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Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Bored Chemist: If you bear in mind that a multiplier is one form of a mixer, then you just described how a spectrum analyser works. Of course there are enough fiddly details to it to fill several fat books, but that is the basic principle.
The complex (I and Q) IF solution you describe is not used in spectrum analysers, AFAIK. Up until very recently, it was easier to just use a dual conversion superhet with a high quality filter to suppress the image response. The first IF can be several GHz.
However, modern DSP power has made complex IFs much more attractive. You get free software nowadays like SpectrumLab, which is basically a FFT analyser that works with a PC soundcard. It will accept I and Q inputs on the left and right channels, and you can buy accessory boards with digitally generated I and Q local oscillators. This lets you transform an arbitrary 48kHz wide slice of radio spectrum down and view it on your computer screen. It can even demodulate stuff digitally and play it through your PC speakers, AFAIK. When used with a quality 24-bit soundcard, the digital dynamic range is pretty stunning, and it's quite usable as a software radio transceiver.
Registered Member #135
Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735
Lots of good suggestions here how to 'in circuit' test frequency response. I'm going to have to play with your suggestions.
Lets try to move toward real-time spectral analysis. The goal is to see a TC spectrum in real-time as it operates while using an antenna of sorts to pick up its frequency signature.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
I'm sure I remember Richie Burnett doing that a while back, just using a regular spectrum analyser with an antenna, placed near the TC. His graph showed three peaks: one in the middle at the secondary free resonance, and two at the coupled resonances. You get the two resonances while the spark gap is lit and the third free one after it quenches.
Registered Member #193
Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022
Long ago I wrote a bit of software that calculated Fourier transforms the hard way ie by multiplying by sine and cosine waves and integrating numerically. It took days to run anything like a spectral analysis. However, if you use the idea I put forward it will run in real time. I just pointed out the idea because Hazmatt already had most of the bits. I'm sure that my mobile phone can do a much better job than my old program did but that's progress for you (and the FFT algorithm). The "official" answer is to get a spectrum analyser from ebay but, even second hand, they get expensive for anything above audio frequency.
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Scope and Generator Spectrum Analysis
