yellowbeard wrote: ↑Tue May 27, 2025 8:18 pm
You should be stretching the coils to make it go lower, and the KV1330 has a a much higher capacitance at 4 Volts - it should go lower with that part. Whats the minimum frequency you can get with it unlocked?
Stretching the coils will reduce their inductance, so you will got
UP in frequency!
Squeezing the coil windings together will increase their inductance, so frequency will go
DOWN.
It should be remembered in that circuit that all the coils have to be identically sized for best results and minimum ½f breakthrough. When correctly constructed with accurately matched transistors, inductors, capacitors and resistors, the breakthrough of unwanted signals is really tiny. Physical layout is also crucial - it has to be perfectly symmetrical.
The basic design of the oscillator also optimises currents through the transistors to keep their shot noise to a minimum, giving a remarkably "quiet" carrier, that when unmodulated just sounds like a "hole" in the white noise. The circuit also has a few other, less obvious benefits, including half frequency generation - eliminating the risk of RF feedback from the nearby PA into the oscillator.
The circuit - as constructed here by "Sinus" and by thousands of constructors of the NRG Kits - is open to improvements. One modification I made to the Pro III version was to use FETs as the "sniff" transistors, to sample the oscillators, and minimise the loading on the oscillator, and improve its isolation, because of the extremely high input impedance of the FETs. I did a further redesign to eliminate the trimmer capacitor, which increased the available capacitance "swing" of the tuning varicaps by increasing the range of bias voltage available by means of use of a DC-DC converter that stepped the available tuning voltage to as much as 32V.
Also - just for the sake of experimentation (and for amusement), I built a version of the oscillator using medium power RF devices (configured as Darlington pairs to keep the h
fe high enough), and built an oscillator that delivered over 32 Watts! The spectral purity was remarkably good, and with the addition of a few poles of lowpass filtering, it would be "clean" enough to use as a complete low power transmitter! Unfortunately, loading on the oscillator would have to be
very accurately matched, and it would be difficuly to tune with varicap diodes (without running into reverse-breakdown issues with the Varicaps). It really was a very silly experiment!
I would still like to know if the oscillator will go that low free running, and what the voltage is on the FFM pin of the PLL...