AM modulation index

[Rutger Stoots]

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

 

[Phil Allison]

"Rutger Stoots"

I'm a software engineer and not really comfortable with electronics.

** That, at least, is bloody obvious.

Have no idea what the word " basics " means either ??

Try you wacky question on " alt.electronics.trolls" anytime.




........ Phil

 

[Rutger Stoots]

I didn't say I was uncomfortable with English, nor did I attempt to be
impolite.

 

[DecaturTxCowboy]

I didn't say I was uncomfortable with English, nor did I attempt to be
impolite.

Certainly not being impolite in my response, but reminds me years ago
when I worked with a software engineer. We both had respective signs on
our office doors.

"Take your silly assed hardware questions next door -->"

"<--Take your silly assed software questions next door"

 

[Anthony Fremont]

I didn't say I was uncomfortable with English, nor did I attempt to be
impolite.

Just ignore the philth, he/she/it's not very sociable, maybe sociopathic
but not sociable.

BTW, top posting is kinda frowned upon in most cases since it's
considered rude. Not in this case, but in most cases. ;-)

 

[John Larkin]

I didn't say I was uncomfortable with English, nor did I attempt to be
impolite.

Ignore Phil. He's angry with everybody.

Wouldn't a narrowband notch filter help? I mean, to supress most of
the carrier. Inside a wider filter that lets the modulation through,
of course.

The equivalent to a notch would be a PLL that generates a local copy
of the carrier that's then subtracted from the signal. This wouldn't
be easy, but then you don't have an easy problem, maybe not a
possible-to-solve problem.

Or, as others have suggested, just envelope detect it and work from
there.

What's the context? Where's this signal coming from, and what is it
used for?

John

 

[jgreimer]

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

How many bits is your ADC? That will determine how much of the carrier has
to be removed.

My suggestion would be to pull a small amount of signal from the carrier and
phase-lock an oscillator to it. The oscillator should have very low levels
of amplitude and phase noise. The phase of the oscillator should be
adjusted until it is 180 deg. out of phase with the carrier. If it is
amplified until its voltage approaches that of the carrier, summing the two
carriers will reduce the carrier to modulation ratio.

Rather than amplify the oscillator voltage to 250 V, it might be easier to
go through multiple stages of summing.

 

[Rutger Stoots]

John Fields, John Larkin and jgreimer (and others)

First of all, thanks for your moral support.


The circuit is for reading out an FDX transponder. The ADC is 16 bits to
keep it within my budget, not only for the ADC itself but also the chain of
digital hardware after it.

I thought of the PLL (and even a DLL) suggestion already, but it works out
that it is very difficult to do. Certainly for 250 Volts, I have to
attenuate that first, from there every little phase shift and pinch of noise
ruins all the nice work, it simple isn't good enough.

Maybe the notch will work, I'll try that one.

B.T.W.: There's one thing I'm sure of, I know the exact frequency of the
signal. The transponder is (faintly) modulating the carrier I'm transmitting
by sucking up, now and then, a teeny little bit of the EM wave I generate
myself. That gave me the idea of the DLL I mentioned. Does that ring any
bells at your end?

Thanks so far,
Rutger

 

[John Larkin]

John Fields, John Larkin and jgreimer (and others)

First of all, thanks for your moral support.


The circuit is for reading out an FDX transponder. The ADC is 16 bits to
keep it within my budget, not only for the ADC itself but also the chain of
digital hardware after it.

I thought of the PLL (and even a DLL) suggestion already, but it works out
that it is very difficult to do. Certainly for 250 Volts, I have to
attenuate that first, from there every little phase shift and pinch of noise
ruins all the nice work, it simple isn't good enough.

Maybe the notch will work, I'll try that one.

B.T.W.: There's one thing I'm sure of, I know the exact frequency of the
signal. The transponder is (faintly) modulating the carrier I'm transmitting
by sucking up, now and then, a teeny little bit of the EM wave I generate
myself. That gave me the idea of the DLL I mentioned. Does that ring any
bells at your end?

