On Tue, Aug 23, 2011 at 7:05 PM, Kok Chen <chen@mac.com> wrote:
>
> The Omni V and Omni VI were "direct FSK" rigs and the keying signal shifts an
> RF oscillator with a trim cap. This frequency keyed RF is done at an
> intermediate frequency and mixed to the operating frequency. Yes, there is
> also an image, but presumably very low level and no inside a ham band.
> Because of the fragile trim cap, you can often identify Omnis back then by
> how far off their shift frequency is when the trim cap value changes over
> time, or over temperature :-).
>
There is a chapter in my Hallicrafters HT-37 manual entitled "Notes on
RTTY Operation of the HT-37 Transmitter/Exciter." This transmitter
(and some others of its day) had a feedthrough cap going to the
cathode of the VFO tube. By diode switching a trimmer in and out of
the circuit you could shift the frequency by an amount determined by
the trimmer setting. I can't imagine using such a beast - between the
VFO drift and the temp/mechanical effects on the trimmer it would
likely require frequent adjustment.
> By the time of the FT-1000MP generation, Yaseu had changed to a more direct
> version of FSK, by keying the direct digital synthesizer chip, which is how
> many rigs from that generation do it. Many DDS chips come with a FSK and PSK
> pin, and you can find them in RF synthesized generators such as the Novatech
> 409.
>
When I redesigned the VFO for my Heathkit DX-60B
(http://k4dsp.homeip.net/~doug/HG-10D/HG-10D.html) I used a DDS chip
and made provision in the firmware for FSK. Unless somebody claims
otherwise I claim to be the only person to ever operate RTTY using a
DX-60B. One of my more dubious achievements.
However it's done, the new rigs are many times better than the rigs
from yesteryear when it comes to FSK.
73,
Doug K4DSP
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