[aprssig] AFSK vs PSK

Stephen H. Smith wa8lmf2 at aol.com
Sun Nov 24 17:22:21 EST 2013

On 11/24/2013 1:36 PM, Andrew Rich wrote:
> Very interesting
> That might be why
> Balloon projects use FSK
> Where the carrier Freq is shifted between two distinct F1 and F2
> Reception is done on SSB ? correct me if I am wrong.

It can be.  However, every hz error in the RF frequency (either at the 
transmitter or at the receiver) on SSB translates into a hertz error in the 
recovered audio tones.  At HF it's practical to keep SSB frequencies within 
5-10 Hz or better long-term.

At VHF and especially UHF, it's difficult unless you are using some sort of 
exotic frequency control such as a rubidium frequency standard or some such.

Typically direct FSK at VHF/UHF is received in an FM receiver with what amounts 
to a tracking voltage comparator connected to the FM discriminator.   If the 
receiver is off-frequency or drifts, the comparator will continue to "straddle" 
the center between the two RF frequencies involved.

Or you directly demodulate with a DSP receiver system.

> Is this what they cal true RTTY ?

Classic 5-bit Baudot RTTY uses this kind of two-frequency FSK, but so does HF 
packet, AMTOR, Pactor I, etc.   In the beginning (early 1950s), RTTY used a 850 
Hz shift between the mark and space (1 and 0) frequencies.   Today, all these 
modes use a shift of between 170-200 Hz.  (Classic RTTY uses the 170 Hz shift, 
while all the newer modes use the 200 Hz shift.)

> Like CW but jump high and low

Exactly.   Some rigs actually have a logic level 0/5 volt "FSK input" for 
direct FSK, that will pull a key-down CW carrier side to side 170-200 Hz.

In the early days, a diode switch controlled by the data stream would 
connect/disconnect a small additional fixed capacitor across the main variable 
capacitor of the transmitter VFO, to "pull" the frequency 170 Hz.

Today, this is commonly done by using a varactor diode, biased by the data 
stream, to shift the frequency of a synthesizer's master crystal oscillator 
slightly.  Or if the synthesizer can settle fast enough, by actually commanding 
the synthesizer to jump between two frequency steps 200 Hz apart at the data 

On other rigs, you feed an AUDIO tone that shifts side to side 200 Hz into the 
rig's mic jack on SSB, which creates (if the carrier and opposite sideband 
suppression is good) exactly the same effect. Typically the audio tone pair 
used is either 2100/2300 Hz or 1600/1800 Hz.


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