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| Two Tone Test with Scope and SA |
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| G0HZU:
If it helps, I've been doing critical IMD tests like this all my career. In my case I would normally testing an RF downconverter. I designed lots of RF downconverters from about 1990 onwards. These were for early versions of SDR receivers for gov/military use back then. The ADC/DSP back end was in a huge rack system in those days. Very different to today. The usual spec for IMD back then varied from -72dBc to -80dBc for the downconverter but obviously some margin was required beyond this. The aim of the overall system was to digitise the RF spectrum up to several GHz as quickly and cleanly as possible. Often a high performance spectrum analyser was used at the IF in place of the ADC/DSP during dev work. If IMD tests are done at narrow spacing then this can be inside the ALC bandwidth of the sig gen and this will degrade the performance unless extra isolation is included somewhere. Also, a conventional spectrum analyser will usually have degraded IMD performance below about 50MHz and also the IMD of the analyser can be worse on narrow spans as both test tones travel further together down the signal path of the analyser. In those days the top spectrum analysers for IMD performance were the HP 8568B, Advantest TR4172 and the Marconi 2382 analyser. It's fairly standard to have ALC in a classic RF lab sig gen because the ALC system is often also used for the AM modulation. This usually means the ALC bandwidth is at least 20kHz although some sig gens can turn the ALC bandwidth right down if AM isn't needed. I'm not aware of any mainstream lab RF sig gens that run open loop unless the ALC is deliberately disabled. I guess sig gens like this do exist but I've not used one unless you include really old or low cost RF signal generators or RF capable function generators. I've got quite a few RF sig gens here and I've used many different types at work over the years. With care it's possible to achieve -100dBc IMD at the RF combiner output. More is possible but it becomes much more challenging to prove it. When testing really high performance HF receivers the IMD3 SFDR can easily be >110dB so this requires a very good test setup indeed. |
| rf-messkopf:
--- Quote from: rf-messkopf on June 20, 2022, 10:07:56 am ---Not all signal generators have an ALC. Some of the old Marconi ones always run open loop, I think the 2024 is an example. I'll repeat the test with two function generators and see if that makes any difference. --- End quote --- Correction: From the block diagram, the Marconi 2024 does have an ALC. But it cannot be switched off, I think, I've never seen such an option in the menu of mine. --- Quote from: Performa01 on June 20, 2022, 10:56:45 am ---Without the ALC, it's hard to see where there is a difference. If we ignore modulation, there should not be any substantial differences between the output stages of a signal generator and an AWG. We have an output amplifier, which is of course prone to intermodulation when fed with external signals and a step attenuator (which usually covers a much wider range in the signal generator, but nothing that cannot be rectified by using an external device for additional attenuation). My Siglent AWGs are somewhat old school because we hear several relays click when dialing the output amplitude within its range of -56 dBm and +25.5 dBm (at frequencies up to 40 MHz). Yet I suspect that there will be some additional electronic attenuator to manage the fine steps in between. Since this is just a switch, i.e. a negligible serial resistance within the signal path, I do hope that it won’t cause any significant distortion. --- End quote --- Usually, rf signal signal generators have a class A output stage, whereas the classical function generator has a class AB complementary output stage with a separate DC path. No idea how the output amplifier in one of these modern x GSa/S arbitrary function generators looks like. The electronic attenuator/PGA for fine control of the output level usually is further up in the signal path, before the output amplifier, and likely will not see much of an externally applied signal. --- Quote from: G0HZU on June 20, 2022, 11:32:24 am ---If IMD tests are done at narrow spacing then this can be inside the ALC bandwidth of the sig gen and this will degrade the performance unless extra isolation is included somewhere. Also, a conventional spectrum analyser will usually have degraded IMD performance below about 50MHz and also the IMD of the analyser can be worse on narrow spans as both test tones travel further together down the signal path of the analyser. In those days the top spectrum analysers for IMD performance were the HP 8568B, Advantest TR4172 and the Marconi 2382 analyser. --- End quote --- Right. The 115 dB intermodulation free dynamic range of the FSEA30 is specified for inputs from 50 MHz onwards. But as I said above, I would expect that the intermodulation products I have seen would depend on the attenuator setting if they were generated in the first mixer. And if they were generated further down the signal path at an IF, then they would also be a problem for input frequencies above 50 MHz. Perhaps this could be checked by varying the reference level. Anyway, some further tests are necessary. Currently I suspect the signal generators and/or the splitter. |
