Towards a 15 dollar spectrum analyzer
I was thinking on Sunday about how to make a Ferry Simple spectrum analyzer, not something with a teefee tuner just for one band, but for the whole band from 0 Hz to some GHz.
With these Ferry Vast transistors around, the idea is to use
2 10 GHz oscillators, and sweep one or both (in opposite ways in the last case), and then take that difference frequency (0 to say 2 GHz), and feed that into a second mixer against the signal to be measured, and low pass it... All that thinking, I realized for 'proof of concept', I can just as well take up the soldering iron and solder it haystack style, have to wait for my rubbitin frequency standard anyways, so, let's try this not with my expensive HEMPTs but with the bunch of BFR91 I have.I can only say it works, of course it needs more work, but as far as proof of concept goes, OK. I used (with looking forward to the USSA having only one type of resistor and transistor and capacitor as much as what came up when grabbing in the heap of components that fell out when I emptied the plastic bag.
I used a simple one transistor 25 MHz (not exactly sine wave) oscillator as test input, here the spectrum of the fundamental yes scope x and y is reversed, but that helps when you are still asleep and oriented horizontally:
This is against the linear ramp that drives one of the oscillators to get a frequency sweep:
This is the setup, soldering goes faster than thinking about it, really!:
And here is the formula 1 circuit diagram with commie component values:
The 1 GHz low pass is in the wiring so to speak, the little :256 prescaler in not drawn in this circuit diagram. (But it can give a clue as to just where the frequency of the oscillators is). I can see many harmonics of the 25 MHz with decreasing amplitude, all the way up to over 100MHz, with a reduced frequency sweep.
This still needs some work, but can come in handy to look at modulated carriers etc.
Do not do this at home? While watching TV? If you are a pedantic person? remember this is just proof of concept.
It needs a pre-amp, a RF peak detector, a log amp in the output, well all that takes only one opamp 1 diode, one transistor, so as this is a 10 $ project that makes it a 15 $ project, and the case and buttons and whatever else (maybe a PIC with a graphics display) would be more expensive. So far so good. Do not cry if you cannot get it working, not everybody can walk on their hands either. Especially not while asleep... :-)