Garey Barrell <[EMAIL PROTECTED]> made an utterance to the drakelist gang
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Pierre -
There probably isn't "one" good answer! On the surface, it seems that
crystals were/are less expensive than good filters. Early Drake filters
were only 4 pole, probably costing considerably less than mechanical
filters at the time, and had to be upgraded to 8 poles in the later
TR-4s and T-4XB/C to meet more stringent FCC signal purity requirements.
Two Filter scheme (Drake) -
1. Only one crystal required, but two "different" 9 MHz (TR-3/4) or
5.645 MHz (T-4any) filters.
2. Suppressed carrier frequency is the same, regardless of USB or LSB.
3. CW is obtained just by shifting the oscillator "somewhere" within
the passband of one of the filters, the shift resulting in the "CW
transceive" offset. Precise shift not critical, allowing simple
switched capacitor shift rather than another crystal.
4. Allows transceive with single patch cable between units.
One Filter scheme (Collins) -
1. Two crystals required, plus a third for CW offset (probably because
of low frequency), but only one 455 kHz filter.
2. PTO must also be shifted "exactly" the same amount as the carrier
oscillator to keep suppressed carrier frequency the same on USB or LSB.
3. Requires patching of carrier oscillator and PTO between T and R,
complicating switching between transceive and separate control.
Drake "phase-locked" the carrier oscillator (T) and 2nd Converter LO (R)
in the C Line because the two crystal oscillators didn't "track"
perfectly over temperature variation in the earlier models. (Color
coded crystals were an "interim" fix in the late A and B Lines.) This
provided stable transceive without the switching complication of the
Collins system.
Probably other considerations that I'm not thinking of! :-)
73, Garey - K4OAH
Atlanta
Drake C-Line Service Manual
<http://www.k4oah.com>
F3WT wrote:
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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