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Hi Erik,

Still having fun with my new Tiny SA.

My main reason for purchasing to was to help locate RF noise sources, especially from Switched Mode Power Supplies.

I have a question relating to the expected performance below 1.5MHz.

I noticed that when operating the Tiny SA in Auto agc mode, there was a distinct series of amplitude steps in the noise floor, that is very apparent below 1.5MHz when there is no input connected.



Tuning the agc off (no tick in box) produces this trace, which is flat as a result of the overall reduction in sensitivity.



Turning the agc on (tick in box) produces this trace, which is probably the most realistic of the three, and makes operation down to about 600KHz possible. When the input is connected it is possible in this mode to see signals down to 100KHz, but they are masked by the higher level noise floor.



Is this behaviour as expected ?

Regards,

Martin

Yes, it is intentional and unfortunately a consequence of some limitations of the tinySA.
The last picture is indeed the most realistic but it gets ugly when you add a rather strong signal at say 100kHz because the automatic AGC gets confused and creates all kind of artifacts. Just try it.
To prevent these artifacts the tinySA (when set in automatic AGC mode) overrides the AGC and increases the amplification stepwise (stepwise as there are only a very limited set of amplification settings possible) when sweeping from zero Hz to 1.5MHz. This stepwise increase in amplification creates a stepwise reduction of the noise level and these are the steps you see.
So as long as you have no strong input signals (everything below say -50dBm) you can set the AGC to on (last picture you showed) but if there is a strong signal below 1.5MHz the better setting is the stepwise gain override? as shown in the first picture.
?

Hi Erik,

OK that's fine.

The signal levels when using a close field probe are likely to be quite low, so I'm happy to put the AGC into manual mode when hunting noise sources below 1.5MHz.

Thanks for the quick reply, much appreciated.

Regards,

Martin