Thanks so far,
Rutger

You're generating the original 100 KHz carrier, and the transponder is
essentially selectively reflecting it? If that's the case, bandpass
the received signal (to improve gross s/n) and then sum it with a
tweaked copy of your prime signal source. Tweak the phase and
amplitude of the summed local signal to reduce most of the carrier in
the received thing; some simple feedback loops should work, maybe even
trimpots.

But the dynamic range and s/n situation still sounds nasty.

John

 

[Rutger Stoots]

You're generating the original 100 KHz carrier, and the transponder is
essentially selectively reflecting it? If that's the case, bandpass
the received signal (to improve gross s/n) and then sum it with a
tweaked copy of your prime signal source. Tweak the phase and
amplitude of the summed local signal to reduce most of the carrier in
the received thing; some simple feedback loops should work, maybe even
trimpots.

But the dynamic range and s/n situation still sounds nasty.

John

The transmitter coil is in essence just a loop antenna burried in the floor,
one end is grounded, the other end connected to the transmitter output. The
transponder is lying in this loop, detects the carrier and then, after
picking up some energy for it's electronics, short circuits it's own (very
small) coil in the rithm of it's unique (digital) ID, thus slightly
modulating the EM field. I'm measuring over the same transmitter coil, so
there is no separate receiving antenna. That's how the incoming signal
originates. That clarifies how I know the frequency exactly.

As stated, the modulation index is very small. If anything metal in the
neighbourhood moves, you get phase shifting and interfering AM (even a
little FM) modulation, although (hopefully) not of the same frequency as the
transponder's modulation. So there you are, it looks ghastly. That's why I
want to filter and demodulate the signal digitaly, but then again, before
going digital I have to boost up the transponder's AM index.

 

[Phil Allison]

"Rutger Stoots"

The transmitter coil is in essence just a loop antenna burried in the
floor,
one end is grounded, the other end connected to the transmitter output.
The
transponder is lying in this loop, detects the carrier and then, after
picking up some energy for it's electronics, short circuits it's own (very
small) coil in the rithm of it's unique (digital) ID, thus slightly
modulating the EM field. I'm measuring over the same transmitter coil, so
there is no separate receiving antenna. That's how the incoming signal
originates. That clarifies how I know the frequency exactly.

As stated, the modulation index is very small. If anything metal in the
neighbourhood moves, you get phase shifting and interfering AM (even a
little FM) modulation, although (hopefully) not of the same frequency as
the
transponder's modulation. So there you are, it looks ghastly. That's why I
want to filter and demodulate the signal digitaly, but then again, before
going digital I have to boost up the transponder's AM index.

** Why does it not surprise me, even one tiny bit, this Stoots dude is:

" a software engineer and not really comfortable with electronics " ????





........ Phil

 

[John Larkin]

The transmitter coil is in essence just a loop antenna burried in the floor,
one end is grounded, the other end connected to the transmitter output. The
transponder is lying in this loop, detects the carrier and then, after
picking up some energy for it's electronics, short circuits it's own (very
small) coil in the rithm of it's unique (digital) ID, thus slightly
modulating the EM field. I'm measuring over the same transmitter coil, so
there is no separate receiving antenna. That's how the incoming signal
originates. That clarifies how I know the frequency exactly.

As stated, the modulation index is very small. If anything metal in the
neighbourhood moves, you get phase shifting and interfering AM (even a
little FM) modulation, although (hopefully) not of the same frequency as the
transponder's modulation. So there you are, it looks ghastly. That's why I
want to filter and demodulate the signal digitaly, but then again, before
going digital I have to boost up the transponder's AM index.

Hell, you're describing a metal detector. There are pretty much three
kinds:

Oscillator, where the sensed object pulls the frequency of an
oscillator which uses the sense loop as its tank. Usually detected by
listening to the heterodyne.

Bridge: the loop is driven by an oscillator/amplifier and is part of
an AC bridge. The bridge is tuned for null, and a detected object
upsets the bridge balance.

