Add-on BFO & TenTec kit questions

[TassieJay]

Hi all,
The idea of turning some SSB-less but otherwise decent receivers into something more useful via an external BFO has recently grabbed my attention.

I hear that many people have issues with the TenTec universal BFO kit because they find it necessary to retune the slug on the BFO everytime they change frequency on the radio. Why is that? I thought the TenTec BFO kit just oscillated at a *fixed* 455 kHz (well, a little bit tunable by the pot) and injected that into the last IF stage of the host receiver... surely no re-tuning would be needed?

A DIY circuit using a fixed 455 kHz ceramic resonator also looks interesting:
FlashWebHost.com - SSB ADAPTOR. Resolve SSB Signal on Broadcast Receiver.
With some simple mods, that ceramic resonator could be capacitively 'pulled' off frequency by use of a varicap diode to provide some tuning ability, in a similar way to the TenTec kit.

One last question - if using either of the above circuits and you wanted to install them permanently inside a host radio - assuming there's the room available to do that - what's the best way to mix the BFO into the last IF stage? One arrangement I've seen used a ring diode mixer or a S042P mixer IC, but could it just be injected into the last IF stage at some point and let any non-linearity of the detector do the mixing? Or is that too simplistic?

 

[nanZor]

I've used the Ten-Tec BFO kit and it works ok, but it works best in an enclosure, otherwise hand-capacitance tends to detune it making it hard to tune sometimes.

Also the injection level can be critical, and an internal connection can be made, rather than wrapping it around the antenna. This 455khz injection level could just be made variable with a resistor or some other circuit like you suggested.

I had good luck with a Grundig S450, but the stability with the simple rats-nest of exterior wiring and capacitive coupling made it impractical for long term use. By all means shield it, but the rigs you are coupling it to may not be very stable in themselves on ssb, and the vfo kit while usable, tends to drift a little too.

Still, it is a lot of fun to hear ssb coming from am-only shortwave radios but in addition to some instability/drift, the wide bandwidth can also drive you crazy if you are used to rigs with tighter filtering.