wideband FM demodulator

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

SDR

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

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

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

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

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

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

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.

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

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.

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

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

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

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

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.

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

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

Easy for you to say!