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Life is full of learning opportunities and I'd like to share one I had today.
I'm trying to measure the signal quality of a SI5351 based signal generator set to 12MHz and using a 30dB attenuator at the input of the SA. Resolution is set to 15kHz
What I see is a real mess, the 12MHz signal is there at -23dBm but also a vast amount of strong spurs at about all possible frequencies.



The SI5351 is not a very bad signal generator so I'm puzzled why it is that bad.
With 12MHz input and the first IF of the SA at 110MHz I should have plenty of bandwidth headroom I think.
So I connected a rather clean 12MHz signal from an old analog signal generator
That looks much better!! The spur at 8.3MHz is coming from some leakage because of the rather long cable between the signal generator and the SA but for the rest it is about clean.?


Then it occurred to me that I am not using a low pass filter before the first mixer of the SA so I add one that should do -40dB at least above 100MHz and reconnect the SI5351
The picture changes completely. There are many spurs, but at 70dB below to the 12MHz signal so this measurement is consistent with this artikel:?




So I learned today that harmonics go a LONG way.....

For the technicaly inclined, In the SA I'm using two ADF5351 LO's to steer two AD831 mixers with first IF at 110MHz and resolution filter of 15kHz at 10.7Mhz followed by the log detector
These AD831 mixers (on 12$ modules from Bangood.com) are VERY good, unfortunately thy don't go above 1GHz. I'm considering shifting the first IF to 500Mhz to make maximum use of them
The two ADF5351 seem to create much cleaner LO signals in the SA then one SI5351, that is why I started to investigate the output of the SI5351

Mixers tend to mix everything available, not just the stuff you expect.
And with some mixers at least, the unwanted (and wanted) products can get reflected back in
to make another set of permutations.? So a LPF is critical, and a high side LO is a very good idea.

But your SA is working great!??
Plenty good for checking emissions on ham gear.
Thanks for showing those plots, the LPF mattered way more than I would have expected.

Jerry

I assume that's an ADF4351.

Jerry

Erik,
Thank you for the post. I also have a couple of the AD831 modules and want to try them for the SA project. How exactly are you connecting the LO to AD831's?
If putting a LPF on the signal input cleans up the plot so much, does a LPF on the LO to the first mixer produce the same improvement?
Congratulations on getting a working instrument.
73 Brian VK4BAP

I did a bit more testing, configuration now is:

Low pass filter
AD831 mixer with first LO as ADF4351 (directly connected, no need for low pass filter as the AD831 makes it a square wave anyway)
10dB amplifier to compensate for loss in first IF filter
110MHz first IF filter 2MHz BW(this could be at any frequency your LO's can reach) with loss of about 10dB
AD831 with second LO as either SI5351 (connected with 10dB attenuator, for max first IF at about 300MHz (yes, its out of spec but it works)) or ADF4351 (directly connected)
10.7MHz resolution filters
Log detector

There is no visible performance difference between SI5351 and ADF4351 as second LO.
The second LO signal level is not critical, as long as its sufficient
The AD831 are very forgiving on LO level, according to the datasheet it should be -10dBm

I need to investigate the 3rd order intercept of the first mixer as I seem to get worse performance then predicted (datasheet states +24dBm) so probably still something wrong in my configuration

Tested with 1st IF at 500MHz? and I get 15dB less sensitivity.
Compared 110MHz IF filter and 500MHz IF filter on my VNA and they have about the same loss. AD831 are 50 ohm matched on both RF input and IF output
Possibly?the AD831 IF output does not like that high frequencies (datasheets states 200MHz max)
Forgot to test without IF filter.....

It may be worthwhile to use a 6dB attenuator between mixer LO port and low pass filter. That way, the LO port always sees 50 ohms. It requires 6dB more LO signal, but gives 12dB more isolation.?
All the ports of a mixer should be considered as both input and a output port. Even tho you use a gilbert cell mixer, using attenuators to improve the isolation should help.
Remember that shielding is important when doing a project like this, feedthrough capacitors and coax connectors help keeping the unintended signals out of the spectrum analyser.?

What graphing library are you using? it looks great!? This is a interesting project and I will be following along.?


73 de Thomas LA3PNA.?

The pc software is visual c++ using .net objects. The graphing is using the standard .net chart object.
Much faster then drawing the chart with lines yourselves and much more funtionality. Of course some things are not possible and you have to accept