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Two Tone Test with Scope and SA
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RoGeorge:

--- Quote from: G0HZU on June 15, 2022, 02:25:08 pm ---where the significant IMD gets generated
--- End quote ---

Distortions can only appear when a circuit is not linear.  A perfectly linear circuit (an ideal LTI - Linear Time Invariant system) can not have any distortions.  In frequency domain, an ideal LTI system will show at its output exactly the same spectrum as it was fed at its input.  The output can only differ in amplitude (the ideal LTI system can amplify or attenuate, but it can not produce new frequencies).  Or else said, an ideal linear system won't have any intermodulation products.

In this topic, intermodulation is only a technique that can evidentiate very small non-linearities that might happen inside the oscilloscope, distortions so small that otherwise would be close to impossible to notice by just looking at the waveform in the time domain.

Intermodulation products get generated anywhere there is a non-linearity.  In the case of an oscilloscope for example, the frontend amplifier is not ideal, it will have some small nonlinearity, therefore some intermodulation products will appear.  Later, the signal get sampled by the ADC then displayed as a spectrum, where we notice the intermodualtion products.  Or, it can be some other components than the input amplifier, where the signal is distorted, even the ADC itself will add some distortions.  It's a test for the whole oscilloscope (assuming the input signal is perfectly clean, with only 2 frequencies).

The TL;DR is, new spectral components will appear anywhere there is a non-linear response.  Non LTI system means the circuit will distort the signal, and we will notice that as intermodulation.  No intermodulation can appear in an ideal, linear amplifier.

I like this video about "Linear and Non-linear Circuits" from "The Signal Path" channel because it tells about intermodulation both in theory and in practice, with experimental measurements to build an intuition:

G0HZU:
That's very impressive from the Siglent scope. Below 40MHz the Tek analyser runs the input direct to the ADC via an attenuator and presumably a preamp when needed. If I were to try for sweet spot drive levels I think I could get better than -90dBc IMD but I'm not sure I'd trust what is displayed. The main niggle with the Tek analyser is that it has a few low level internal spurious signals even with no input so it has to be used with some care and sympathy for its limitations.

I had a go at retesting the single/dual ERA1 MMIC amplifiers this evening and this time I did it quite formally by measuring each one for gain with a VNA and also I checked they were the same for IMD. I also used a precision 0.1dB step attenuator to set up the correct attenuation between the two amplifiers. See the images below. On the VNA the small signal gain was about 12.3dB for both amplifiers and the IMD performance seemed to be identical. The spectrum analyser plot below shows the IMD for both ERA1 amplifiers when tested standalone and trace 3 is when they are in series with the 12.3dB attenuator in between them. The marker shows the IMD change was very close to 6.0dB when the two amps were in series.

I've used a decent Agilent lab VNA for the gain tests and I set the drive level correctly to capture the correct small signal gain for the IMD tests. I also used the VNA to set the step attenuator within 0.1dB of 12.3dB loss.

So I think this is a good experiment and demonstration of the equations provided by Agilent and R&S. It shows the expected 6dB increase in IMD.
rf-messkopf:
This thread got me curious. To see where I'm at, I first tried two ways to combine the outputs of two signal generators (a R&S SML and a SMU200A, both with ALC switched to sample-and-hold to avoid any interference between them): A Mini-Circuits 3 dB combiner/splitter, model ZFSC-2-6-N+, with a 10 dB pad at each input, and a 6 dB resistive combiner, also with 10 dB pads at the inputs. I tested at 10 MHz with a frequency difference of 1 kHz, and with 10 dBm input power for the 3 dB combiner, and 13 dBm for the resistive combiner.

With the 3 dB combiner, I measure the 3rd order products at 75 dBc, and with the resistive combiner, they are more than 10 dB higher on a high dynamic range spectrum analyzer (R&S FSIQ 26).

I'm pretty confident that in the case of the 3 dB combiner, the 3rd order products are for the most part not generated by the analyzer, because I choose the input attenuator setting high enough that they are no longer moving when I vary it. Here the 1 dB attenuator option comes in handy.

I haven't investigated this any further, but I suspect that the higher IMD with the resistive combiner is due to the lack of isolation (only the 10 dB pad plus the 6 dB insertion loss of the combiner) between the two signal generators, and the IMD is actually produced in the generators.

Then I tried two tones 20 kHz apart at 1 MHz on a R&S RTM 2054 oscilloscope. That is an older model with an 8 bit ADC. Also, its built-in FFT is a bit compromised, so that for more comprehensive tests I would have to pull the samples on a PC and do the FFT there. However, at that frequency with 100 mV/div I measure the 3rd order products at about -60 dBc (see attachment).

I also verified with the analyzer that they are at least 80 dB down at the scope input.
David Hess:
How would this measurement be made while avoiding the limits of the analyzer?  Notch out the fundamentals?
RoGeorge:
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.

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.  :)
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