Thursday, 3 March 2011

CW / AM TRANSMITTER

The circuit is almost ridiculously simple, and construction is even simpler. It is basically a single transistor oscillator with a very low output impedance, suitable for driving the base of another transitor amplifier stage.

cw-am_01

The oscillator uses a single coil and crystal. The coil is tuned to the output frequency, which may correspond to the crystal frequency, or a harmonic, for example you my like to use:

The above table does NOT contain rules. You may have a 3.5MHz CW crystal, but there is no reason why you cannot tune the oscillator to 7MHz. It will work with the fundamental, 2nd harmonic, 3rd and 5th. The output power will reduce at higher harmonics and frequencies are chosen.

The power amplifier is quite straight-forward. The amplifier is biased as a linear amplifier with the collector load matching the 200 Ohms collector impedance to 50 Ohms with T2. L1 and L2 plus the three capacitors for an output low-pass filter.

Modulation is applied to the base of TR2. The MOD link is removed and pin 2 is varied from 0vDC to 12vDC to vary the TX output power from zero to full power. Pins 1 and 2 may therefore be coupled directly to a morse key and you have an HF bands QRP CW transmitter. The capacitor across the 10K resistor will also adjust the keying envelope. Leave it at 1n0 for radio control use, but increase it to 100nF for good morse. On the band this transmitter sounds surprisingly clean and well formed. Since the oscillator is continuously running, there is absolutely no chirp. Adjusting the tuning of T1 will adjust the frequency by a couple of KHz.

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