to estimate the absolute level at which the radio will overload. It is a single number that, with a simple calulation using IP3 and the noise floor, will tell you what level off off-channel signal will cause intermodulation at any receiver output level that you deem to be useful to know. That may be meaningless to you, although I can't understand why. It is not meaningless to most receiver designers, and although some think in terms of noise floor and dynamic range, most receiver designers tend to think in terms of noise floor an intercept point. Both say the same thing, except that IP3 says it in a way that applies to the entire operating way of the receiver.
> Your measurement of DR3 is technically accurate, but practically
> of little value since most radios today are reaching their limit, at
> least close in, due to the synthesizer. You say you can have two
> CW signals 2 and 4 kHz away that are not going to interfere with
> reception until they are X dB above the noise floor. In reality, in
> the example of the IC-7600 or the IC-7410, the radio fails at
> X-10 dB with one signal 2 kHz away. The point is the radio fails at
> X-10 dB, not at X dB.
That is exactly what the reciprocal-mixing test will tell you, if you are using that radio under circumstances where those weak products are at the noise floor of the receiver, and not at the noise floor of the system, set by external noise. Very few receivers are used at their noise floors in the real world. That measurement of DR3 and the resultant calculation of IP3 can be used to determine the failure point of that receiver at NF+10 dB, NF+20 dB, etc, in a way that the simple reporting of a "noise limited" value cannot do.
> It measured the point when the radio was degraded. Who cares
> why it was degraded.
It is technically oversimplified to not care why it was degraded. In one case, it is degraded by a first-order phenomenon. In another case, it was degraded by a third-order phenomenon. If you are going to use a "dynamic range" number to predict anything about what a receiver will do at higher operating levels, you very MUCH need to know whether the effect measured will vary at a first-order or a third-order rate. Simply knowing it degraded, without knowing why is of almost no use whatsoever, other than at the very unique and almost-never-encountered case of the receiver being used at received on-channel signal levels at its front-end noise floor. If you can't extrapolate a measurement to the practical levels at which a receiver will be used, that measurement is of little practical use. I disagree strongly that anyone should not care why the receiver was degraded.
> For that matter, a single signal can make it fail on reciprocal
> mixing, while it takes at least two signals to cause a third-order
> product. Of course a band full of signals can also add up in total
> power to cause distortion products.
I agree, which is why the three dynamic ranges are all important. The single-signal case is measured with reciprocal mixing. The TTTODR and IP3 measurements capture the two-signal case with a product falling in the desired passband and the GCDR measurement determines the level at which any strong signal elsewhere in the band will cause gain compression of the linear function of the receiver.
> With most radios today you could decide to leave out all the
> blocking data and DR3 data and simply publish the reciprocal
> mixing data at a whole bunch of spacing.
I strongly disagree with that premise. Reciprocal mixing data would indeed show how a radio degrades from a single signal at the noise floor, and it could be extrapolated well to stronger signal levels. But that is a first-order extrapolation that would tell you nothing about third-order effects. One could have a noiseless receiver that had poor intermodulation performance that would make it almost useless on the bands, yet the reciprocal-mixing data could look great.
> I am being slightly facetious, but QST has spend 30 years
> extolling dynamic range, but now due to the way radios perform,
> the worst case data is in one little line almost no one
That's more than slightly facetious, and being facetious rarely has value any way. The TTTODR and GCDR tests tell you a lot about the way a radio works. So does the reciprocal-mixing test. If "no one understands" it, then what is needed is for an education process about the importance and usefulness of that test to occur. That education, however, would be inaccurate if it were to tell people that the ONLY thing that matters in a receiver is the reciprocal-mixing test, because each of the effects measured in the other tests affects the receiver's use in different ways, under different circumstances. Those ways and circumstances should be explained, instead of incorrectly dismissing the tests as yielding "bogus" results, because that is simply technically wrong.
The measurements in the presence of noise are being made because the resultant data are useful at the typical signals levels at which receivers are often used in the real world. Most desired signal levels are much stronger than the noise floor, or even stronger than the signal levels used for gain-compression testing, so measurements that can be extrapolated to practical levels are very much necessary and appropriate.
> The League has hardly mentioned the concept of reciprocal
> mixing (RM) for five years. Now suddenly the League decides it
> should be emphasized, and that is progress.
"Suddenly" being four years ago, right? Starting in the mid 1980s, ARRL started to do the transmit phase-noise tests, and it was the League's belief that by measuring the LO in transmit, one had at least a relative idea about the receive noise as well. True enough, although when ARRL chose to start measuring IP3 and gain compression in a way that was less limited by LO noise, it was very necessary to add a RM test, which it did four years.ago.
> How are you going to explain that the blocking value really
> doesn't have any meaning since the RM will almost certainly
> predominate ...
I am not going to explain that, because it is simply not true that the gain-compression value has no meaning. The levels used for the gain compression test are quite a bit above the noise floor of the receiver, chosen to fall within the linear range of receivers, but not near the compression point. Their "noise limited" value would be dependent only on the level of the test signal. But when gain compression of the receiver occurs, it occurs across the entire usable operating rang of the receiver, even when the receiver AGC is active. (AGC is a desired gain compression, saturation overload is not). So that measured value very MUCH has meaning, because it applies at much stronger levels where noise would not be a factor.
> How are you going to get the OEMs to focus on better
> synthesizers since 95% of the data will have that effect taken that
> out of the equation?
