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Friday, 24 January 2014

Minima - Full Circuit TX - Take II


Following on from my last post, having looked at this spectrum plot:

We needed to understand what the RF peaks on the left were; there seems to be one at 5.something MHz and another at 8.something.

The first thing I did was throw the cable I was using to connect the audio generator to the mic amp in the bin - it was clearly a pile of dingos kidneys. I then connected the audio sig gen to the 'scope and set it for about 40mV peak to peak at 1.2 (ish) KHz.

I then repeated the sweep above:

So, this tells us a few things - primarily that these RF peaks are related to the audio and the drive level - I was probably overdriving the audio amp in the earlier scan.

Now, if I then focus in on the peak we can still see at 5.something MHz and exaggerate the problem by increasing the audio drive I see this:

Now, these multiple peaks look very suspiciously like they are spaced by the audio frequency to me, so I increased the audio sig gen frequency to 3.5KHz:

I can reduce the amplitude of these peaks by setting the balance pot on the bias for the KISS mixer - but this is about as low as they will go.

This will all mean something to those far cleverer than I, but I actually suspect that there isn't really a problem as long as the audio levels are sensible.

What do you recon, fun, egh?

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