wideband FM demodulator

We're thinking of sending a wideband (say, dc to 50 MHz) signal using FM. The carrier could be in the 500 MHz to 1 GHz range maybe.

I can get the VCO. What's a good way to do the FM discriminator these days?

I could homebrew a quadrature detector, with some resonator and an XOR gate or a mixer, but there must be some easier way to do it. Not being an RF type, I'd appreciate suggestions.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics
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John Larkin
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For bandwidths that wide, I'd be looking at a delay discriminator. In the lab that would be a coax cable and a MCL mixer.

Cheers

Phil Hobbs s

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Dr Philip C D Hobbs 
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Phil Hobbs

SDR

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  When I tried casting out nines I made a hash of it.
Reply to
Jasen Betts

How do you get down to DC with a delay discriminator?

Reply to
Steve Wilson

You have a scope with an isolated ground. Take it apart and find out how it works.

Reply to
Steve Wilson

So long as your analog signal chain bandwith is sufficient to pass the carrier frequency + /- modulation index you can undersample it to recover the modulated signal directly.

Alternatively if you lock a PLL to the frequency modulated carrier signal then your demodulated baseband signal is just whatever the instantaneous VCO control voltage give or take a phase offset

Both of these should work down to DC

Reply to
bitrex

Probably a Tek TPS2000. I would expect a pll demodulator. Not hard to do with today's ic's.

Reply to
Steve Wilson

A ring mixer can go down to DC on its low-frequency port. When the delay is 90 degrees, the output will be zero.

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-TV
Reply to
Tauno Voipio

Computing arctangents at John's bandwidth will be sweaty, and it's the way to detect angle modulation (FM/PM) on a SDR.

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-TV
Reply to
Tauno Voipio

John Larkin wrote

PLL Foster Seeley Ratio detector If you are really good at fast pulses (I think you claimed that) use a fixed length oneshot at zero crossings of the 500 MHz, and lowpass the pulses to 50 MHz. Disadvantage of tha tmethod is that any interrupts in the signal will cause huge spikes, so you need to describe your transmission path.

Reply to
<698839253X6D445TD

Doesn't help. You need a reference frequency which doesn't exist on a delay discriminator.

Reply to
Steve Wilson

The reference is in the delay.

You take two copies of the amplitude-limited input signal, delay one of them and mix together. The low-passed mixer output is zero on the ferquency where the line delay is 90 degrees.

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-TV
Reply to
Tauno Voipio

Never mind. At 500 Mhz, a 500 ps delay would provide the necessary phase shift for a quadrature demodulator.

Reply to
Steve Wilson

Yes. At 500 Mhz, a 500 ps delay would provide the necessary phase shift for a quadrature demodulator.

Reply to
Steve Wilson

Yes. At 500 Mhz, a 500 ps delay would provide the necessary phase shift for a quadrature demodulator.

Reply to
Steve Wilson

Never mind. At 500 Mhz, a 500 ps delay would provide the necessary phase shift for a quadrature demodulator.

Reply to
Steve Wilson

Nah. A 1/4 wave delay is much simpler and has wider bandwidth. At 500 MHz, a 500 ps delay will provide the necessary phase shift for a quadrature demodulator.

You need a fairly wide frequency modulation for reasonable demodulated signal amplitude. I'm trying to decide if that will produce significant distortion but don't have time to model it in LTspice.

Reply to
Steve Wilson

John Larkin wrote

An other way, using same one shot, is if you have say 1GHz, and a sweep from 0.9 to 1.1 GHz, mix it down with 800 MHz, lowpass 200 MHz (only want the diff, not the 1.9 GHz) Now you have a sweep from 100 MHz to 200 MHz. Trigger a fixed time oneshot on each zero crossing, lowpass

Reply to
<698839253X6D445TD

It's drifty and super noisy Its sensitivity is harder to predict It requires way more circuitry

For sufficiently permissive definitions of 'fine'. ;)

Cheers

Phil Hobbs

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Dr Philip C D Hobbs 
Principal Consultant 
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Reply to
Phil Hobbs

Easy for you to say!

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics
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John Larkin

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