VFO TX/RX switching - frequency sideslipper?

[G6Tanuki]

I'm looking at building a classic "table-top" 25-Watt 80M AM transmitter - 807 or 6146 PA, pair-of-6L6 as the modulator and a Woden UM1.

I'm thinking of using a transistor VFO, something like the classic "synthetic rock" circuit.

While looking at the VFO design I had an idea.

Traditionally the VFO has been de-powered on receive to avoid any carrier-leakage desensitising the receiver (or causing an annoying heterodyne with stations you're trying to receive) but I've never liked this since it varies the amount of self-heating of rthe VFO and this can cause drift.

What came to me at about 00:15z today was to leave the VFO running all the time, but to include a PIN-diode-switched capacitor across the tuned-circuit and controlled by the TX/RX/mute line, so when on receive the PIN-diode is forward-biased and the additional capacitor comes into play, sideslipping the VFO a couple of hundred KHz away from the receive-frequency to somewhere it can't cause trouble.

On TX the PIN-diode is reverse-biased so the extra capacitor is isolated.

I don't see any reason why this wouldn't work - but I've never seen it mentioned in any of the usual ham-radio mags or handbooks, which to me means either the idea has been tried and it doesn't work, it works but someone's got a patent on it and some good lawyers, or nobody's thought of it before!

What think you, good people?

[Bazz4CQJ]

Originally Posted by G6Tanuki

I'm looking at building a classic "table-top" 25-Watt 80M AM transmitter - 807 or 6146 PA, pair-of-6L6 as the modulator and a Woden UM1.

Just below the surface, I suspect that very many of us want to build such a rig, or something very similar.

Sometime ago, I considered the possibility of allowing a receiver BFO (455kHz) to be continually powered, but using a varicap diode on it to pull it sufficiently off frequency so that it was effectively 'switched off'. On paper, it actually appeared possible to set up a circuit where the BFO was functionally tuned by the varicap when needed as a BFO, but then a separate switch put sufficient DC on it to push it "out of bounds".

I got as far as buying the varicaps.... and now it's somewhere on the to do list.

B

[G6Tanuki]

The idea of mixing the BFO and Rx LO to generate the Tx carrier is well-established [the 19-set used it]; equally the idea of using a common 450-470KHz "IF/SSB-generator" strip and filters in a transceiver with one VFO/LO.

Alas that idea won't work here because my 'default' HF receiver is a RA217D so there's no convenient "LO" related-to-transmitter-frequency source to be extracted-and-mixed... which is in a way a shame considering the efforts Racal went to in order to make their tunable LO so stable.

[frsimen]

That sideslipping idea works perfectly well. I have used it a number of times and I think the circuit featured in ARRL publications such as Solid State Design for the Radio Amateur.

From memory, I used 1N914 diodes for the switching.

[Radio Wrangler]

I think the RA217/1217 receivers use the same frequency plan as the RA117, and the MA79 exciter would work with that, so you could use the Rx VFO (for the 2-3 MHz tuneable IF) and either mix it with a crystal oscillator for your band or else build a reversed wadley converter.

G3ROO did a transceiver mod for the FRG7!

David