FSK

[Gorgon]

The receiver simply uses a PLL to make the appropriate decision on what data was sent.

I know that, but what are the frequencies for the "Mark" and "space" tones?

300/300 Bd (All number in Hz)
Bell 103 calling: 1270(1)/1070(0) answer: 2225(1)/ 2025(0)

V21 calling: 980(1)/1180(0) answer: 1650(1)/1850(0)

75/1200Bd
Bell 202 Calling:387(1)/487(0) answer: 1200(1)/2200(0)

V23 Calling: 390(1)/450(0) answer: 1300(1)/2100(0)


This is taken form the K-series design manual from Silicon Systems.

TOK

Hi mstechca,
You're asking for frequencies and 'Mark' and 'Space' . In the listing above the frequency with a (1) is the 'Mark', and (0) is the 'Space'. I'm sorry I didn't spell it out

I think you'll find your two frequencies in the Bell 202 standard.

TOK

 

[mstechca]

Is the difference between FSK and AFSK just the way data is handled?

and what are the frequencies (in hertz, kilohertz, whatever) of the mark and space tones?

 

[lord loh.]

Is the difference between FSK and AFSK just the way data is handled?

and what are the frequencies (in hertz, kilohertz, whatever) of the mark and space tones?

FSK- Frequency shift keying is where frequency is shifted from mark to space to correspond to the incoming modulating data 1 and zero respectively.

ASK is Amplitude Shift keying and the amplitude is varied from -V to +V to correspond to the incoming modulating data 1 and zero respectively.

Technically you can use any frequency. But standards are probably specified for compatibility. If you do not want compatability, you may use any frequency. ie. if you are designing both the transmitter as well as the receiver.

 

[Nigel Goodwin]

Is the difference between FSK and AFSK just the way data is handled?

and what are the frequencies (in hertz, kilohertz, whatever) of the mark and space tones?

FSK is just a carrier, and you switch the carrier between two different frequencies. AFSK uses a single carrier frequency and you modulate it with one of two audio tones, one for mark and one for space - commonly used frequencies have been posted previously in this thread.

 

[mstechca]

FSK is just a carrier, and you switch the carrier between two different frequencies.

thats what I thought. and how do I determine these two different frequencies FSK uses? I can't just go random.

 

[upand_at_them]

Packet radio has a universal frequency of about 144Mhz, so what is it for FSK?

The standard tones for packet radio are 1200Hz and 2200Hz. Bandwidth determines what frequences can be used.

Mike

 

[Nigel Goodwin]

FSK is just a carrier, and you switch the carrier between two different frequencies.

thats what I thought. and how do I determine these two different frequencies FSK uses? I can't just go random.

The frequency shift used depends on a number of things, mainly the bandwidth available, and the data rate required. As someone commented eariler, it's mainly a question of matching the transmitter and receiver - for FSK it's EXTREMELY important that both are highly stable, you would normally use crystal control on both tx and rx.

 

[mstechca]

Let's say I'm looking for 9600bps.

FSK has two carrier frequencies. We will assign them as x Mhz and y Mhz.

I make the assumption that one carrier frequency represents "mark" and the other one, "space".

Do I transmit 2200hz on one carrier frequency and 1200hz on the other carrier frequency?

I can agree that bandwidth determines the second carrier frequency, but how do I determine the first one?

Is there a manual that lists FSK carrier frequencies?

 

[mstechca]

According to one post in google groups, one user stated to feed a DC voltage into the carrier frequency.

Does this mean that any signal fed into one carrier frequency declares that "mark" is being transmitted? and any signal fed fed into the other carrier frequency declares that a "space" is being transmitted?

"any signal" in this case means a high-pitched tone (data going at high speed, I think).

 

[audioguru]

FSK is two audio tones that modulate a single carrier, you can hear it. 1200Hz and 2200Hz are low frequencies that were used at 300 (up to 2400?) baud over low-bandwidth and noisy telephone lines. All kinds of improvements in modulation allows 56k over telephone lines and transmitters. They use data-compression, error-correction and phase modulation. They use a lot more than only two audio tones.

Search Google for modems.

 

[akg]

in fsk is the signal is decoded using pll or from filter o/p ?? i think both can be used and faster being filter type .right?

 

[audioguru]

It would be a tedious exercise to make a modem IC with discrete parts.

 

[mstechca]

Nigel says:

FSK is just a carrier, and you switch the carrier between two different frequencies. AFSK uses a single carrier frequency and you modulate it with one of two audio tones, one for mark and one for space - commonly used frequencies have been posted previously in this thread.

Audiogurusays:

FSK is two audio tones that modulate a single carrier, you can hear it. 1200Hz and 2200Hz are low frequencies that were used at 300 (up to 2400?) baud over low-bandwidth and noisy telephone lines. All kinds of improvements in modulation allows 56k over telephone lines and transmitters. They use data-compression, error-correction and phase modulation. They use a lot more than only two audio tones.

