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Three-band single-crystal conversion oscillator

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The interest in crystal-controlled converters for the higher-frequency bands prompts me to send you the accompanying circuit. The conversion oscillator technique might be of value to those whose wallets may be described only in two dimensions.

The 3.5 Mc crystal is used on the 3rd, 5th, and 7th overtones to cover, respectively, 20, 15, and 10 meters. Oscillator switching is accomplished by switching trimmer capacitors in the Butler oscillator tank circuit. On all these bands the receiver tunes forward since the fixed conversion oscillator is on the low side and the i.f. starts at approximately 3.5 Mc. Since the overtones are not harmonically related to 3.5 Mc. there is a slight displacement of the lower band edges from the 3.5 Mc tuning position of the receiver. This amounts to about 8 kc for 20 meter, 9 kc for 15 meters, and 10 kc for 10 meter with the 3505 kc crystal (Peterson type Z-2) now in use; a 3502 kc crystal of the same type gave a larger offset for 20 meter and a larger dispersion between 20, 15, and 10 meters.

Fig 1
Fig. 1. W4TKR's single-crystal oscillator circuit for covering 14, 21 and 28 Mc in a crystal-controlled converter. The 3rd, 5th and 7th overtones are used for the three bands. The oscillator tank circuit should tune to the oscillation frequency in each case-approximately 10.5 Mc for 14 Mc, 17.5 Mc for 21 Mc, and 24.5 Mc for 28 Mc. Separate tank circuits of ordinary design can be used, but W4TKR simply switches trimmer capacitors with a single coil, as shown. This requires that L1C1 tune to 24.5 Mc with a low value of capacitance at C1, in order to achieve a reasonable L/C ratio at 10.5 Mc. The value of C4 should be adjusted to give the desired coupling to the mixer. (Note: The grid of the left-hand triode should connect to the top of the 20 k resistor.)

It is interesting to note that the 7 Mc band could be beat down to 3.5 Mc by using the fundamental frequency of the crystal, but the oscillator signal would come directly through the mixer at the crystal frequency and so would the second harmonie of the crystal, which falls in the 7 Mc band. Three possible solutions suggest themselves: One is to use a second crystal somewhat lower than 3.5 Mc for the 40 meter band only. Another is to accept more of a dispersion of the higher-frequency bands and use a single crystal with a frequency a little below 3.5 Mc. The third method would be to use a crystal with small dispersion among the high-frequency hands but one which would permit tuning 40 meter with the oscillator on the 3rd overtone (about 10.5 Mc). In this case the 7 Mc band would tune backward, from about 3.5 Mc to 3.2 Mc. It has been assumed that the overtones will be lower than the harmonics of the fundamental, which is my experience with the two crystals mentioned.

A converter such as this with some 40 meter provision would be a valuable accessory for one of the 3-6 Mc Command-set receivers.

James A. Murray, jr., W4TKR.