Local Oscillator Overtone




RCA 40602 dual-gate FET which exhibits high gain, low noise,and very little reverse capacitance. This low reverse capacitance allows you to build a high-gain amplifier at this frequency without having to neutralize. The second gate provides a convenient way of varying the stage gain so that the converter can be properly matched to your receiver i-f. The gain of this stage is 28 dB with the i-f gain control at maximum and 0 dB at the minimum position. The output of the FE is a double-tuned circuit centered at 30 MHz. it provides a flat response from 28 to 32 MHz with high skirt selectivity. It should be noted that the i-f amplifier following this type of passive mixer must have the lowest noise figure possible as it will directly add to the overall converter noise figure. The use of the FET in the i-f amplifier also allows one to take advantage of the FET's overload capabilities and low crosstalk and intermodulation products.

The mixer's oscillator injection is obtained from a "no multiplication'* local oscillator. It uses a seventh overtone crystal at 116 MHz in a common-base Colpitis

Mhz Transverter Schematic

Fig, 6* PC layout (component side of,board).

oscillator circuit. The cost of the crystal is about S7.50, but it eliminates the multiplication stages and tuneup required for the tripled 38.666 MHz approach. The "no multiplication" approach also eliminates many undesireabie harmonics from being fed into the mixer which would cause unnecessary image and "birdie" problems. This oscillator circuit is easy to get started and it will supply the mixer with about 10 mW of drive. This circuit has been used with crystals from various manufacturers

Fets 40602

Fig, 7. PC layout (copper side of board) 24 73 MAGAZINE

f44 148 MHi RF IN

f44 148 MHi RF IN

Magazine March 1970 Dip Meter
Fig. 8. 2 meter hot carrier diode converter.

and by others in their converters and it always plays right off. So don't fear getting this I E 6 MHz oscillator circuit to work.

The printed circuit board layout is shown in Figs. 6 and 7, By using it and the components recommended, duplication of this converter should not be a problem. The use of a passive mixer and an i-f amplifier thai doesn't require neutralization makes tuneup very simple.

Alignment of the down converter is a simple matter- First, get the oscillator working and then peak the input filter and the i-f' amplifier with a 2 meter signal. The oscillator is started by tLining C4 and using a wavemeter or the diode position on a grid dip meter as a signal indicator. Once you've established that the oscillator is running, feed a 2 meter signal into the converter input ;nul peak the input filter and the i-f

30 MHz i-iT

+1 -2

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