Precision QRP Transmitter

Posted on Feb 5, 2014

This little transmitter is suitable for less demanding Doppler propagation measurements. It is designed for use on 80m, but can be easily adapted for 40m or 20m. The output is about 500mW of unmodulated carrier, with a stability of about one part in 106, or ±3Hz on 80m. With care (for example mounting the unit in a diecast box, packed in an outer

Precision QRP Transmitter
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box of expanded polystyrene) stability will be even better. The transmitter consists of a TCXO (Temperature Compensated Crystal Oscillator) reference at about 14MHz, followed by two divide-by-two CMOS dividers, and a Class E FET power amplifier. Follow the Schematic Diagram as you read the description below. At the left of the drawing is a 14MHz TCXO. These can be obtained on a range of suitable frequencies, sometimes surplus (for example 14. 85MHz units are used in many cellular phones). Any frequency from 14 to 15MHz will be suitable for 80m use (depends on the Amateur band allocation in your country). If you build this transmitter, MAKE SURE that the oscillator you use is a TCXO, not just a packaged oscillator. Many old computer cards include such oscillators, and some will be on 14. 31818MHz - very tempting, but don`t be fooled - they are not stable enough. The output of the TCXO is typically AC coupled, and a clipped sinewave about 1V p-p. This is insufficient to drive CMOS directly, but if biassed to half-supply by two 100k resistors, will reliably clock the first 74HC74 divider, U1A. This D flip-flop is arranged to divide by two through the expedient of connecting the /Q inverting output to the D input. If you are lucky enough to locate a TCXO with CMOS output, just omit the two resistors. The second flip-flop, U1B, is also arranged to divide by two. In this case the /CLR (reset) input (pin 13) is held high by a...

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