20-03-2019, 05:53 PM
(This post was last modified: 20-03-2019, 05:53 PM by pamphonica.)
Welcome also.
If you really need sweeping, I would experiment by adding a variable capacitance diode across the main frequency-setting capacitor VC1, fed with a ramp waveform via an RF choke . Maybe add a cap in series withe the varicap so the DC cannot upset the FET oscillator bias.
You probably only need a few 10s of pF swing on the varicap diode if you are for instance sweeping a 465KHz IF by 10KHz or so. You would obviously need to make a ramp generator to produce the necessary voltage sweep across the diode.
Be prepared for modern varicaps to be absolutely tiny, surface mount devices. Best soldered carefully to something like a tiny scrap of veroboard with wire tails as they won't take repeated soldering and desoldering .
Have fun!
Jeremy
If you really need sweeping, I would experiment by adding a variable capacitance diode across the main frequency-setting capacitor VC1, fed with a ramp waveform via an RF choke . Maybe add a cap in series withe the varicap so the DC cannot upset the FET oscillator bias.
You probably only need a few 10s of pF swing on the varicap diode if you are for instance sweeping a 465KHz IF by 10KHz or so. You would obviously need to make a ramp generator to produce the necessary voltage sweep across the diode.
Be prepared for modern varicaps to be absolutely tiny, surface mount devices. Best soldered carefully to something like a tiny scrap of veroboard with wire tails as they won't take repeated soldering and desoldering .
Have fun!
Jeremy
Jeremy. G8MLK. BVWS Chairman. British Vintage Wireless and TV Museum Friend.
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Pamphonic Website ______________ BTTT OB Truck Restoration
