Raypsi:
- Thank you for the hint, the CATV tuners cover 54 to 900 MHz (5 to
~40 is for the upstream), an impressive 16:1 range, but the L.O. is
much narrower in relative values. Suppose 1st IF at 1200MHz, the L.O.
would sweep 1254 to 2100, 1.67:1, not enough for me.
- "The TRF front end also". Are you sure?. I dismantled a Jerrold and
the incoming CATV hit directly the balanced modulator thru a fixed
high-pass. But even if it really tuned 54-900, it would be easier than
my needs because the CATV amplitude is much lower than in an
oscillator.
- "Roll the Q down"?. It wouldn't help, I just need to vary 9:1 the LC
product.
- "Series varicaps"?. Sorry... they are back-to-back in series
already!.
Doug:
Thank you for encouraging me about the heterodyne way, and the links.
Yes, I think I will go this way for my gen-purp lab generator, but for
the dipper I still need the coil to oscillate right at the measured
frequency.
Brian:
I'm afraid this is more complicated than choosing a long time
constant, I have already tried in the simulation. The problem seems to
stem from the fact that the effective FET gm is dependent not only on
applied bias but also on RF amplitude. The error amplifier senses the
amplitude is high, applies more -Vgs, gm decreases, amplitude
decreases, but this lower amplitude makes effective gm to fall even
more, and as a result oscillation ceases and then starts again, and
again, and again. I think I will have to prevent the FET from working
in class C and forget about Vgs as a control means.
Again, many thanks to all of you. As soon as I can come back to this
subject I will let you know the results.