Building a Transceiver, more thoughts

recently ran into a few videos on YouTube featuring the Collins KWM-380, and the Heathkit SB-104A transceivers.  Both of these radios are contemporary, the years in which they were made overlap.  However there is QUITE a HUGE difference in the price of these rigs, the Heathkit being nearly an order of magnitude less expensive than the Collins.

Both rigs have quite simple front panels with few controls (compared to today’s rigs!).  Both have an analog S-Meter that also monitors other parameters during transmit, and seven segment frequency displays without an analog dial.

The SB-102 reads out the operating frequency to the nearest 100 hz by use of a frequency counter.  It computes the on the air frequency from the sum and difference of the crystal HFO frequency in the first conversion, the second conversion VFO frequency, and the carrier oscillator-bfo frequencies.  The displays use neon plasma type devices.  It is not easily possible to add WARC band coverage to the SB-104 (it might be possible to give up some of the 10 meter band positions and make some major modifications to add different crystals and band pass filters on these positions.  The 12 meter band would be the easiest to add, and perhaps the 15mhz WWV position could be modified to work on 17 meters.)

The KWM-380 shows its operating frequency to the nearest 10 hz on red 0.56″ seven segment  LED displays.  A PLL frequency synthesizer is used in the first conversion, and digital logic (micro-processor) is used to control the PLL and displays.  The KWM-380 did not have a band switch, this was done by using a tuning rate of 1 mhz / step.  As the KWM-380 was a feature reduced version of government radio, it was possible for Collins to add transmit ability on the WARC bands to the radio via a firmware update.  Owners of these radios were able to also add the 60 meter band many years later!

The Heathkit transceiver uses much the same conversion scheme as the earlier members of the SB10x family, a variable first IF centered on 8.5 mhz, and a second IF at 3.3 mhz.  Two different crystal filters in the second IF provide bandwidths of 2.1khz, or 400khz.

The KWM-380 up-converts to a first IF of 39mhz, and then down to a second IF of 455khz.  A third conversion is made to 6.255 mhz., and a final conversion back to 455khz.  Several 6.255 mhz crystal filters with different bandwidths may be selected.  The purpose of the final two conversions is to move the bandwidth of the 6.255 mhz filter in reference to the the carrier oscillator frequency.  This allows for removing the influence of a close by signal, at the expense of an non optimal placement of the bfo signal.   It should be pointed out that many of the later Japanese radios offered this ‘passband’ tuning feature, along with a notch filter AND a variable bandwidth feature.

I’m giving some though to some kind of ‘old school’ approach to a transceiver design that would look something like a cross between these two rigs, at least from the front panel.  I like the idea of large LED’s for the frequency display (either 0.6″ or 0.8″ in size, I have some of both).  I’d use an AD995x series DDS instead of a VFO or PLL, which would give better performance than either the Heath LMO or the Collins synthesizer.  I have some 9mhz crystal filters and also some 455khz Collins filters in the parts bin, so a variable bandwidth IF setup is planned for.  I don’t know if I’d use a 45mhz first IF (which would allow for general coverage on receive) or use the 9mhz IF as the first conversion.  Both could be included and switched in as desired.  I have two different cabinets I could use, a large (almost relay rack sized) one, and a smaller box (that’s still a bit larger than many current Japanese transceivers).  I could fit the rig, speaker, and power supply in the larger box, but I’m partial to the smaller one.




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