| mawyatt:
--- Quote from: rf-messkopf on June 20, 2022, 02:39:24 pm --- Usually, rf signal signal generators have a class A output stage, whereas the classical function generator has a class AB complementary output stage with a separate DC path. No idea how the output amplifier in one of these modern x GSa/S arbitrary function generators looks like. The electronic attenuator/PGA for fine control of the output level usually is further up in the signal path, before the output amplifier, and likely will not see much of an externally applied signal. --- End quote --- Interesting discussions!! The AWG output amps are usually wide-band higher performance op-amp types with very little introduced non-linearity. TI and AD have created op-amps especially for this type of use. The mentioned "relay clicking" when using built-in two tone signals from some of the modern AWGs (For example Siglent's SDG2042X and 6022X) hint that this is combining the separate tones from each channel signal path in the analog domain and not the digital domain, which may provide better results than a simple digital combine. An interesting IMD test might employ a couple wide-band quality linear Power Amps buffering each individual tone source before combining into a single two tone signal. Since the required two tone signal is low relative to the linear PA output capability, maybe this PA wouldn't introduce much additional non-linearity and it's output wouldn't be effected as much by the "other" signal, thus reduced IMD. We've utilized high power wide-band low noise linear PA types as receiver front ends for high input dynamic range in "special" type receiver applications, and might prove useful here. Best, |
| 2N3055:
--- Quote from: mawyatt on June 20, 2022, 03:28:58 pm --- --- Quote from: rf-messkopf on June 20, 2022, 02:39:24 pm --- Usually, rf signal signal generators have a class A output stage, whereas the classical function generator has a class AB complementary output stage with a separate DC path. No idea how the output amplifier in one of these modern x GSa/S arbitrary function generators looks like. The electronic attenuator/PGA for fine control of the output level usually is further up in the signal path, before the output amplifier, and likely will not see much of an externally applied signal. --- End quote --- Interesting discussions!! The AWG output amps are usually wide-band higher performance op-amp types with very little introduced non-linearity. TI and AD have created op-amps especially for this type of use. The mentioned "relay clicking" when using built-in two tone signal from some of the modern AWGs (For example Siglent's SDG2042X and 6022X) hint than this is combining the separate tones from each channel signal path in the analog domain and not the digital domain, which may provide better results than a simple digital combine. An interesting IMD test might employ a wide-band quality linear Power Amp buffering each individual tone source before combining into a signal two tone signal. Since the require two tone signal is low relative to the linear PA output capability, just maybe this PA wouldn't introduce much additional non-linearity and it's output wouldn't be effected by the "other" signal, thus reduced IMD. We've utilized high power wide-band low noise linear PA types as receiver front ends for high input dynamic range in "special" type receiver applications, and might be useful here. Best, --- End quote --- Mike, SDG series from Siglent definitelly combine in digital domain... Relay clicking is from mundane reason. If amplitude of single channel is right there on the edge where it would go between output ranges, once you combine amplitudes it will be large enough for output relays to "switch to higher gear".. Best, |
| mawyatt:
--- Quote from: 2N3055 on June 20, 2022, 04:06:17 pm ---Mike, SDG series from Siglent definitelly combine in digital domain... Relay clicking is from mundane reason. If amplitude of single channel is right there on the edge where it would go between output ranges, once you combine amplitudes it will be large enough for output relays to "switch to higher gear".. Best, --- End quote --- Thanks, that's why I said "hint" since didn't know for sure. Suspect Siglent had done the proper evaluation for the Digital vs. Analog combining of signals from each channel and weighted the pros and cons. Starting with 16 bit DACs likely helps as well, which gives good resolution. Best, |
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