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The Clapp oscillator, or some variant of the Clapp-Gouriet circuit, currently is just about top favorite for VFO use by hams. Fig. 1A is representative, and the values shown are typical for the 80-meter band. Because of the small tuning capacitance involved, the circuit does not lend itself readily to application of the conventional type of vernier control. The latter usually consists of a variable condenser of several pF in parallel with the main tuning condenser. However, in the Clapp circuit, a vernier of this type, covering only a few kc. for easy setting to zero beat, becomes impractically small. With the values shown in Fig. 1, a change of 5 pF. will cause a change in frequency of about 50 kc.

Fig 1
Fig. 1. A Clapp VFO with typical tuning values. B - Ct is a vernier covering only a few kc. on 80, useful in setting VFO accurately to zero beat. C2 and Ca, along with the usual fixed parallel condenser, Cs, produces a tuning curve that favors the higher-frequency bands.

Accordingly there is need for another solution. The answer is suggested by one of the important virtues of the Clapp oscillator - its relative immunity to slight capacity changes associated with the tube. A 50-pF. variable, connected as shown at C1, Fig. 1B, gives a well-nigh perfect vernier - a total QSY of about 6 kc. The tuning is almost precisely linear if a straight-line-capacity condenser (semicircular plates) is used. The h.f. c.w. man may prefer to use a 25 pF. variable to limit the QSY on 14 Mc. to about plus or minus 6 kc.

Fig. 1B also includes a bandspread scheme that may appeal to many. The tuning element consists of a 150 pF. variable, C2, in series with a fixed capacity of 50 pF (C3). The tuning characteristic is shown in Fig. 2(A). The straight line B shows linear tuning for comparison. With linear tuning, the tuning rate on 80 is about 5 kc. per dial division. When multiplied for the higher-frequency bands, this means a tuning rate of 10 kc. per dial division on 40, and 20 kc. on 20 meters. With series tuning the characteristic is essentially linear out to about 3600 kc., with tuning rates of about 1.65 kc., 3.3 kc., and 6.6 kc. respectively on 80, 40 and 20. From 3600 to 3650 the rate is 5 kc. per dial division on 80, or 10 kc. per division over the range of 7200 to 7300. The latter is just about the same as the linear scale. Over the remainder of the range up to 4000 kc., useful exclusively for 80 meters, the average rate is about 12 kc. per dial division. For the ham who works several bands with the same VFO, the advantages of this tuning system are obvious. Combining this bandspread circuit with the vernier provides a tuning system that is easy to adjust to meet any requirement.

Fig 2
Fig. 2. A - Typical tuning curve for the band-spread circuit of Fig. 1B. B - Linear tuning.

O.J. Russell, G3BHJ.