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| Two Tone Test with Scope and SA |
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| G0HZU:
Usually the way to get good results is to choose a combiner with high isolation and to try and maintain that isolation through having a good port match at the sum port of the combiner. Also, try and use a lowpass filter on the sig gen outputs. Try and use a spacing frequency that is higher than the ALC bandwidth of the sig gens as this can minimise crosstalk effects between the sig gens. Another thing to try is to use the attenuator lock/hold feature on the sig gen (if it has this feature) as this can keep some internal attenuation inline when trying for higher output levels from each sig gen. I have lots of RF combiners here and for use below 200MHz my favourite one has 6dB through loss but over 48dB isolation from LF through to about 200MHz. If I have to use a resistive combiner for wideband stuff I have one here that has about 9.5dB through loss and just under 20dB port isolation. Don't overlook that fact that a typical microwave spectrum analyser will typically have compromised distortion performance below about 20-50MHz. This is because of the image and LO breakthrough that will degrade the distortion performance of the analyser below about 20-50MHz. So the analyser might become the weakest link unless care is taken with the attenuator settings within the analyser. It should be possible to get >100dB SFDR using the above techniques if care is taken with the setup. |
| rf-messkopf:
--- Quote from: G0HZU on June 19, 2022, 11:47:58 am ---Usually the way to get good results is to choose a combiner with high isolation and to try and maintain that isolation through having a good port match at the sum port of the combiner. Also, try and use a lowpass filter on the sig gen outputs. Try and use a spacing frequency that is higher than the ALC bandwidth of the sig gens as this can minimise crosstalk effects between the sig gens. Another thing to try is to use the attenuator lock/hold feature on the sig gen (if it has this feature) as this can keep some internal attenuation inline when trying for higher output levels from each sig gen. --- End quote --- The datasheet of the ZFSC-2-6+ I was using states about 30 dB isolation at 10 MHz typical. Plus there was a 10 dB pad at each input, so I would expect about 50 dB of isolation. No idea if that is enough. I haven't gotten the VNA out to actually measure it though. The ALC was turned off on both generators, so that it would not cause any trouble. --- Quote from: G0HZU on June 19, 2022, 11:47:58 am ---Don't overlook that fact that a typical microwave spectrum analyser will typically have compromised distortion performance below about 20-50MHz. This is because of the image and LO breakthrough that will degrade the distortion performance of the analyser below about 20-50MHz. So the analyser might become the weakest link unless care is taken with the attenuator settings within the analyser. --- End quote --- True, but I have set the attenuator of the analyzer high enough that the IMD products would not move with a variation of the attenuator. Therefore I suppose that what I'm seeing is not generated within the analyzer. --- Quote from: G0HZU on June 19, 2022, 11:47:58 am ---It should be possible to get >100dB SFDR using the above techniques if care is taken with the setup. --- End quote --- I wonder if the IMD products are generated in the splitter. It is a core and wire model, rated from 2 kHz to 60 MHz, and the core might introduce some distortion. I should check that by varying the drive level at the inputs. --- Quote from: David Hess on June 19, 2022, 04:18:59 am ---How would this measurement be made while avoiding the limits of the analyzer? Notch out the fundamentals? --- End quote --- If you can make a filter that is steep enough to notch them out without disturbing the 3rd order products. --- Quote from: RoGeorge on June 19, 2022, 07:20:45 am ---The two tone testing is particularly useful when the bandwidth is limited, because this technique is shifting the harmonics close to the two main tones, so their amplitude is preserved. Otherwise (with a single tone) the harmonics will fall out of the DUT's band and will be attenuated by the DUT's frequency response, appearing much smaller than they really are, or not visible at all. In the case of a 10MHz test signal a combiner and an SA, there is no bandwidth limitation (I expect the passive combiner to work up to many hundreds of MHz, same for the SA) so the SA can observe the harmonics of a single tone directly, at 2f, 3f, etc. without the need of a second frequency. It might worth measuring the harmonics of each tone alone, one by one (observed at 20, 30, 40MHz, etc), with the second generator disconnected, in the hope that this measurement will show only the purity of each signal, and will exclude any intermodulation it might happen in a second generator. --- End quote --- This is true, at least in theory. If you model the output voltage of the two-port DUT as a nonlinear function and expand it as a Taylor series, i.e., $$v_{\rm out}(v_{\rm in})=\sum_{k=0}^\infty a_kv_{\rm in}^k,$$ and consider it up to order 3, then in a single tone excitation the products at \(3f_0\) are proportional to \(a_3\), and the same is true for the third order products at \(2f_1-f_2\) and \(2f_2-f_1\) in a two-tone test. So the harmonics contain information about IMD products. However, this assumes that the Taylor series is an accurate model, and there are no frequency dependent effects. In practice, I suppose there is no way around a two-tone measurement. --- Quote from: RoGeorge on June 19, 2022, 07:20:45 am ---An alternate way to know if the intermodulation is produced by the outputs of the generators (acting as mixers) might be to add a directional coupler between each generator and the combiner, so to reduce any signal from outside that is trying to travel into the generator. In theory, it should work. In practice, I have close to zero experience or equipment to try this. :) --- End quote --- True, but I guess that a highly isolating combiner is more promising. |
