Thursday, 3 March 2011

Some Rules on VFO Stability

Shortened  list of VFO  rules:

1) Keep RF power output  to  a minimum. (Depend on subsequent stages to build it up--WA8MCQ)
2)  Feedback to maintain  oscillation should he minimized.
3)  Shield the Ls and C's of  the VFO tank components.
4) DC supply instability and ripple should he minimized.
5)  Decouple DC coming into the oscillator carefully.
6)  Lightly couple  the  oscillator to the buffer.
7)  The huffer  stage should have  constantinput impedance.
8)  The  huffer  stage  should have  low reverse gain.

9)  The  fT specification  of  the oscillator device should be  at  least 20 times  the operating  frequency.
10) There should he a means of stabilizing oscillator amplitude.
11) Use  low-temperature coefficient, low loss  caps  such  as  NPO,  silver-mica, polystyrene.
12) Use parallel caps where RF current is high.
13) Use  good quality  air  variables:  brass plates, double-end bearings.
14) Use a solid air-wound coil on a ceramic, glass or plastic form.
15) Use Q-dope on home-made coils.
16) For coils with slugs, use low-temperature coefficient iron-powder material.
17) Keep slug penetration into the coil to a minimum.

18) Inductors  and  capacitors  in the VFO tank shouldn't  encounter any airflow.
19) Keep stray inductance and capacitance low:  short  lead  length, wide  ground return path  (big  ground plane),  no
double-sided PC board construction.
20) Reduce mechanical vibration:  mount all critical  components securely; wiring should  be  solid, heavy gauge wire.

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