RF Signal Generator




Circuit Schematic
The RF Signal Generator is a Colpitts Oscillator coupled via a 68pF capacitor in a Seiler configuration. Frequency is selected by a
switch and a variable capacitor. The oscillator output is buffered and an agc loop holds the buffer output at 1.5 volts pk-pk.
The switched attenuator has an output impedance of 50 ohms and provides an output voltage selectable in 10dB steps.
The frequency counter sync output is fed by a separate fixed attenuator.

Component Values
Some component values are parallel or series combinations.

Frequency Range
400kHz-15MHz

Output Level into 50 ohm Load
-40dBm, -30dBm, -20dBm, -10dBm, 0dBm
(adjust R6 for 1.265V pk-pk off-load)



Circuit Board, Component Side



Circuit Board, Copper Side

The circuit was constructed on Vero stripboard. Three busbars link all tracks, except where cut, to produce a ground plane.
The same board was used for various builds and modifications. It has many blank holes and much redundant tinning.



Interior View

The controls can be seen on the rear of the front panel. The inductors are mounted directly on the frequency band switch and the
attenuator resistors are mounted on the attenuator switch. Only the 130pF gang of the variable capacitor is used. All controls
are linked to the circuit board via headers and sockets for quick release. The battery is plugged into the circuit board at left.
The output socket connects to a header on the attenuator behind the pillar and the sync socket connects to the header at right.



Front Panel View

The front panel controls are left to right: Frequency Band Switch, ten bands covering 400kHz to 15MHz, Tuning Dial, unmarked
because the frequency generator will be used with matching radio frequency counter, and Attenuator Switch with five positions
for -40dBm, -30dBm, -20dBm, -10dBm and 0dBm. The Indicator LED helps avoid battery wastage.



Rear View

The rear panel has BNC sockets for output and frequency counter, on/off switch and a drawer to hold the 9V PP3 battery.





My thanks to ZL2PD for his kind help and encouragement.



< back