2-coil. A driven coil blasts out a local field, and a separate coil is
the pickup. The pickup coil can be oriented at 90 degrees to the main
coil to reduce the constantly-received signal. A portion of the
transmit signal is tweaked in amplitude and phase and subtracted from
the signal from the receive coil, to null it out in the absence of
detected objects.


All of these can be made sensitive in the ballpark of
parts-per-million. The oscillator version is out for you, but a bridge
arrangement might work. A slow servo could null the bridge, and a
phase-sensitive detector could recover the faster data.

Still, there will be noise, including amplitude and phase noise in
your oscillator/amplifier itself, and the signal levels you're
considering are tiny.


Could you do the RFID thing, have the transponder passively double the
excitation frequency? Or just use the stored energy to respond on a
totally different frequency? That would change everything.

John

 

[John Larkin]

** Why does it not surprise me, even one tiny bit, this Stoots dude is:

" a software engineer and not really comfortable with electronics " ????



....... Phil

Too bad you're all insults and no ideas. But that's in character:
ideas are fun, and you never let yourself have fun.

John

 

[Jim Thompson]

Too bad you're all insults and no ideas. But that's in character:
ideas are fun, and you never let yourself have fun.

John

John, Ever hear the term "shunning"?

If EVERYONE would simply shun Phil, the problem will vanish.

But as long as you persist with the joust the rest of us have to put
up with the crap.

...Jim Thompson

 

[John Fields]

"Rutger Stoots"


** Why does it not surprise me, even one tiny bit, this Stoots dude is:

" a software engineer and not really comfortable with electronics " ????

---
Unless you're blind or illiterate, why should it surprise you?

He stated it right up front, posted to seb, described what he was
trying to do and asked for help without an attitude.

You, of course, with your nasty-assed misanthropy egging you on, had
to attack him from the very beginning instead of trying to help.

Why do you do that?

 

[John Larkin]

John, Ever hear the term "shunning"?

If EVERYONE would simply shun Phil, the problem will vanish.

But as long as you persist with the joust the rest of us have to put
up with the crap.

...Jim Thompson

Don't get snippy with me, young Jim.

I'm only increasing Phil's injected crap level incrementally. Ignoring
him will probably not help a lot, since he comes to newsgroups
specifically to distribute insults.

I wonder why.

John

 

[Jim Thompson]

Don't get snippy with me, young Jim.

I'm only increasing Phil's injected crap level incrementally. Ignoring
him will probably not help a lot, since he comes to newsgroups
specifically to distribute insults.

I wonder why.

John

You've never heard of "Usenet Filters" ??

The only time I see Phil's insults is when you reply to them.

I guess I just need to kill "AM modulation index", rather than reading
it to see if I can help ??

...Jim Thompson

 

[Rich Grise]

The transmitter coil is in essence just a loop antenna burried in the
floor, one end is grounded, the other end connected to the transmitter
output. The transponder is lying in this loop, detects the carrier and
then, after picking up some energy for it's electronics, short circuits
it's own (very small) coil in the rithm of it's unique (digital) ID,
thus slightly modulating the EM field. I'm measuring over the same
transmitter coil, so there is no separate receiving antenna. That's how
the incoming signal originates. That clarifies how I know the frequency
exactly.

Well, if the transponder stores up energy for its response, then just
pulse the transmitter long enough to charge it up, then listen for its
response at whatever frequency - it could even be the same frequency.

That's how they do radar and ultrasonic, you know.

Good Luck!
Rich

 

[Rich Grise]

Unless you're blind or illiterate, why should it surprise you?

He stated it right up front, posted to seb, described what he was trying
to do and asked for help without an attitude.

You, of course, with your nasty-assed misanthropy egging you on, had to
attack him from the very beginning instead of trying to help.

Why do you do that?

It's called "trolling", and he seems to be doing a pretty effective job
at it.

Thanks,
Rich

 

[Meindert Sprang]

Hell, you're describing a metal detector. There are pretty much three
kinds:

Nah, he's describing standard RFID technology. I wonder whether there aren't
any canned solutions around.

Meindert