Valve frequency multipliers (followup)

Air core inductors, simple coils, would have best Q at your frequencies.

All the 150 Mc/s coils are air-cored but the 50 Mc/s ones could have been either air or ferrite cored - or so I thought - wrongly!

The formers were some NOS ones that had about 40 turns of wire on them, bfeore I stripped them and wound on 7 turns for 50 Mc/s. That looks as though they were originally intended to work somewhere around 1 Mc/s and presumably the ferrite core would have been optimised for that frequemcy range.

I am now beginning to wonder about the other inductors in the crystal 'pulling' circuits, which are running at about 15 Mc/s and come from the same batch. The behaviour of the oscillator had been somewhat strange at times, so the cores could be causing trouble there too.

I've wound high-Q inductors on a Sharpie.

Probably not: the wire was layered, whereas all the 455 Kc/s I.F. transformers I have seen were wave wound with Litz wire to give the highest possible 'Q'. - and they had a lot more than 40 turns.

I'm not after a particularly high 'Q', but if the coils are stupidly 'flat', that might explain why I had such difficulty getting a good signal out of the oscillator. I'm in unknown territory with this circuit, as the crystal frequency has to be modulated on transmit and pulled on receive. The partially-complete circuit of the Xtal oscillator and multiplier chain is at:

Decent coax is pretty good too--to make an inductor at 150 MHz takes only a few inches of coax with a short at the other end.

I like to tune coaxial stubs using thumbtacks--you stick it through the shield and the center conductor. It survives very well, so you can do this many times on one piece of coax.

Cheers

Phil Hobbs

That's an excellent idea, I hadn't thought of doing it that way. It might be a bit tricky 'tapping' such an inductor for various loads.

You tap it Colpitts-style when necessary. That isn't as good as tapping a tightly-coupled coil such as a gapped toroid, because the impedance change comes only from the tank Q, rather than tank Q plus transformer action.

Cheers

Phil Hobbs

If you use an 'open' transmission line, such as a trough line, then you do get a transformer effect. Most UHF television tuners used trough lines with tappings to match the aerial impedance to the transistors and still retain a reasonable 'Q' for tuning.

Leak even did it at VHF in their FM tuner but I don't know whether it gave any advantages (I believe some of their designs were actually made under licence from the BBC and weren't designed by H.J. Leak at all.)

You do get different impedances at different points, but that’s just on account of the circulating power, as in a Colpitts. It falls apart completely at low Q.

With tight inductive coupling, as in an ideal Hartley, the coupling ratio is nearly independent of Q.

Cheers

Phil Hobbs