Astro Modulation Question
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Astro Modulation Question
I have an academic question for the group. What is the difference between c4fm and cqpsk. Can someone give me a thumbnail description of the two modulation schemes? Any information would be appreciated.
OK, here you go:
In both modes, the signal is converted into "symbols" of 2 bits per symbol - hence there are 4 possible symbols. 4800 of these symbols are transmitted each second - hence APCO25 is a 4800 baud system (NOT 9600 BAUD!).
In C4FM, the symbol selects an instantanious FM deviation of either -1800 Hz, -600 Hz, +600 Hz, or +1800 Hz from nominal carrier frequency. That signal is routed through a shaping filter to limit the frequency content, and then used to modulate a standard FM transmitter. As a result, the signal has a total bandwidth of roughly 12.5 kHz, and constant amplitude - in other words, pure FM, no AM component.
In CQPSK, the symbol selects a phase shift of the carrier by -135, -45, 45, or +135 degrees. This results in the carrier being both frequency modulated as well as amplitude modulated. However, by allowing the signal to be AM as well as FM modulated, the occupied bandwidth is reduced to 6.25 kHz.
Due to the selection of the carrier phase shift, a CQPSK signal "looks like" a C4FM signal at symbol time - in other words, if you take the same symbol stream and generate a C4FM and a CQPSK signal, the two signals will look identical once every 1/4800 second - at the time you sample the signal to see what the signal is. At that time, the carrier will be at -1800, -600, 600, or1800 Hz instantanious deviation from nominal center. Thus, a properly designed demodulator will demodulate both a C4FM and CQPSK signal.
The C4FM signal, because of the lack of AM, can be generated with non-linear class C amplifiers, which are very efficient. The CQPSK signal requires the RF amplifiers to be very linear (class A, AB, or B) to prevent compression of the AM information.
In C4FM, the symbol selects an instantanious FM deviation of either -1800 Hz, -600 Hz, +600 Hz, or +1800 Hz from nominal carrier frequency. That signal is routed through a shaping filter to limit the frequency content, and then used to modulate a standard FM transmitter. As a result, the signal has a total bandwidth of roughly 12.5 kHz, and constant amplitude - in other words, pure FM, no AM component.
In CQPSK, the symbol selects a phase shift of the carrier by -135, -45, 45, or +135 degrees. This results in the carrier being both frequency modulated as well as amplitude modulated. However, by allowing the signal to be AM as well as FM modulated, the occupied bandwidth is reduced to 6.25 kHz.
Due to the selection of the carrier phase shift, a CQPSK signal "looks like" a C4FM signal at symbol time - in other words, if you take the same symbol stream and generate a C4FM and a CQPSK signal, the two signals will look identical once every 1/4800 second - at the time you sample the signal to see what the signal is. At that time, the carrier will be at -1800, -600, 600, or1800 Hz instantanious deviation from nominal center. Thus, a properly designed demodulator will demodulate both a C4FM and CQPSK signal.
The C4FM signal, because of the lack of AM, can be generated with non-linear class C amplifiers, which are very efficient. The CQPSK signal requires the RF amplifiers to be very linear (class A, AB, or B) to prevent compression of the AM information.
This is my opinion, not Aeroflex's.
I WILL NOT give you proprietary information. I make too much money to jeopardize my job.
I AM NOT the Service department: You want official info, manuals, service info, parts, calibration, etc., contact Aeroflex directly, please.
I WILL NOT give you proprietary information. I make too much money to jeopardize my job.
I AM NOT the Service department: You want official info, manuals, service info, parts, calibration, etc., contact Aeroflex directly, please.
Wowbagger,
Excellent description.
Obviously the filter characteristics are the key to the bandwidth.
Now - how about LSM? Would that be sort of a wider CQPSK?
In other words with different filtering the bandwidth could be 12.5 KHz but by using CQPSK modulation [hence the 'Linear'] the symbol width would be greater than 12.5 B.W. C4FM, thereby increasing the tolerance to intersymbol interference and thus allowing greater site separation in a simulcast environment?