That is a fair question. I do note that when ARRL started to publish transmit phase-noise values, manufacturers started to make less noisy rigs. The most valid point, never quite said and lost in the discussions about "bogus" testing (far from a technically appropriate term, under any circumstances, btw), is the point that the reciprocal-mixing test is an important measure of dymamic range and should be given more emphasis. The point that the colored graphs do not include it is very well taken and it is my recommendation to the editor that we start doing so at the earliest opportunity. It may take a bit of doing to go through old data and determine the best ranges for the graph. I have also recommended that we change the reciprocal mixing entry in the test table to "reciprocal-mixing-noise dynamic range," to better reflect its import.
Later this weekend, I am going to post this exchange on the new ARRL forum, as I think that it's important that this be aired to our membership. There has been a lot of public discussion about some of these issues on various lists and such, and rather than going at this issue on each of those lists, signing up for lists I really don't need to receive, ARRL chose to create a forums area on its own web page. I think that refuting some of the technical misinformation I have seen posted can best be done by simply posting ARRL's rationale, as I have done. This discussion will add to that a lot, as it brings up the questions that will be asked by some and it helps to emphasize the reasons that reciprocal mixing may be more significant than some hams have realized. ARRL has been reposting some of the technical questions it receives on the forums, but I do so keeping the person asking the question anomymous, unless I have permission to post their question with their name and call sign, as recieved.
Ed Hare, W1RFI
ARRL Laboratory Manager
225 Main St
Newington, CT 06111
|Oct 19th 2011, 14:06|
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The purpose of receiver testing is to find a couple of numbers by which one can predict the outcome of a direct comparison of two radios on the same. Is one better than the other?
Today we can compare radios under difficult interference conditions by use of wideband recordings. SDR softwares can be tested directly and analog receivers can be tested by use of a (good) SDR transmitter that generates RF on a suitable ham band from the recording.
Not only the usual problems, selectivity intermodulation and reciprocal mixing show up. Also other problems like AGC sensitivity to pulses and poor sound quality for strong SSB signals become evident.
This page: http://www.sm5bsz.com/lir/agctest/agctest.htm has loudspeaker output files from these transceivers: TS-520, R-4C, FT-1000, FT-2000, FTDX-5000, K3, IC-706MKIIG, IC-7000, FT-221R and IC-202. The following SDR softwares are tested: Linrad, Winrad, WRplus, SpectraVue, HDSDR, SDR-radio, Rocky, PowerSDR, G31DDC, perseus.exe and SDRMAXIV.
High performance SDR hardwares are ideal in this test but low cost hardwares have limitations. These low cost SDR hardwares are tested: SDR-14, Softrock, AFEDRI SDR.
It is interesting to note how a low cost SDR like Softrock outperforms old radios like R-4C and FT1000 as well as modern low cost radios like IC706MKIIG and IC7000.
In the near future anyone interested can perform qualified receiver testing by use of a laptop and something like this: http://www.sdr-one.com Just play a 96kHz .wav file to the unit by use of whatever audioplayer. A small set of test files would be enough to rank receivers with respect to dynamic range below 50 kHz frequency separation.
|Oct 23rd 2011, 13:37|
Joined: Apr 4th 1998, 00:00
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Interesting thread and thanks for sharing this exchange, Ed.
The oft-referenced Sherwood Engineering Receiver Test Data table purports to rank receivers based on a single metric--Dynamic Range Narrow Spaced (dB) @ 2 kHz. This has given a sort of a "King of the hill" bragging right to whomever owns the receiver that currently tops the list. For a time this was the Elecraft K3 (disclaimer: I've owned and used a K3 as my primary transceiver for just over a year).
As of late 2010 the Yaesu FTdx-5000D was placed at the top of the list with identical wide and narrow spaced dynamic range figures. The only reason I can determine as to why is that the K3 achieved the narrow spaced value with its optional 200 Hz IF filter while the '5000 achieved it with an included filter. Fair enough.
A look to the other numbers in the table seem to reveal a different story. Working from the right side of the table since this is where the ranking is determined, we see that Ultimate Filter dB on the '5000 (footnoted as Phase Noise limited) appears to be 15 dB worse than on the K3. Local oscillator noise is 3 dB higher. 100kHz Blocking (dB) appears to be 13 dB worse on the '5000 than on the K3. Other values are reflective of the apparent design goals of each radio.
Taken as a whole, it seems to me that based on Mr. Sherwood's testing that the FTdx-5000D is not entirely deserving of its top placement in the table. For Mr. Sherwood's purposes, it is. I think this is not often well understood.
Clearly, selection of a radio hinges on more than a single entry in a single test table, although I will admit that Mr. Sherwood's table helped me evaluate the K3 when I set out to purchase one last year. I am pleased with my purchase and have no "buyers remorse". The K3 is in all ways a superior performer to the FT-920 it replaced--hardly a high-end radio in its day and less so now--as would probably be expected. And while narrow spaced dynamic range is important, I have used enough receivers with filter blow-by that the FTdx-5000D's poor showing in this area would be a cause for concern for me, especially considering the price and desktop real estate the '5000 demands.
73, de Nate >>
|Dec 15th 2011, 21:04|
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Is there a book or pamphlet that explains how to interpret the test specs given the product review?
|Dec 19th 2011, 01:34|
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I'd suggest visiting our product review page which has two articles you can download that explain how to interpret product reviews.
ARRL Senior Lab Engineer
|Aug 12th 2012, 20:05|
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