Can someone suggest a chip for a 2400 MSK modem (which is FSK at 2400
baud with the frequencies being 2400 and 1200 Hz). I'd like a simple
chip that is suitable for a radio application.


I'd like something simple, like the old (discontinued) MX614 chip.


Thanks,


Art

Mebart wrote:

Can someone suggest a chip for a 2400 MSK modem (which is FSK at 2400
baud with the frequencies being 2400 and 1200 Hz). I'd like a simple
chip that is suitable for a radio application.


I'd like something simple, like the old (discontinued) MX614 chip.


Thanks,


Art

Decoding MSK as FSK will give you way less than optimum bit error rate
vs. noise. Unless the MSK is coming in on an FM channel you should
consider decoding it as PSK.

DsPIC + code -- that's a single-chip solution, and in current production!

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

Decoding MSK as FSK will give you way less than optimum bit error rate
vs. noise. Unless the MSK is coming in on an FM channel you should
consider decoding it as PSK.


I did find a chip that does 2400 baud MSK. But, why should I use a PSK
method?

PSK transmits one frequency, but inverts the amplitude of the
modulation to represent a 1 (or 0).

PSK has an advantage in bandwidth, which has less noise due to the
narrower bandwidth. But, can a PSK modem actually decode MSK better?

The ap is amplitude modulated VHF and I want the best weak signal
perfirmance I can get. The AM carrier wastes some transmit power and
has a disadvantage over a single sideband supressed carrier. But, it
is still many db better than a very wasteful fm mode.

Anyway, I can't control the transmitters, so I'm stuck with AM.

Having the carrier received with the sidebands establishes the center
frequency, so the tuning in the receiver is not quite so critical. AM
is really quite practical in this case.

Can you tell me some more about decoding the signal as though it was
PSK?

Thanks,

Art

Mebart wrote:

Decoding MSK as FSK will give you way less than optimum bit error rate
vs. noise. Unless the MSK is coming in on an FM channel you should
consider decoding it as PSK.



I did find a chip that does 2400 baud MSK. But, why should I use a PSK
method?

PSK transmits one frequency, but inverts the amplitude of the
modulation to represent a 1 (or 0).

PSK has an advantage in bandwidth, which has less noise due to the
narrower bandwidth. But, can a PSK modem actually decode MSK better?

The ap is amplitude modulated VHF and I want the best weak signal
perfirmance I can get. The AM carrier wastes some transmit power and
has a disadvantage over a single sideband supressed carrier. But, it
is still many db better than a very wasteful fm mode.

Anyway, I can't control the transmitters, so I'm stuck with AM.

Having the carrier received with the sidebands establishes the center
frequency, so the tuning in the receiver is not quite so critical. AM
is really quite practical in this case.

Can you tell me some more about decoding the signal as though it was
PSK?

Thanks,

Art

It acts as offset quadrature PSK with the bits weighted to be 1/2
cosines instead of rectangular pulses. You decode it just the way I said.

If you can find them the following two papers are quite useful. One of
them (the one from the IEEE Comm. Soc. IIRC) is easy enough that you can
just about design a demodulator straight from the paper.

[1] R. De Buda: "Coherent Demodulation of Frequency Shift Keying with
Low Deviation Ratio" -- IEEE Transactions, 1972, COM-20, pp. 429-435.

[2] S. Pasupathy: "Minimum Shift Keying: A Spectrally Efficient
Modulation" -- IEEE Communications Society Magazine, July 1979, Vol 17,
#4, pp 14-22.

And if you live in Worcester, MA, and you're wondering why I know:

[3] Tim Wescott: "A DGPS Radiobeacon Receiver for Minimum Shift Keying
with Soft Decision Capabilities" -- Master's Thesis, Worcester
Polytechnic Institute, 1990.

Stop by the library there -- I believe they'll have a copy.

Here's a summary of the reasoning (taken from Chapter 3 of [3] -- I like
the author). This is much better with pictures (the clearer paper of
[1] and [2] uses pictures, as does [3]):

The way that MSK works is to have FSK signal with a frequency shift that
is exactly 1/2 the baud rate. Thus the phase shift from one bit time to
the next is 90 degrees (1/4 of a circle).

Now consider a quadrature, offset PSK with 1/2 cosine weighting (really,
do). Thus even-numbered bits are transmitted as either positive-going
or negative-going cosine waves multiplied by the carrier, each one
lasting for exactly 1/2 cycle of the cosine, and the cosine having a
frequency of (bit rate/4). Odd-numbered bits are transmitted as either
positive-going or negative going _sine_ waves multiplied by the
quadrature of the carrier.

