[ARC5] Test Oscillator - Lambda conficuration.

[Leslie Smith]

Hello List.
About one month earlier some-one posted an inquiry about oscillators
suitable for use as a VFO.

I have been working on a circuit based on the Lambda (negative
resistance oscillator) configuration, and in particular the frequency
stability of this circuit.

I have a circuit of the prototype available for any-one interested.
The interest in this circuit, setting it apart from conventional VFO
circuits (Clapp, Vackar & co) comes from the wide range of oscillation
without change to component values.  Specifically I have had my test
circuits oscillating from audio (eg 850Hz) to mid HF (eg 10MHz) with the
only change being to swap the tank circuit coil and capacitor.  The
circuit does not use a tapped coil and has no feedback capacitor
network.  One side of the tuning components is tied to the common rail.

Stability.  The circuit drifts at the rate of 315 Hz per degree C, using
the components in the test configuration.  I changed the coefficient of
drift from negative to positive by adding a 22pF capacitor across the
main tank capacitor.   A 7pF N750 compensating capacitor would give a
lower drift, although I've not confirmed this by measurement yet.

At constant temperature (19 C) the circuit on my bench, beside me as I
write this, has drifted 29 Hz in the last 31 minutes - at 5.5 MHz
(approx).

In the longer term I have several tasks for this oscillator, one being a
test and alignment oscillator for the ARC-5 sets.  While I get lower
drift figures and do a little more work, if anyone is interested in this
circuit (and drift figures) send an e-mail and I'll reply with a
circuit. (Format: png) 

 

  Leslie Smith
  vk2bcu at operamail.com

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