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If you have a transmit capable SDR, you may have heard that you need
to filter its output, before transmitting to the world. Certainly
before amplifying the signal.
I have a TinySA Ultra spectrum analyzer, and will here show
you some screenshots about just how true that is.
I tested this with my USRP B200, transmitting a pure carrier
around 145MHz and 435MHz.
Oh, and a word of caution: If you want to replicate this, make sure
to add an inline attenuator, to not damage your spectrum analyzer. I
had a cheap 40dB one, but the values in the graphs have been adjusted
to show the real signal strength, as if I hadn’t.
tl;dr
- Harmonics can be almost as strong as the fundamental. You need
to filter these.
- Transmitting at maximum output gain may cause lots of unwanted
signals right around your fundamental. You cannot filter
these. You need to not generate them.
Harmonics

Reducing the output gain did not meaningfully fix the problem. The
best I saw from using half output gain was to make the strongest
harmonic 9dB less than the fundamental. That’s way too strong.
I added a cheap band pass filter (FBP-144), which made Continue reading