Excellent description.
Obviously the filter characteristics are the key to the bandwidth.
Now - how about LSM? Would that be sort of a wider CQPSK?
In other words with different filtering the bandwidth could be 12.5 KHz but by using CQPSK modulation [hence the 'Linear'] the symbol width would be greater than 12.5 B.W. C4FM, thereby increasing the tolerance to intersymbol interference and thus allowing greater site separation in a simulcast environment?
LSM vs. Phase 2
You are correct - the only difference between LSM and Phase 2 CQPSK is the filtering. I'm not at liberty to divulge the exact difference, but from a bandwidth perspective there really isn't much difference - as in, "I cannot see any difference either in the frequency domain or in the I/Q domain".
Both signals are about 6.25 kHz wide.
We've scratched our heads over the reasoning for the difference ourselves here, but.... That's what they want, that's what they get.
And I'd HOPE I know the difference, having written both a data slicer and modulator for both forms!
Both signals are about 6.25 kHz wide.
We've scratched our heads over the reasoning for the difference ourselves here, but.... That's what they want, that's what they get.
And I'd HOPE I know the difference, having written both a data slicer and modulator for both forms!
This is my opinion, not Aeroflex's.
I WILL NOT give you proprietary information. I make too much money to jeopardize my job.
I AM NOT the Service department: You want official info, manuals, service info, parts, calibration, etc., contact Aeroflex directly, please.
I WILL NOT give you proprietary information. I make too much money to jeopardize my job.
I AM NOT the Service department: You want official info, manuals, service info, parts, calibration, etc., contact Aeroflex directly, please.
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ASTROMODAT
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Astro modem
It does not.
It is that simple.
An Astro modem operates at 9600 BITS PER SECOND.
BITS PER SECOND != BAUD, no matter what some people think.
A baud is a symbol per second. A symbol can be a bit, many bits, or even a fractional bit.
The confusion started long ago, with 300 baud modems. They sent one bit per symbol, so the baud rate was equal to the bit rate.
Joe Computer user got used to the idea that 300 baud == 300 bits per second, and so he thought that baud meant "bits per second".
The the 1200 bit per second modems came out. They were technically 600 baud, with 2 bits per symbol. And if you recall, they were labeled "1200 bps", not "1200 baud". In fact, go look at any major online computer hardware site at the modems. Look at how they are spec'ed, even now. You will NOT find "baud" on the page - only bps. If CompUSA said they had a "56kbaud" modem they would be nailed for false advertising.
However, "bits per second" is three words and three syllables. "Baud" is one. Laziness caused Joe Computer User to say "I gots me one o' dem dere 1200 baud modums". And so the misapplication of the term stuck.
Now, there is a furthur possilble confusion: your RS-232 port operates at 1 bit per symbol, so for it, baud == bits per second. So, if you are talking to a 56kbps modem (8000 baud, 8 bits/symbol, with some bits "robbed" for in-band signaling), you need to set your RS-232 port for at least 56kbaud to have your serial port feeding the modem at 56 kbps.
Saying "APCO-25 is a 9600 baud system" is just as WRONG as saying "I hooked one of them thare linears up to my 2 meter rig" when the amp in question is a class C FM only amp.
I get really picky about this because we hear a lot of customers who don't have a CLUE what they really have installed:
US: "OK, we need to figure out what options you need. What kind of system do you have?"
Them: "It's a Smartnet system."
US: "OK, you need the Smartnet option."
<time passes>
Them: "This piece of $#!7 won't connect - my radios won't go into service."
Us: "You SURE you have a Smartnet system?"
Them: "OF COURSE I'M SURE! I'M NOT AN IDIOT! Motorola said it was a Smartnet 9600 baud system!"
Us: "<groan> OK, what speed is the control channel?"
Them: "9600 baud."
Us: "OK. You don't have a Smartnet system. You have an APCO-25 system. You need the APCO-25 option."
