27MHz CW transmitter
Posted on Nov 18, 2012 10329
Under: RF Transmitter Circuits
The original TX was designed to be only PULSE Modulated for a proportional R/C system, but I later used the same circuit to make an HF bands Amplitude Modulation (AM) transmitter using PA-Base Modulation. It eliminated the need for a modulation transformer; it can be driven with a simple Op-Amp amplifier. Frequency range of the original prototypes were as low as 1MHz and the top frequency was 40MHz. The circuit is the same, just the crystal and coils are changed. The CW output power is about 200mW at 29MHz. It is only 70mW Amplitude modulated, but modulation peaks can rise to 200mW with 80% modulation.
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The transmitter is quite small, only about 60mm. x 35mm. so it easily forms a single transmitter module inside the case of any other peice of equipment, such as an R/C transmitter or a Hamradio portable rig. This is the PCB version. The final PCB has had a couple of changes around the oscillator stage. 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. Position the components on the board and solder them. I don't think I can add more to that, except to say that you should follow the PCB board component layout I have given. Vertically mounted resistors have a long wire and a short wire. The long one should not have any RF on it. The circuit is quite forgiving in that it can be built birds-nest/ugly-bug style and still have a clean output. Mine had all unwanted harmonics better than -45dBc. The transistors can be 2N2222 or BC547 up to about 20MHz. After that the power starts to fall. Changing TR2 to BC108 is great to about 35MHz. Above 35MHz then BSX19...