The NOGAnaut QRP Transmitter

 

The crystal oscillator is the simplest form of transmitter. Normally, oscillators are used to drive buffer amplifiers and power amplifiers, which provide increased output, as well as prevent the output circuit from adversely loading the oscillator.

Most transistors exhibit a characteristic impedance different from the 50-ohm impedance of a well-tuned antenna system. An improper match between the impedance of the transistor and the load (e.g. antenna system) can cause severe power degradation, and worse, can seriously affect the signal, including shifting the oscillator frequency in unpredictable ways.

In the NOGAnaut transmitter, the 2N2222A transistor, which exhibits a characteristic impedance of approximately 200 ohms, is matched to a 50-ohm load via the pi-network filter composed of C1, C2 and L2. The values of these components were chosen to provide a close match between the 200-ohm transistor and a 50-ohm antenna (it is therefore critical that a good 50-ohm antenna system be used with this transmitter). It so happens that these values also form the familiar half-wave harmonic filter, thus satisfying FCC spurious emissions requirements.

Figure 1. NOGAnaut 80M Transmitter Schematic.

Capacitor C5 provides the necessary feedback to begin oscillation. You may find that you can operate your NOGAnaut without this capacitor--stray capacitance in the circuit provides a certain amout of feedback without C5. However, it was found during development of this circuit that the oscillator can have troubles starting at times, therefore it is recommended that you leave C5 in the circuit.

The 0.01 uF capactor, C3, serves as a DC-blocking capactor. At 3.6864 MHz, this capacitor is essentially a dead-short to RF, but blocks the DC current from flowing into the load.

This is a familiar Colpitts oscillator, operated in "common-base mode." The usual base-bypass capacitor is replaced by the capacitance of the crystal. With a 15V supply, this transmitter has been measured to deliver as much as 134 milliwatts into a perfectly matched 50-ohm load ("your mileage may vary"). With a 9V supply, about 20-50 milliwatts should be expected.

The transmitter is keyed by interrupting the positive supply voltage. You can modify this to be grounded keying, if necessary (just interrupt the negative supply voltage instead of the positive voltage). This may be necessary if you use a keyer that expects grounded or negative keying.

For a very good description of crystal oscillators, check out Solid State Design for the Radio Amateur by Wes Hayward, W7ZOI, and Doug DeMaw, W1FB. This is one of the most popular amateur radio books ever written and is packed full of practical information about how solid state circuits behave. It is published by the ARRL, and can be purchased directly from them, as well as from many electronics retailers.

Further information about pi-network filters can be found in The ARRL Electronics Data Book, by Doug DeMaw, W1FB, also published by the ARRL. This book contains most of the nuts and bolts of basic circuit design, and is a must for any ham shack.

http://www.nogaqrp.org/projects/noganaut/circuitdescription.html