From: Sean Breheny <shb7@cornell.edu>
To: Microcontroller discussion list - Public. <piclist@mit.edu>
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Date: Mon, 18 Jan 2021 15:18:09 -0800
Subject: Re: [EE] Lossy bandwidth compression with analog circutry?
Thread-Topic: [EE] Lossy bandwidth compression with analog circutry?
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Hi Jason,

There are many ways to do what you are asking. One way (maybe not the
most practical) is to use a length of highly-dispersive transmission
line (or dispersive delay line or SAW structure). This would convert
frequency-domain into time-domain (in other words, if you put a 1
millisecond burst of the entire radio spectrum in one end, you'd get a
longer (let's say 10 milliseconds) burst out the other end where the
lower frequencies would come out first and the higher frequencies
later, or vice-versa. You could then use a demodulator to produce a
power vs frequency output which was represented as a voltage vs time.
Some old radars did things like this and so did early spectrum
analyzers. The section of transmission line needed would be very long
- probably at least 10s of meters even when made of very special
ultra-low velocity factor materials.

Of course another, simpler way would be a wideband tunable filter
which you sweep across the spectrum. This would produce a similar
output (frequency vs time). This may be easier to do if you first
upconvert your 0-100MHz to, say, 1 GHz to 1.1GHz and then use a
YIG-tuned filter (YTF) or dielectric resonator filter.

Could also be done with clever use of a comb filter and undersampling.
In other words, you intentionally produce a highly-aliased version of
the signal but preceed the sampler with a comb filter which selects
out several narrow ranges of frequency so that the aliased copies
don't overlap in the frequency domain. Tough to make sure everything
lines up with no gaps or unintentional overlaps.

All of the above, though, would be harder to implement today than a
digital solution. They would require careful tweaking and quite
possibly temperature compensation to make them workable. I've seen
1970s microwave test gear where a large part of the system consisted
of arrays of op-amps and multi-turn pots to create complex calibration
curves (e.g. for linearizing the tuning of a VCO) in an analog
circuit, which all had to be hand-tuned for each unit at the factory.

Sean

On Sat, Jan 16, 2021 at 6:17 PM Jason White
<whitewaterssoftwareinfo@gmail.com> wrote:
>
> Hi everyone,
>
> Is it possible to "compress" the bandwidth/frequency spectrum of a signal
> using an analog circuit? (with or without loss of information) For exampl=
e
> taking 100Mhz of radio spectrum and shrinking it (preferably without
> involvement of digital electronics) into a 1Mhz span for monitoring
> purposes. Specifically to look at received power vs frequency vs time in
> order to identify approximately when and at what frequency transmissions
> occur at over a long period.
>
> I think the closest equivalent of this would be having a receiver scan
> "quickly" and looking at each "slice."
> A second might be to do some sort of "frequency binning" in the analog
> domain.
>
> Are there other ways?
> --
> Jason White
> --
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