Sweep Generator Frequency Vs Varicap Sias

theoretical PLOTS

find some way to vary the dc potential across the varicap repetitively. The schematic diagram is shown in Fig- 3.

Q1 is a unijunction transistor relaxation oscillator that generates a sawtooth waveform. I his sawtooth is used to provide the voltage variation across the varicap to provide the sweep function. The potentiometer R1 sets the sweep speed and can adjust the sweep rate from under one sweep per second up to about 50 sweeps per second. The sawtooth from the unijunction is coupled into Q2 which feeds the varying dc voltage to the varicap. Q2 also provides a sweep output that can be applied to an oscilloscope as will be described later. Potentiometer R2 adjusts the amount of voltage variation applied across the varicap which controls the amount of frequency shift or sweep width.

The oscillator section is straightforward. The 25pF air variable capacitor CI is used to set the center frequency around which the oscillator is to be swept. The frequency set capacitor is kept at a small value so that the relatively small variations in varicap capacity have a sufficient effect on the frequency and thus the sweep width. At the higher frequencies not much capacity variation is required, and the width of sweep can be adjusted by R2 if it becomes too great.

Q4 is used as a straight amplifier to boost the output signal to a usable level and to

provide isolation between the oscillator and whatever load might be applied. The unit described provided about 2.5 volts rms on the 40 meter band and about a volt rms on 20 into a high impedance load.

The unit shown in the photograph was built in a 2lA x 2% x Minibox. As was mentioned earlier, this project started out as an expe;irnent and a larger enclosure might have been better since the parts became rather crowded after several additions and modifications were made. The center frequency adjust capacitor CI is mounted on the front apron of the box with an octal socket tor the plug in coils mounted immediately above it on top. The sweep width potentiometer R2 is mounted on top toward the rear On the rear apron are the sweep rate control RI, a BNC rf output connector and a universal binding post for the external sweep output connection. Originally it was planned to use a fixed sweep rate, but when a variable rate was found to be useful the rate control took the spot that was intended for an on-off switch for an internal battery. As a result, two binding posts were mounted for connection to an external battery or power supply.

The circuit is wired up in typical transistor construction fashion on a phenolic board. The unijunction circuitry is toward the rear and the oscillator/output amplifier

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