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
