SECTION 4 — Local Oscillator
Click for the Master Index to this project
This year, I built and designed 3 local oscillators, however, this VXO leads in simplicity. Click for my synthesizer for Jupiter receivers.
Above — Entire schematic for the VXO.
2 standard value crystals in “super VXO” format provide a delta F of 135 KHz; just enough to steer around any interference on or around 20.1 MHz. Stability tests showed average frequency variation at room temp is 1/10 of 1 hertz. No need to ovenize crystal oscillators for Jupiter reception.
You may wish to increase the delta F by increasing the inductor size and/or employing a variable capacitor with a wider delta C. It’s important to test the output in a ‘scope and counter to ensure reasonable frequency stability and that it remains oscillating across the swing of the variable capacitor.
For the main oscillator, I employed the medium power BC337 transistor — a device with low flicker noise. Mine are original Fairchild parts [ now ON Semiconductor ] as terrible bootleg copies are widely sold now.
Vojtěch Janásek keeps an interesting thread on low noise AF transistors including the BC337. Click for his pdf page link.
Follwoing the Colpitts oscillator, an emitter follower’s output port provides a good input match for a common base RF amp. The CB amp provides strong reverse isolation between the oscillator and its load. Again, another emitter follower transforms the high output Z of the common base amp to a low Z which gets low pass filtered and buffered with a 4 dB pad.
Above — Output of the VXO in a DSO.
Above — Simulation of the low-pass output filter in GPLA from EMRFD.
This VXO looks pretty unremarkable — but adopts careful RF bypass of DC power lines in every transistor amp and runs just enough current to minimize distortion as the signal moves through each RF amp.
The enclosure looks ugly — it’s a re-purposed box from some old project. I threaded in a bolt to block the hole seen on the right side in the reverse view photograph.