Looking for a easy to build RF generator to test and adjust my home made HF transceiver I found this nice project from www.vintage-radio.com. The device is very basic but suits my needs to test the band pass filters and the receiving unit of my rig. It has also a built-in AM modulator with an internal audio generator providing about 800Hz tone or you can plug-in an external audio signal to test and repair radio receivers.
The frequency range of the generator can cover the whole HF spectrum depending on the value of the inductors and the variable capacitor. With the components used my device can cover 450kHz to 22Mhz, in four ranges (described further below).
The oscillator is a Colpitts type, with a high gain FET transistor followed by a buffer stage. The output level can be adjust by a potentiometer up to 3Vrms. The audio stage is an RC oscilator and with the values shown in the schematic it can generate about 800Hz. A switch is used to select either to modulate with this signal either with an external one, that should be about 1.5Vrms. The whole circuit is powered by a 15V regulated supply.
Circuit diagram, PCB, as well as other functional details you can find in the original document here.
The entire assembly is fitted into a PVC box, KM-85 type. Ideal was to use a metal one, but this one was in hand at the moment, so some screening I need to do later to the circuit board and transformer, althouh during testing and measuring it was quite stable as it is now.
The variable capacitor used is from an old radio, Orizont S620A. I bought it for 1 euro from scraps. It has a multiplication of more then 4 times, very good for fine frequency tuning. With the inductors used, I managed to get the following frequency ranges:
- f1: 450 – 1600kHz, L1=220uH
- f2: 1000 – 3750kHz, L2=47uH
- f3: 2.3 – 10MHz, L3=10uH
- f4: 6.2 – 22MHz, L4=1.2uH
Note that the inductors should be placed radially on the rotary switch, otherwise there’s a chance of interaction between them, even with the ones not in use. Also, connections between the main circuit to the switch and capacitor should be made out of short length wires.
Because of the high multiplication rate of the tuning capacitor it was almost impossible to do printed scale on the front panel, so I added an extra BNC output connector to use togheter with the digital frequency counter. This one is connected to the output of the oscillator, before the level potentiometer, via a 100n capacitor.
Testing the AM modulation stage is quite simple, no extra equipment needed, just an ordinary AM radio receiver. After both frequencies are matched and the modulation switch is turned to internal, a tone must be heard into the radio. I tried also to connect an external audio signal. First I used some music from my laptop’s audio output, but no sound in the receiver, this maybe because the signal is to low. After, I tried a more powerful source, from a CD player headphone output, and then I could hear the music quite clear in the receiver.
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