07-12-2016, 08:54 AM
(06-12-2016, 10:56 PM)Kalee20 Wrote: Looks good!
The trouble with single-stage modulated oscillators is that there's a tendency to get some FM. The envelope can look lovely on a 'scope, you can have trapezium figures with super straight sides, but if there's FM going on too, when it hits the receiver's passband you get slope detection, which combined with the intentional AM, gives weird tuning and distortion effects. Your circuit with a crystal should, hopefully, reduce this at the expense of being stuck with whatever frequency the crystal is.
The idea of an audio preamp with another Russian rod valve is really the only way to go, who'd want to drive it with a tr*ns*st*r? A third valve, configured as a detector, could demodulate the RF and allow a bit of negative feedback to reduce distortion.
I'll watch with interest!
Thanks Kalee.
I fully agree with you regarding getting some FM modulation, one reason I use crystals. I also would think another rod pentode would be the best for an Audio preamp, current consumption certainly would never be an issue on the HT side and the use of a high capacity Ni-Mh cell for LT will mean many hours of "broadcasting". I have been aiming for an average of 1 volt for the filaments, a freshly charged battery will give 1.1v via a series resistor and this drops down to 0.9v with no change in performance, once down to 0.6 the oscillator can drop out.
The addition of an RF buffer is also a good idea and the RF can be detected, filtered and fed back to the audio pre stage, possibly to one of the G1's on its own, this would be a neat solution and would make the whole little transmitter very linear indeed.
I did NFB in the EH90 micro TX a few years ago and the improvement is well worth the effort.
I am going to visit a sick neighbour today who is in hospital so it could be tomorrow before I update the thread.