| rf-messkopf:
Okay, I had another go at it. --- Quote from: rf-messkopf on June 19, 2022, 04:30:38 pm ---The datasheet of the ZFSC-2-6+ I was using states about 30 dB isolation at 10 MHz typical. Plus there was a 10 dB pad at each input, so I would expect about 50 dB of isolation. No idea if that is enough. I haven't gotten the VNA out to actually measure it though. The ALC was turned off on both generators, so that it would not cause any trouble. --- End quote --- I measured the isolation of the ZFSC-2-6+, see the attachment. I get about 33 dB at 10 MHz with the sum port terminated in a high quality load (the orange Trc3 trace), and about -18 dB isolation with 12 dB return loss at the sum port. provided by a 6 dB attenuator with one port left open (the blue Mem4[Trc3] trace). So I can confirm the 50 dB of isolation with the two 10 dB pads. I also measured the IMD again, with two 10 dB pads at the inputs of the ZFSC-2-6+, plus a 6 dB pad at the sum port to improve termination. The generators were set to a level of 15 dBm, and the RF attenuation of the analyzer was set to 40 dB. Again, I observe no dependence of level of the 3rd order products when I very the attenuator around 40 dB. I also tried two analyzers, a R&S FSIQ26 (20 Hz to 26.5 GHz) and a FSEA30 (20 Hz to 3.5 GHz), with consistent results, see the attachments. I'm still not sure though if the 3rd order products might to some extent be generated at the IF stage of the analyzer, or if it is all due to the splitter. When I set the FSIQ26 to FFT mode, I get the same results (only much faster, with 400 ms sweep time). |
| G0HZU:
I can only really offer general advice to the forum on stuff like this as I've not used the R&S gear you have there. Trying for a narrow tone spacing like 1kHz is quite challenging if you want to try for (say) -80dBc IMD. This is well inside the normal ALC bandwidth of a sig gen so this means the sig gens will be very prone to crosstalk issues. One option is to turn off the ALC or to select a lower ALC bandwidth. It looks like your sig gen can change the ALC in other ways as well. I would try and avoid doing IMD tests at such a narrow tone spacing but if it has to be done then I'd recommend a high isolation combiner, try using the attenuator hold option in the sig gen and turn down the ALC bandwidth if the sig gen permits. The attenuator hold option can typically introduce an extra 10dB isolation per signal generator if used correctly. See below for a plot from my old Tek analyser. This analyser doesn't sweep the LO and the plot below is taken using an FFT. To get about -90dBc IMD the tone levels have to be 10dB below the reference level on the display. This is why the tones are so low compared to the reference level. I don't really trust this analyser when exploring IMD levels as low as this. I'd normally use something else. I've added a second plot below where I've turned up the sig gens such that I test with two tones at 0dBm each into the analyser. This gives similar results. |
| rf-messkopf:
--- Quote from: G0HZU on June 19, 2022, 08:03:23 pm ---One option is to turn off the ALC or to select a lower ALC bandwidth. It looks like your sig gen can change the ALC in other ways as well. --- End quote --- I used the sample-and-hold function of the ALC, i.e., it measures the output level once, adjusts, and then holds that level, with the control loop turned off. --- Quote from: G0HZU on June 19, 2022, 08:03:23 pm ---I would try and avoid doing IMD tests at such a narrow tone spacing but if it has to be done then I'd recommend a high isolation combiner, --- End quote --- One option might be to use a 4-way splitter, because it has higher isolation between opposite ports than adjacent ones. With this splitter one could achieve more than 50 dB of isolation between opposite ports at 10 MHz, which could be further increased with attenuators. Unfortunately I don't have a suitable 4-way splitter here. And most datasheets don't quote the IMD performance of splitters. I've only seen that for high power splitters intended for cellular radio applications. --- Quote from: G0HZU on June 19, 2022, 08:03:23 pm ---See below for a plot from my old Tek analyser. This analyser doesn't sweep the LO and the plot below is taken using an FFT. To get about -90dBc IMD the tone levels have to be 10dB below the reference level on the display. This is why the tones are so low compared to the reference level. I don't really trust this analyser when exploring IMD levels as low as this. I'd normally use something else. I've added a second plot below where I've turned up the sig gens such that I test with two tones at 0dBm each into the analyser. This gives similar results. --- End quote --- Thanks for running it. I also didn't see much difference with different drive levels. But I don't mean to hijack the thread with this discussion. Maybe the additional data point for the R&S RTM2054 is of some interest. |
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