SIGLENT SSA3021X: Seeing Harmonics -- Too Much Non-linearity?

[mikemxyzzy]

Was doing some testing and seem to be seeing non-linearity.

For example, here is a simple setup:

[SIGLENT SDG1032X @ 4MHz]--->[4MHz LPF]-->[Step Att]--->[SSA3021X]

The function generator is set to 4Mhz, 50 Ohm output, amplitude adjusted to get 0 dBm on the spectrum analyzer. The 4MHz low pass filter is to make sure I'm not seeing harmonics from the generator, only the 4MHz signal.

I see a response at 8MHz (first attachment). When I add 5dB of attenuation the 4MHz signal goes down by 5dBm but the 8MHz goes down by more, approximately 10dB (second attachment). WIth 10dB attenuation it is the same proportion: 4MHz down 10dBm, 8MHz down approximately 20dBm (third attachment).

This looks like non-linearity producing the 8MHz response.

Does this make sense? Is this to be expected for the SSA3021X?

Thanks!

[SWR]

It's probably your signal generator producing more harmonics at higher amplitude levels?

[tautech]

The 4MHz low pass filter is to make sure I'm not seeing harmonics from the generator, only the 4MHz signal.

I strongly suspect you failed and it is indeed a harmonic of the fundamental.

Sanity check:
I suggest you sweep over a much wider span to see further harmonics at multiples of the fundamentel.

[dmills]

0dBm into the first mixer of most analysers is WAY too much, turn the input attenuator up at least 10dB or so, level at the first mixer should generally be in the -10 - -20dBm region at least on my Anritsu.

One good way to tell if you are seeing SA mixer products is to deliberately attenuate the input by a few dB, if the suspected harmonics go down by MORE then the amount of attenuation you drop in then they are at least partly internally generated, if they go down by the same amount as the attenuation then they are present on the input.

This sanity check is also (in a slightly different form) useful to verify that network analysers are behaving.

[tautech]

0dBm into the first mixer of most analysers is WAY too much, turn the input attenuator up at least 10dB or so, level at the first mixer should generally be in the -10 - -20dBm region at least on my Anritsu.

One good way to tell if you are seeing SA mixer products is to deliberately attenuate the input by a few dB, if the suspected harmonics go down by MORE then the amount of attenuation you drop in then they are at least partly internally generated, if they go down by the same amount as the attenuation then they are present on the input.

This sanity check is also (in a slightly different form) useful to verify that network analysers are behaving.

The OP is using the default -20dB internal attenuation in each of his screenshots so really we don't know if his external attenuation is linear or not.

[Performa01]

This looks like non-linearity producing the 8MHz response.

Does this make sense? Is this to be expected for the SSA3021X?

As is generally known, the first component in a spectrum analyzer after DC block (optional), input attenuator and pre-selector (optional) is the mixer.
Mixers are a non-linear devices by nature, so they will indeed generate harmonics of the input signal.

The CP of the SSA3kX is a bit low at -5dBm and that means that accurate amplitude measurements are possible up to some -10dBm at the mixer input, but for distortion measurements the signal levels need to be much lower than that.

When in doubt, it's always a good idea to have a look at the datasheet:

Second harmonic distortion: fc>50 MHz, mixer level -30dBm, attenuation = 0 dB, preamp off, 20°C to 30°C , typ. -65 dBc

For your specific test with a signal level of 0dBm and 20dB internal attenuation this means you need another 10dB attenuation to get a mixer input level of -30dBm and then you can expect a 2nd harmonic distortion of -65dBc typical.

Looks like your device is a lot better than that, even though your signal frequency was way below the specified range of >50MHz.

[rf-loop]

For OP your SA is very ok with these measurement results.

For set these values to same space with others:
Here example: very good (old State Of Art class) well known laboratory SA what still can use for compare or least tell what is "normal". (Also I have owned and used this amazing SA previously) 

This image show specs limits and also there is graph about "typical" situation in this old "state of art"(1) class SA..

(1) "State Of Art" is highest class and then next below it is "High End"

Of course there is also available many others SA's  specs and also reading material about fundamentals. But now just this tiny example.

[mikemxyzzy]

Thanks for the help everyone. This is my first experience with a spectrum analyzer and needed a sanity check.

[rf-loop]

I recommend this for reading. It also tell lot of how SSA3000X wirks when (of course need look things related ti full digital IF systems).

No need understand all but it can use also as basic "handbook".

Keysight AN-150 pdf

It have lot of good information. Basic fundamentals but also some practical details.
I will recommend this to everyone. It explain lot of things and also based to right theoretical facts.

Example figures on page 57 show how signal mixer level affects.