It is a simple Colpits oscillator, but it has a 78L05 5v regulator (not shown) as the supply feed. The 22K and 33K resistors bias TR1 emitter to about 3v DC, which means that TR1 emitter can feed the CD4040 counter input directly when the synth is fed from a 10 volt supply. This is the “sense” output. TR2 is a buffer with an additional 4K7 base feed resistor to further buffer the oscillator from the output stage. The output from TR2 emitter is about +10dBm.
L1 determines the frequency and you need change this component, plus a few caps, for other frequencies. This circuit will cover from 500kHz to 15MHz with nothing more than component changes. L1 was wound on one of those surplus RF coils for PCB mounting, but a “recovered” transistor IF can is the same thing.
Note that these values are given as a guideline only and may vary quite a lot, depending upon the coil formers you use. The trick is to wind one coil, fit the caps and then measure the frequency. You can usually just adjust L1. But if the frequency was, say 20% too high, then increase L1 turns and capacitor values by 20%. Easy! My figures were tested at 1MHz and 5MHz. I used L1=35 turns, C1=47pf, C2=47pf, and C3=100pf. This gave me 5MHz to 5.5MHz by setting the ferrite slug.
On the main board reference osci
llator section, there are two capacitors in the reference oscillator section: “C+” and “C-“. Under normal circumstances these are not fitted, and in place of “C+” is a wire shorting link. But if you want to “trim” the complete synthesiser you can do this by fitting capacitors. “C+” increases the reference. The smaller the capacitor so the higher is the frequency. Value as low as 12pf can be used. “C-” lowers the refrence frequency. The larger the capacitor so the lower is the frequency. You can fit up to 150pf here. This should be done with reference to a frequency standard, such as WWV on 5MHz.
This VFO shows you the principle involved for other VCOs, with or without a prescaler. The only thing you have to remember is that the synthesiser board uses a 10v supply, but 5v prescaler chips use 5v logic. You need some sort of logic level converter. I used two resistors in my prototype, but a transistor level converter would be more elegant, and faster, as detailled in the section “Putting It All Together”.
The above circuit is perfect f
or VHF use, up to about 450MHz. It has a very wide frequency ratio. TR1 and TR2 can be BF959 for frequencies above 200MHz, 2N2369 for frequencies above 60MHz, and BC547 for frequencies below 30MHz. I must stress that this circuit is a suggestions and has only been built on a “rats nest” board, but there is as yet no PCB available for it. “Point-to-point” wiring on copper-clad board is probably the best construction method for frequencies above 30MHz. Typical values for L1 using 0.15mm Dia. enamelled magnet wire on a 5mm Dia. air-dielectric former are: 50MHz 30-turns, 75MHz = 28-turns, 145MHz = 5-turns, and 300MHz = 1.5-turns. For 450MHz L1 should be about 1/2 turn, 12mm Dia. using 0.5mm Dia. wire.
