"Robert Donnell" <[EMAIL PROTECTED]> made an utterance to the drakelist gang
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I feel like the two-filters in the transceivers was a continuation of the theme
of the 2A/2B receivers, where changing sidebands was actually moving the filter
response, with respect to the BFO.
You've all probably seen some of the rigs from other makers, with a single
filter and analog VFO. When you changed sidebands, you had to either use a
different tuning mark, or recalibrate against the 100 kHz crystal, to account
for the carrier being shifted about 3 kHz between upper sideband and lower
sideband. With Drake rigs this was not necessary, since the carrier oscillator
stayed put. The first Drake transceiver I owned was a TR-3, and I felt like
the dual filters gave the radio an upscale flavor, in addition to the potential
for improved frequency stability in the local oscillator, because the only time
it changed frequency was when it was shifted into the passband for CW
transmission. Shifting the carrier frequency instead of using (and switching)
a second crystal was undoubtedly a cost saving measure, probably to offset the
cost of a second filter, and perhaps to save from having to build potentially
less-reliable or less-stable real-time crystal switching.
Also, shifting the VFO, to compensate for shifting the carrier oscillator,
would have been virtually impossible to make track over the full range of the
VFO, without having a second slug-tuned inductor to either swap between, for
sideband changes, or to offset the VFO the proper amount at all points in the
tuning range, when the alternate sideband was selected. That sounds like my
idea of a technician's nightmare. I'm betting just getting one slug-tuned
inductor to track a 12% frequency range change with linear frequency change vs.
slug insertion was not trivial to learn how to do.
In any event, I thought the dual-filter arrangement more elegant than the
alternative.
73, Bob, KD7NM
---------- Original Message ----------------------------------
From: F3WT <[EMAIL PROTECTED]>
Date: Tue, 20 Dec 2005 22:26:05 +0100
>
>F3WT <[EMAIL PROTECTED]> made an utterance to the drakelist gang
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>Hi Garey,
>
>Very clear explanation below!
>
>I have a related question I was wondering about for a while:
>
>Why were TR4s - and others by he way - designed with 2 sideband
>filters and 1 Xtal ( 9MHz ) rather than todays set of 1
>sidebandfilter and 2 Xtals (SSB) or 3 including CW?
>Was it a problem then shifting VFO without loosing stability?
>73, Pierre - F3WT
>
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