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AM modulation index

Hi,

I'm a software engineer and not really comfortable with electronics. May be some of you could help me with the following problem.

A have an AM signal, the carrier is about 250 volt (ca 100 kHz) and the modulated (digital) block wave (ca 1 kHz) is about 1 mVolt. That is an AM index of about 1:250,000! I'd like to (under)sample it with an ADC and then demodulate it (Hilbert transform). Because of the enourmous difference in amplitudes it is not possible to do this in one go. So I'm looking for an analogue way to enhance the modulation index before digitizing the signal.

Does anyone of you out there have a suggestion how to improve the modulation index of an AM signal?

Rutger

Reply to
Rutger Stoots
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I think you could use a traditional AM peak detector, followed by a high-pass filter to reject the 100VDC. You'd still have considerable ripple at 100kHz which you'd need to reject. You _may_ be able to deal with this by sampling the result at the peaks of the 100kHz wave.

I think you're going to find that you'll be generating way more than 1mV of noise with any demodulation scheme you cook up -- be prepared for a long, hard trip from where you are to where you need to be.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

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Reply to
Tim Wescott

do you have an unmodulated version of the carrier avaialble to you in the system?

you need a way to subtract or null out the carrier first

Mark

Reply to
Mark

No I don't, but I could create it, because I know the exact frequency, not the phase nor the (exact) amplitude.

Sorry, I posted this question at sci. electronics.basics also.

Rutger

Reply to
Rutger Stoots

--- Yes, that's a problem, but one that can be easily fixed by crossposting. That is, in your newsreader, in the area where you enter which newsgroup you want to post to, enter _all_ the newsgroups you want to post to.

That way everybody will read what everybody is posting from any of the crossposted newsgroups, eliminating having to go from newsgroup to newsgroup to follow the thread.

I've taken the liberty of crossposting this to what are probably appropriate newsgroups. You may want to do the same thing with your other posts as they get answered.

-- John Fields Professional Circuit Designer

Reply to
John Fields

I think if your carier is that strong, you have both phase and frequency. Take the signal into a comparator to make it a square wave with fixed amplitude, feed this into a mixer as LO, feed the original signal into the HF port and get the demodulated signal from the output of the mixer.

If you reduce the signal amplitude to 2.5V, your modulation will be 10uV. So after the mixer, you'll have a DC voltage of a few volts, with a 10uV modulation on top of it. Feed that through a bandpass filter and amplify the result.

Meindert

Reply to
Meindert Sprang

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