Take your bitstream, and make two signals a_i(t) and a_q(t). The signal
a_i(t) is +1 if it's corresponding bit is 1, -1 if the bit is 0, and it
only changes on boundaries of t = 2*k*T where T is the baud rate and k
is the bit number. The signal a_q(t) is similar, but it changes on
boundaries of t = (2*k+1)*T.

Then the transmitted signal is equal to

s(t) = a_i(t)*cos(pi*t/(2*T))*cos(2*pi*f*t) +
a_q(t)*sin(pi*t/(2*T))*sin(2*pi*f*t) (1)

where f is the carrier frequency (1200Hz in your case).

If you grind through those high school trig identities (1) becomes:

s(t) = cos(2*pi*f*t + b(t)*pi*t/(2*T) + phi(t)) (2)

where at any given moment b(t) = a_i(t) * a_q(t) and phi(t) = (a_i(t) +
a_q(t))*pi/2. Assuming I haven't messed up my expression for phi(t)
(the value I gave for it in my thesis is clearly a typo, I'm guessing
here) this is a phase continuous FSK signal who's frequency is the
carrier + 1/4 the baud rate for two consecutive 1's or two consecutive
0's, and carrier - 1/4 baud for a 1 followed by a 0 or visa-versa.

That's what MSK _is_, from a OQPSK sense, there's more about
demodulating it -- let me know if you can't find the articles, or
anything sensible on the web -- I'm thinking of scanning this stuff in...

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

OK, I can see it working better in principle, but, without software
writing, how do I do it???

I've never seen a PSK modem chip. I had a tough time finding an MSK
chip! It seems that software is the only presently accepted way to
create a psk decoder. I have googled for PSK MODEM, and found mentions
of it, but I cannot find a hardware solution.

Do you know of a chip that does it (public domain)?

Thanks.

Art

Mebart wrote:

OK, I can see it working better in principle, but, without software
writing, how do I do it???

I've never seen a PSK modem chip. I had a tough time finding an MSK
chip! It seems that software is the only presently accepted way to
create a psk decoder. I have googled for PSK MODEM, and found mentions
of it, but I cannot find a hardware solution.

Do you know of a chip that does it (public domain)?

Thanks.

Art



Without software, no. When I built the radio for my thesis I started
with an analog demodulator which occupied a 4.5x6 inch prototyping board
-- I think it had 6-12 chips on it, and it had problems with thermal
stability which is why the final version of the radio used a processor
for the demodulation.

I'm eyeing my thesis, and my brand-new scanner. I keep thinking "36
pages? NO!". If I get the spare time I'll scan this in but right now I
have a shortage on both time and round tuits.

You indicated on another one of your postings that you had found a
suitable FSK chip -- this will work for you, but with far more noise
sensitivity. But: if you started with that chip you could get a working
device _now_; later on when you got all of the other bugs worked out you
could circle around to the detector and upgrade it to something better.

What's this for, by the way?

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

Hi Tim,

Hi Tim,

It's for an ACARS (aircraft packet) decoder, which is 2400 baud MSK.
The planes use an AM transmitter, so it should be quite good with
respect to signal to noise ratio (compared to FM). It will only have
to receive.

My hope is to have a single decoder chip, on a board with a small
stand alone board containing a PIC (to extract the useful information
from the raw data of the modem output) and a non-volatile RAM chip to
store the extracted data for later retrieval.

Since the signals I'm interested in copying are 'dx' signals, the
ability to decode a slightly weak(er) signal is important. If I can
get good copy with a 6 db poorer s/n ratio, I'd like to use the better
method (if possible).

The overall purpose is to monitor the ACARS frequencies at 131 Mhz to
determine when/if the 2 meter band is open past it's normal range.
Many of the ACARS packets contain longitude and latitude data from the
planes GPS. Hence, when I start hearing planes more than 300 miles
away, I can start listening and transmitting on 2 meters.

I spent the last couple of hours on google, looking for single chip
psk modems-they don't exist. I did find a 400 pin super computer chip
that has been programmed to function as a PSK modem, but it's much
more than I need, both in terms of $$ and complexity. There are some
Motorola DSP EVM modules that have been programmed for stand alone
PSK, but they are horribly expensive, obsolete and draw way more
current than I want to deal with.

There were a few 'rumors' of PIC implementations being developed, but
I can't imagine a PIC with it's very limited A/D converters as being
very useful for weak signal work.

I actually have a DSP EVM here (the original one used for the DSP-10
transceivers, an Analog Devices EZ-Kit Lite DSP module). It's in the
box, has never been used. I can't program the beast-I'm software
challenged. More info on the DSP-10 is at:

http://www.proaxis.com/~boblark/dsp10.htm

There is an ACARS software decoder that needs to have a dedicated
computer and soundcard-I'd like to have something more 'stand alone'
though, hence my interest in rolling my own modem).

Regards,

Art