Fig. 3. Details of rigid capacitor mount.
was a slow thermal drift each time the oscillator was turned on during a QSO. This sort of short term frequency variation is common with tube circuitry and high levels of oscillation. Drift with the semiconductor vfo is practically non-existent on both a short and long term basis.
In normal operation the vfo is allowed to run continuously and a subsequent stage in the transmitter is keyed. This does not allow break-in operation aithough the vfo has been keyed and only a very slight chirp can be detected. The chirp is dependent on the load presented by different settings of exciter tuning and suitable adjustment of the multiplier stages does eliminate the chirp almost entirely. This sort of arrangement is by no means a sound solution to the chirp problem, however, and for best operation you should run the vfo continuously or provide a suitable method of differential keying.
This vfo should be used to drive a frequency multiplier in the companion transmitter and not straight through on 3.5 MHz. For eighty meter operation, a half frequency design of 1.8 MHz should be built. Since I wanted operation only on 40 meters and above ! chose the 3.5 MHz output frequency.
The above precautions are almost standard practice for ham vfo's, and the many attempts to bypass them have been futile, In other words, the old handbook warning against trying to key a vfo still holds.
This oscillator has been a very worthwhile replacement for the old tube model in the station transmitter. As mentioned, the output was adequate to drive my rig, but some transmitters in the Novice class might require a little more drive. The output level is dependent on supply voltage; increasing the battery to 12 or 1 SV will provide additional drive. However, the supply should not be much over 20V unless a 2N697 is used at Q4 in place of the 2N706 shown on the schematic diagram. Different bias resistors for Q5 as well as a larger dropping resistor for the zener diode would be required if the supply is increased. An untuned vacuum tube buffer amplifier might be more effective to couple into an existing transmitter if the output of Q5 is not enough alone. . . .W9ZTK
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