Them: "But it's NOT APCO-25. It's Smartnet."
Us: "We'll send somebody out."
<time passes>
Us: "OK. I just installed the APCO-25 trunking option. Watch...."
2975: <in APCO-25 control channel mode>
Radio: <connects>
Us: "You have an APCO-25 system. See?"
Them: "But we were TOLD it was a Smartnet system!"
It is that simple.
An Astro modem operates at 9600 BITS PER SECOND.
BITS PER SECOND != BAUD, no matter what some people think.
A baud is a symbol per second. A symbol can be a bit, many bits, or even a fractional bit.
The confusion started long ago, with 300 baud modems. They sent one bit per symbol, so the baud rate was equal to the bit rate.
Joe Computer user got used to the idea that 300 baud == 300 bits per second, and so he thought that baud meant "bits per second".
The the 1200 bit per second modems came out. They were technically 600 baud, with 2 bits per symbol. And if you recall, they were labeled "1200 bps", not "1200 baud". In fact, go look at any major online computer hardware site at the modems. Look at how they are spec'ed, even now. You will NOT find "baud" on the page - only bps. If CompUSA said they had a "56kbaud" modem they would be nailed for false advertising.
However, "bits per second" is three words and three syllables. "Baud" is one. Laziness caused Joe Computer User to say "I gots me one o' dem dere 1200 baud modums". And so the misapplication of the term stuck.
Now, there is a furthur possilble confusion: your RS-232 port operates at 1 bit per symbol, so for it, baud == bits per second. So, if you are talking to a 56kbps modem (8000 baud, 8 bits/symbol, with some bits "robbed" for in-band signaling), you need to set your RS-232 port for at least 56kbaud to have your serial port feeding the modem at 56 kbps.
Saying "APCO-25 is a 9600 baud system" is just as WRONG as saying "I hooked one of them thare linears up to my 2 meter rig" when the amp in question is a class C FM only amp.
I get really picky about this because we hear a lot of customers who don't have a CLUE what they really have installed:
US: "OK, we need to figure out what options you need. What kind of system do you have?"
Them: "It's a Smartnet system."
US: "OK, you need the Smartnet option."
<time passes>
Them: "This piece of $#!7 won't connect - my radios won't go into service."
Us: "You SURE you have a Smartnet system?"
Them: "OF COURSE I'M SURE! I'M NOT AN IDIOT! Motorola said it was a Smartnet 9600 baud system!"
Us: "<groan> OK, what speed is the control channel?"
Them: "9600 baud."
Us: "OK. You don't have a Smartnet system. You have an APCO-25 system. You need the APCO-25 option."
Them: "But it's NOT APCO-25. It's Smartnet."
Us: "We'll send somebody out."
<time passes>
Us: "OK. I just installed the APCO-25 trunking option. Watch...."
2975: <in APCO-25 control channel mode>
Radio: <connects>
Us: "You have an APCO-25 system. See?"
Them: "But we were TOLD it was a Smartnet system!"
This is my opinion, not Aeroflex's.
I WILL NOT give you proprietary information. I make too much money to jeopardize my job.
I AM NOT the Service department: You want official info, manuals, service info, parts, calibration, etc., contact Aeroflex directly, please.
I WILL NOT give you proprietary information. I make too much money to jeopardize my job.
I AM NOT the Service department: You want official info, manuals, service info, parts, calibration, etc., contact Aeroflex directly, please.
That's what I'd like to know - see some of my other posts.CAPTLPOL wrote:My guess is the real question is why won't the PRO-96 decode the CQPSK audio?
This is my opinion, not Aeroflex's.
I WILL NOT give you proprietary information. I make too much money to jeopardize my job.
I AM NOT the Service department: You want official info, manuals, service info, parts, calibration, etc., contact Aeroflex directly, please.
I WILL NOT give you proprietary information. I make too much money to jeopardize my job.
I AM NOT the Service department: You want official info, manuals, service info, parts, calibration, etc., contact Aeroflex directly, please.