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AM modulation index

R

Rutger Stoots

Jan 1, 1970
0
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
 
T

Tim Wescott

Jan 1, 1970
0
Rutger said:
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
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

Posting from Google? See http://cfaj.freeshell.org/google/
 
M

Mark

Jan 1, 1970
0
Rutger said:
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

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
 
R

Rutger Stoots

Jan 1, 1970
0
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

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
 
J

John Fields

Jan 1, 1970
0
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.

---
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.
 
J

John Fields

Jan 1, 1970
0
---
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.
 
M

Meindert Sprang

Jan 1, 1970
0
Rutger Stoots said:
No I don't, but I could create it, because I know the exact frequency, not
the phase nor the (exact) amplitude.

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
 
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