I'm very confused, because Nigel says with FSK, you switch the carrier between 2 frequencies. Audioguru says that FSK is equivalent to what Nigel said about AFSK (two tones modulating a single carrier).

Who is right?

 

[Nigel Goodwin]

Nigel says:

FSK is just a carrier, and you switch the carrier between two different frequencies. AFSK uses a single carrier frequency and you modulate it with one of two audio tones, one for mark and one for space - commonly used frequencies have been posted previously in this thread.

Audiogurusays:

FSK is two audio tones that modulate a single carrier, you can hear it. 1200Hz and 2200Hz are low frequencies that were used at 300 (up to 2400?) baud over low-bandwidth and noisy telephone lines. All kinds of improvements in modulation allows 56k over telephone lines and transmitters. They use data-compression, error-correction and phase modulation. They use a lot more than only two audio tones.

I'm very confused, because Nigel says with FSK, you switch the carrier between 2 frequencies. Audioguru says that FSK is equivalent to what Nigel said about AFSK (two tones modulating a single carrier).

Who is right?

A simple search soon finds relevent data:

Frequency-shift keying
From Wikipedia, the free encyclopedia.
Jump to: navigation, search
Frequency-shift keying (FSK) is a form of frequency modulation in which the modulating signal shifts the output frequency between predetermined values.

Usually, the instantaneous frequency is shifted between two discrete values termed the mark frequency and the space frequency. This is a noncoherent form of FSK.

Audio frequency-shift keying
From Wikipedia, the free encyclopedia.
Jump to: navigation, search
Audio frequency-shift keying (AFSK) is a modulation technique by which digital data is represented as changes in the frequency (pitch) of an audio tone, yielding an encoded signal suitable for transmission via radio or telephone. Normally, the transmitted audio alternates between two tones: one, the "mark", represents a binary one; the other, the "space", represents a binary zero.

AFSK differs from regular frequency-shift keying in that the modulation is performed at baseband frequencies. In radio applications, the AFSK-modulated signal is normally used to modulate an RF carrier (using a conventional technique, such as AM FM or ACSSB(R)(LM Mode(R)) for transmission.

AFSK is not generally used for high-speed data communications, as it is less efficient than other modulation modes. In addition to its simplicity, however, AFSK has the advantage that encoded signals will pass through AC-coupled links, including most equipment originally designed to carry music or speech.

Applications
Most early telephone-line modems used audio frequency-shift keying to send and receive data, up to rates of about 300 bits per second. The common Bell 103 modem used this technique, for example. Some early microcomputers used a specific form of AFSK modulation, the Kansas City standard, to store data on audio cassettes. AFSK is still widely used in amateur radio, as it allows data transmission through unmodified voiceband equipment.

I don't know if Audioguru is a licenced radio amateur?, but I was G8MMV, I've not been active for a few years now, but when I last was I used Packet Radio on 2m.

 

[audioguru]

I was never a "ham".

 

[mstechca]

audioguru, I suppose you go through 100's and 100's of books related to radio. I think I'll have to go with Nigel on this one, because I have looked at wikipedia before I checked here again, and if a licensed guy is showing me wikipedia, then I guess wikipedia wins.

 

[audioguru]

audioguru, I suppose you go through 100's and 100's of books related to radio.

I haven't read a single book about radio. I think the prof in university talked a little about radios. Many magazines I read in the library taught me stuff about radios.

Are you going to modulate a single RF carrier with AFSK audio tones, or are you going to somehow switch between two RF frequencis with FSK?

 

[akg]

Frequency-shift keying (FSK) is a form of frequency modulation in which the modulating signal shifts the output frequency between predetermined values.

so this becomes equivalent to an FM signal , where the carrier freq is changed WRT to i/p signal.

note:
ok..a google search confirms it ..

 

[audioguru]

so this becomes equivalent to an FM signal , where the carrier freq is changed WRT to i/p signal.

Good point!
FM is a very simple way to make FSK. But how would you detect it, it is DC modulation, isn't it? I guess you could scale down its frequencies with a counter then detect them with a PLL.

The problem is that MStechca's transmitter was an AM transmitter with a little FM on the side.
Now he can't afford a real varactor diode to allow it to produce true FM.
Also he is using an AM receiver.

 

[Nigel Goodwin]

FM is a very simple way to make FSK. But how would you detect it, it is DC modulation, isn't it? I guess you could scale down its frequencies with a counter then detect them with a PLL.

I mentioned earlier in the thread that FSK is generally used with crystal control in both transmitter and receiver, this makes it simple and reliable. A simple FM detector will output different voltages for the two carrier frequencies, and you can easily convert it to logic levels with a comparator.

Many licence free modules use FSK, but they use AC coupling, which prevents a continuous DC output from the receiver.