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I have an oscilloscope/spectrum analyzer that has a maximum sampling frequency of 20MHz and I would like to analyze the spectrum of a higher frequency band (also 20MHz wide) for example 90-110MHZ to see on which frequencies are signal peaks.
Is there a way (some analog circuit) that can convert that higher frequency signal into a lower frequency one and keep some proportion / ratio so I could determine the original frequency. Basically I would like to move tge signal down the spectrum.
Yes, and you already have that circuit. It's called a ADC, and by the mathematical nature of aliasing, you get all frequency components at multiples of your sampling frequency \$f_s\$ aliased into your discrete signal.
That's why every ADC needs an anti-aliasing filter: so that only frequencies in \$[-f_s/2;+f_s/2[\$ even reach the ADC. If you instead just filtered to some other \$f_s\$ (or less) large bandwidth, you still would get undamaged signal from there. This trick is bandpass subsampling, and it's pretty common, especially in IF receivers (superheterodynes).
If you own osciloscope/spectrum analyzer that has max sampling frequency of 20MHz and I would like to analyze spectrum of higher frequency band(also 20Mhz wide) for example 90-110MHZ.
If the oscilloscope or analyser is well designed then it will no-doubt have anti-alias filters built in that restricts the maximum analogue spectrum to somewhat less than 10 MHz.
This means you can't rely on directly under-sampling in that device.
Is there a way(some analog circuit) that can convert that higher F signal into lower F one, and keep some proportion / ratio ? So I could determine original F? Basically I would like to "move signal down" the spectrum.
Given that it is unlikely that the device can under-sample effectively, you should try and use "radio methods" such as mixing and filtering to lower the 90 MHz to 110 MHz band down to near base-band. Note that you are going to be limited to something a little less than 10 MHz base-band-width so, I would recommend a tight filter after the mixer that takes the relevant portion of the down converted signal into base-band. Then use the local oscillator (associated with down-converting) connected to a frequency counter so you know what part of the upper spectrum you are dealing with when you view the signals on your device.
Another term to look for is super-heterodyning (as per fairly standard radio receivers). You also might be able to use a standard FM radio receiver with some modifications: -
The original block diagram is from the superhet link above. I've marked it in red. This will likely get you a band from 88 MHz to 108 MHz (the standard FM broadcast band) with a detector output bandwidth of between 100 kHz and 200 kHz (something you'll need to measure). Good luck.
Spec; Design a BW=20MHz scope into a SA using a sweep 90-110MHZ.
This is done using a VCO & Mixer such that VCO starts above fmax and sweeps up by the VCOmax=fmax+BW required.
The mixer creates both +/- sidebands so the choice of f requires blocking the upper sideband and passing the lower sideband.
Thus VCO sweep should use the Hsync from the scope or V_Hsweep sawtooth to be used to control the frequency.
VCO = 110 to 120 Mz with MHz/V gain and offset TBD to match your sawtooth horizontal sweep signal.
For the FM radio, it is similar except the BW is fixed and smaller rising a 11 MHz centre and ~ 100 kHz BW. So a 100MHz RF uses an 111 MHz VCO.
Baseband noise level will depend on the IF filter BW and noise figure.
