EMRFD Message Archive 461

Message Date From Subject
461 2007-02-12 05:15:32 neomag_magneo FSK in homemade transmitters
Hello group,

This is a subject I have not encountered too often in the publications
or in the web. Personally, since I am using the RTTY mode some 95% of
my time on the air, I have included this every tx design accomplished.
The procedure is quite straightforward and just slightly more
elaborate than in case of a simple cw gear. All my designs have a
separate carrier oscillator in which the crystal frequency is shifted
the 170 Hz as required. I am aware that in most modern transmitters
this is achieved with AFSK by using the balanced modulator and crystal
filter, but I have always been fascinated by the elegancy and
simplicity of true FSK. Moreover, the first two or three transmitters
did not have any SSB capability at all but were used in the RTTY mode
several years. Currently, I am using also a homemade TX setup which is
based on a SSB generator and a transverter for multiband use, but with
a separate FSK modulator.

Now the question: For the next design, I will include both the SSB and
FSK modules, as previously. In EMRFD I have seen a schematic (CW/SSB
TX, I think) in which the CW carrier is filtered with the existing SSB
filter, to remove the spurious signals. Is there any
advantage/problems when doing the same thing with FSK (170 Hz shift)
signal ? I would imagine that a normal 2 - 3 kHz wide SSB filter does
not distort the narrow signal too much, as long as it is centered
(aligned) with the passband. Any experience/comments on this ?

73 de Heikki (OH2LZI)