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I have a circuit with a TFM-3MH+frequency mixer. I need to output a low frequency signal of 10-100Hz.

The signals that I'm sending to the input ports from a signal generator are:

  • Sine wave of frequency 6.9 MHz at LO port (+17 dBm)
  • Sine wave of frequency 6.9 MHz+10Hz at RF port (-39 dBm)

The output of the mixer should be a sum of: frequency sum (6.9 MHz+6.9MHz+10Hz) and a difference (10Hz.)

When I filter out the high frequency component, the signal that I'm getting is:

10Hz:

enter image description here

80 Hz:

enter image description here

Can anyone explain what can cause these glitches in the signal?

How to avoid them?

Can I add some circuit after the output to smooth the signal?

The spec of the mixer is here:

https://www.minicircuits.com/pdfs/TFM-3MH+.pdf

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    \$\begingroup\$ I don't think the glitches are a result of the mixing process, I think it's the way you're using the 'scope. It looks like a previous trace is being partially overwritten by a second trace of a different amplitude Try it on single shot. \$\endgroup\$
    – Neil_UK
    Oct 21, 2021 at 13:54
  • \$\begingroup\$ If you use a higher (audible, say 100Hz) frequency, can you hear these? (as clicks or plops) Is the low pass filter ahead of the scope, or done by some averaging function in the scope? If the latter, I would suspect some sampling artefact in the scope (aliasing?) and want to eliminate that possibility. (Still an analog scope guy here!) \$\endgroup\$ Oct 21, 2021 at 13:56
  • \$\begingroup\$ @Neil_UK I tried it also with a single shot and it's the same effect. Tried it with a different scope too. When I start to increase the frequency difference the effect starts to disappear. I also sent the signal to the ADC and drew the waveform in MATLAB, the same occurrence appears. \$\endgroup\$ Oct 21, 2021 at 14:08
  • \$\begingroup\$ @user_1818839 By increasing the frequency difference the effect fades. Low pass filter is inside the scope. Tried it by sampling it with ADC and drew the waveform, same occurrence. I support your analog preference! \$\endgroup\$ Oct 21, 2021 at 14:15
  • \$\begingroup\$ Make sure the mixer is properly terminated with 50 ohms on all ports, and use a real analogue lowpass filter ahead of your scope. Maybe buffer the scope from the mixer, a digital scope can do funny things to its input impedance. Draw a schematic of what you currently have driving the mixer and how you're loading the IF out. What scope input impedance are you using? \$\endgroup\$
    – Neil_UK
    Oct 21, 2021 at 14:31

1 Answer 1

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In response to this comment

@Neil_UK They weren't phase locked to a common reference. I synced them and the glitches are gone. Thank you for your help! But I must ask, why is important that the starting phase must be the same? Since the signals are at a different frequency, the phase will constantly change during signal generation

It's not important the starting phases are the same. It is important the phases are consistent throughout the experiment. The clue was in the information that the effect reduced as the frequency separation got larger.

Typically signal generators have a high frequency low phase noise internal reference that ensures the phase noise performance in the 1 kHz offset and further out region. This is then locked to an external reference in a fairly narrow loop, 10 Hz is not uncommon, or is allowed to free run. Rarely it's locked to another internal reference optimised for low long term ageing rather than phase noise.

Signal generators are full of non-linearities and unexpected signal sneak-paths, especially the cheaper ones. You don't say what make/model these generators are, so I can only assume the worst. Any signal getting back into the generator has the possibility of mixing in the internal non-linearities of amplifiers, power supply reaction (you name the mechanism, it will be operating at some level) with other internal signals to mix down to low frequencies. Not only that, the mixer will be sending IF and harmonics of the LO back into the generators, and its reflection coefficient will be modulated by the LO, causing more modulated return signal back into the signal signal generator.

The result of all that is that when the two set frequencies are sufficiently close together that the difference frequency is within the bandwidth of the low noise phase locked loop, there's a possibility that the PLL will be modulated with that difference. Now the steps are unexpected in a fully analogue system, but who knows what digital shenanigans are being done in signal generators these days, that could decide to muck about with PLL bandwidths or pre-tunes on detecting that the loop wasn't stable.

As the difference frequency increases, it moves outside the bandwidth of the PLL, stops modulating the reference, and the problem goes away.

When the signal generators are synchronised, the signal paths around the PLL are changed, to positively phase locked to each other.

If you want to experiment some more with the mechanisms, then the first thing to do would be to try again with free-running generators, but fully isolate the two signal generators from each other. Buffer the LO signal with a power amplifier and attenuator to reduce any return signal from the mixer. I'm presuming that with a low output level, the signal signal generator already has a significant attenuator in its output, but put some more amplification and attenuation in there anyway.

I've been involved with building signal generators for much of my professional career. Getting the thing working to produce the required frequency range, resolution, phase noise, power output and harmonic performance takes about 20% of the time. Finding and curing all the signal sneak-paths that cause unexpected mixing and generate unwanted spurious signals takes the remaining 80%.

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  • \$\begingroup\$ Wow, thank you for such a detailed explanation. I was lucky that a man of your experience stumbled onto my post. I didn't know that could be causing the problem. The signal generator is decent, but I guess they also have this common issue. \$\endgroup\$ Oct 22, 2021 at 15:05

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