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DIY spectrum analyzer (RF)

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Mosaic

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Hi all , i've been looking @ making an RF spectrum analyzer to hook to a scope , freq range of interest is in the 1Mhz to around 104-112Mhz range.

Questions:
1) For sensing RFI from PCBs etc, what kind of wideband preamp is suggested?

2) Are there combined VCO or (pref. I2C/SPI controlled) oscillators that can cover the freq band of interest? Hopefully they can also produce the IF by difference mixing?

3) By linking the VCO sweep period to the 'scopes trigger I should be able to view the various freq. amplitudes on a single scope channel?

4) By injecting (mixing) known period, small duty, fast square pulses of peak vertical amplitude (based on the VCO cycle period)matching the scopes display I can create horiz. 'scale' markings (vertically) on the CRT of known frequencies to 'calibrate' the CRT freq. display?

eg. If the CRT is displaying the full 112Mhz Bandwidth of the VCO, injecting pulses every 1/8 VCO period should create vertical lines on the CRT @ 14Mhz, 28Mhz , 42Mhz .... 112Mhz; for a 112MHz VCO bandwidth?

5) If #4 is reasonable, can I then create those same calibration pulses alternating at 2 known amplitudes after each VCO period thereby creating an apparent vertical amplitude reference for aligning the vertical deflection of the scope for more accurate amplitude readings?
Eg. Pulse 1 @ 1V, Pulse 2 @ 2v, pulse 3 @ 1V ...etc

6) #5 can be expanded to multiple known amplitudes if a log. output is used to help visualise the signal amplitudes?

Any advice is appreciated.
 
100:1 tuning range VCOs are not simple to make. it would be better to have a range switch and cover it in bands. your VCO output should also have very tight control of the output amplitude. you can get the calibration signal from a crystal controlled pulse generator. you want a fairly even mix of even and odd harmonics. i would recommend a 1Mhz source for this, which will give you a tick mark every 1Mhz, but you will want to turn the generator off when analyzing signals. i've seen a 10 decade log amp circuit with discrete components, i'll try to post it later. your input preamp needs to have a very high dynamic range. you could use an untuned front end, or a voltage tuned front end, but the voltage tuned front end is more complicated.
 
1) For sensing RFI from PCBs etc, what kind of wideband preamp is suggested?
Probably none.
Gain in front of the first mixer can cause overload problems.
Leave the wideband amp until you have the rest of it working.

2) Are there combined VCO or (pref. I2C/SPI controlled) oscillators that can cover the freq band of interest? Hopefully they can also produce the IF by difference mixing?
As previously mentioned, a 1:100 frequency ratio is a bad idea and unnecessary anyway.
Make the first IF frequency of your analyser higher than the highest input frequency.
For example, if the first IF of the analyser is 200Mhz, then the first local oscillator only needs to run from 200 to 300Mhz and the analyser will tune from 0 to 100Mhz.
A VCO running from 200 to 300Mhz is relatively easy.

3) By linking the VCO sweep period to the 'scopes trigger I should be able to view the various freq. amplitudes on a single scope channel?
Yes, that is what the average spectrum analyser does.
Infact it uses the timebase oscillator output as the drive for the VCO.

4) By injecting (mixing) known period, small duty, fast square pulses of peak vertical amplitude (based on the VCO cycle period)matching the scopes display I can create horiz. 'scale' markings (vertically) on the CRT of known frequencies to 'calibrate' the CRT freq. display?
Once the analyser has been calibrated and assuming that the timebase and VCO are linear, the 10 x-axis divisions on the scope graticule will represent 0, 10, 20...90, 100 Mhz.
a Full time calibrator is not necessary or even desirable.

eg. If the CRT is displaying the full 112Mhz Bandwidth of the VCO, injecting pulses every 1/8 VCO period should create vertical lines on the CRT @ 14Mhz, 28Mhz , 42Mhz .... 112Mhz; for a 112MHz VCO bandwidth?
Sounds like a bad idea.

5) If #4 is reasonable, can I then create those same calibration pulses alternating at 2 known amplitudes after each VCO period thereby creating an apparent vertical amplitude reference for aligning the vertical deflection of the scope for more accurate amplitude readings?
Eg. Pulse 1 @ 1V, Pulse 2 @ 2v, pulse 3 @ 1V ...etc

6) #5 can be expanded to multiple known amplitudes if a log. output is used to help visualise the signal amplitudes?
Use several stages of logarithmic amplifier, usually as the lasy IF amplifier, calibrate it and you have your vertical scale calibrated in dB.

JimB
 
For frequencies from around 50Mhz to 1Ghz, look at the software radio / USB dongles.

https://rtlsdr.org/start is a pretty good site - I bought a USB TV dongle for around £6 delivered which is compatible with the software on that site. There are also designs to extend the range to much lower frequencies.

If nothing else, it's fascinating seeing how much junk there is on the radio frequencies !
 
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