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Friday, 28 June 2013

So, Why would you want one?


It's a great question - we're talking Spectrum Analysers here, by the way - my first offering would be, why not? Second would come, because I can.... but really, let me try and show you.

None of this has any real importance, but you can say that about anything, can't you?

I've just been aligning (read fiddling with) an SSB transmitter that I made a while ago. Now, part of the circuit mixes the BFO frequency (in my case it's about 12.9MHz) with the audio that's being injected. Under normal operation this would be from a mic, but for this exercise I'm using an audio signal generator and injecting a 1KHz tone.

The signals are mixed together in a simple diode based doubly balanced mixer, and the output contains a whole pile of stuff. There's the BFO frequency, the audio frequency, the sum, the difference and all sorts of other harmonics in this signal. On a scope, when the balanced modulator is un-balanced it looks like this:

As you can see the signal is a complete mess, and very hard to see what's going on. This was a single shot take on the signal as there is so much that the 'scope cant really decide what to trigger on.

Here's the same signal on the Spectrum Analyser:

Now, this makes a whole pile of sense. The left hand peak is the LSB, it's exactly 1KHz (the audio frequency) below the next peak which is the carrier or BFO (remember I said the one of the things in the output was the difference between the carrier and Audio frequency). The right hand peak is the USB - this is the sum of the carrier and the Audio frequency. What we actually have here is an AM transmission - a carrier and two sidebands.

So it's very clear here - this part of the circuit contains some components to suppress the carrier (remove it) but the circuit needs to be balanced before this will happen correctly. Using the adjustment available and looking at the Spectrum Analyser I can adjust to this:

So, I can clearly see that the carrier is now well down into the noise. The next part of the transmitter chain is the crystal filter, that will suppress the unwanted LSB to create SSB as what we have here is DSB (double side band). Once again, using a Spectrum Analyser, I can watch the RF output and alter the BFO to place the USB inside the filter passband and the LSB well down the skirt of the filter. Only then will we have what we are trying to create; Single Sideband (SSB).

Simple, egh?

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