Please help dumb digital guy w/ FSK - RF Cafe Forums
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Post subject: Please help dumb digital guy w/ FSK Posted:
Sun Jan 14, 2007 10:24 pm
Joined: Sun Jan
14, 2007 9:52 pm
I am trying to make the receiver portion
of a Universal Access Transceiver (UAT). This is an FSK modulated data-link
for aircraft. The specifics are as follows:
Frequency 978 MHz,
+/- 20 PPM
Delta f/2 +312.5 kHz
-Delta f/2 -312.5 kHz
Type Continuous Phase FSK
Modulation Rate 1.041667 MBPS
I know a lot of RF guys and all of them say “you can
probably get a chip that does all that”. Or another quote is “They make
a chip that does that”. Who is “They”? I have found several FSK/ASK
chips that work up to about 920 MHz. I think these are designed for
Keyless Entry, Car Alarms, Remote Controls and that sort of thing. It
looks to me like I will have to make a circuit out of discrete components.
I don’t think this is a big deal but I am not sure how to get started.
I think I can handle the gain and filtering portion and I can for sure
handle the bit stream once it gets turned into ones and zeros, but I
need help with the demodulation. Does anyone have any specific suggestions
on how to build this type of circuit? Or can someone point me to a book?
I am not too interested in communications theory as I am building a
radio to a standard. I would really like a book with circuit ideas.
From the information I have been able to figure out on the web I
have two ideas how to do the demodulation. I am guessing I am way off
base so any constructive comments are welcome.
Filter, amplify, down convert, split then pass each leg through a filter/rectifier
and apply each leg to the input of a comparator.
(and this is my lame idea)
Filter, amplify, down convert, then square
up the signal w/ a comparator. Send the signal into a fast counter and
use a CPLD to keep reading the counter to determine the frequency.
Thank you in advance,
Post subject: FSKPosted: Mon Jan 15, 2007 1:47 am
Joined: Wed Feb 22, 2006 3:51 pm
you might be able to get a chip to do what you want to do, I suspect
the market isn't large enough for a specialized part.
be able to re-purpose chips for the cellular telephone market, but that's
not at all a sure bet. And "consumer" devices have a deservedly bad
reputation for being discontinued during the production life of industrial/commercial
equipment. Still, you might browse through Qualcomm's offerings.
All of which puts you squarely in the "do it yourself" category.
Generally, the "best" way to demodulate FSK is with a phase-locked
loop (PLL). For weak signals, this is best done by a receiver with the
Filter - amplifier -downconverter (mixer + oscillator)
- amplifier - pll - comparator.
Filter - amplify
- quadrature down-convert to DC, apply the two signals to an ADC, and
demodulate using DSP.
(This isn't as unthinkable as all that
- some of TI's TMS320C2000 series DSP's have multiple high-speed 10-bit
ADC's built in, and still don't cost much.)
The front-end filters
are critical. In most aircraft, there are several transmitters: voice,
transponder, radar, etc. The signal from those must be reduced to the
point where the FSK receiver low-noise amplifier doesn't overload. BUT,
any loss in that filter adds dB-for-dB to the noise figure: bad news.
Given that this isn't a unique system, some manufacturer of surface-acoustic-wave
(SAW) filters might actually have one as a stock item.
are widely available as IC's from companies like RF Micro Devices, Mini-Circuits
Labs, and others; and mixers/IF amplifiers are not hard to get. (Motorola
used to make some nice ones - but they were discontinued when Mot spun
out On Semi, then Freescale Semi.
You might even get a 74HC4046
to do the PLL job (ouch - I'm showing my age! Pay no attention to that
man behind the curtain...)
Bear in mind that in the 900 MHz region,
"breadboarding" is an art requiring minimal lead length, a good ground
plane, and just plain luck, if you don't have experience. Many people
just lay out a pcb, and plan on doing at least one revision.
By the way, the methods you mention are classics: the first one is generally
referred to as a "discriminator" in the literature - of which there's
a lot before PLL's took over. The second one is also a standard - implemented
in analog (yes, an "analog counter"!), it's used in high-end audio.
So your intuition isn't bad.
Me, if I were going to do the project,
I'd take the DSP route - but that has its own challenges - particularly
Post subject: Posted: Tue Jan 16, 2007 1:50 am
Joined: Sun Jan 14, 2007 9:52 pm
Thank you for the input. What a great reply! Great
news, I like the "do it yourself" category the best. I have about a
million more questions for you. I wish I worked with you.
downloaded a data sheet for the 74HC4046. I will read it and try to
figure it out.
I though about trying to use a DSP but I thinking
that with a bit rate of over 1 MBPS. I would have to sample at over
two million samples per second. Is this correct? I am very comfortable
writing low level micro code so if can be done this way that would be
Also I would like to thank you for you advice on the SAW
filter. I will look into it.
Post subject: FSKPosted: Tue Jan 16,
2007 3:09 am
Joined: Wed Feb 22, 2006 3:51
Well, maybe a TI '6000 series DSP would
If your speed were lower, Analog Devices has a transceiver
chip - but it only goes up to 384 kbits/second - not enough. (ADF7025,
I think - but you should check their website rather than depend on my
ability to remember part numbers accurately...)
What kind of
design constraints do you have? Size, power, cost? If cost, is it "cost-per-unit"
or "cost-to-first-unit"? (That is, parts cost or development cost).
"You can have it good, fast, and cheap - pick 2..."
try to write more later - but it's already too late.
Post subject: Posted: Tue Jan
16, 2007 12:14 pm
Joined: Sun Jan 14, 2007
My target is “good and cheap”. I am paying for
development out of pocket so I would like to keep that cost down although
I realize nothing is free, especially good tools. And I would like to
keep "cost-per-unit" down too so that I can keep the price down. Avionics
are relatively expensive so I think I would like to keep parts down
in the $100 or $200 range. The good news is I do have time.