[RTTY] (no subject)

[Doug Hall]

On Tue, Aug 23, 2011 at 7:05 PM, Kok Chen <chen at 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