Australian Amateur Radio Station
|My "favourite" Local Oscillator circuit.
The Local Oscillator circuit below is pretty much straight out of the "ARRL Microwave Handbook". It
utilises a single stage Butler and with a few additions it lends itself to a very useful and predictable
LO for microwave multiplier chains.
A Varicap is added for Voltage Controlled Frequency setting and with the addition of a "sampling
port" this circuit works very well in PLL systems. Most of my EME microwave transverters use this
as the basic LO frequency generation and coupled with the VE1ALQ schemes it works well when
referrenced to a 10 MHz GPS derived signal.
Hints , important points and notes on retro-fitting into
other multiplier chains:
- All capacitors marked * are frequency related. The Values given apply to 90 MHz +/-
- L1 = 0.1uH or thereabouts. The old TOKO "Green" coils with the Al cores were perfect
but the newer TOKO coils in the shielded cans work very well.
- The 4.7pF interstage coupling capacitor CANNOT be any larger than this value or the
oscillator will "misbehave".
- If there are problems with "netting" the crystal on frequency OR getting to restart once
netted the following can / have been tried and do work:
- 100-200 nH of inductance across the crystral
- 10pF in series with the crystal
- 470 ohm resistor in parallel with L1
- the ratio of 12pF to 47pF caps can be altered.
A lot will depend on the "Quality" and ageing characteristics of the crystal but usually 1
of the above or a combination will get the LO starting and netting on to frerquency
- With 0-12V on Voltage tune input, and with the component values given, a typical 93MHz
crystal should pull > +/- 60 Hz.
- All Spurri on the output are < 60dB down on the fundamental
- When using with a VE1ALQ board it was found that a 47 ohm resistor to gnd on the
sample port provides a better response ( ie less signal to the LO input on the VE1ALQ
- The signal output of +4 - +6 dBm is perfect for feeding into a "G4DDK" board / multiplier
chain. This level also works well into other multipliers.
- Those with a G4DDK board or any LO that uses a 2 stage butler oscillator will find that
by adding this circuit in place of the original crystal, the oscillator chain will be more
stable and less subject to any pulling and any other such effects.
- To feed the signal into an existing multiplier chain that uses a 2 stage type Butler, feed
the RF from the LO into the 1st stage (where the old crystal was) via ~ 1nF and add a
1nF bypass capacitor across the emitter resistor on the 2nd stage.
- For improved stability I build the LO in a box with "R2-Pinkbats " insulation inside and
out and place a crystal warmer to keep the temperature in th LO enclosure fairly
constant. Even with a PLL system this helps maintain better stability and relies less on