Javascript® Electronic Notebook by Martin E. Meserve Transmitter Building Blocks
The circuit shown is a standard oscillator of the Colpitts variety. Similar circuits have been used in many ham radio homebrew transmitters. This particular circuit should function well at frequencies from 1500 kHz to 8000 kHz. For stability, C1 and C2 should be disc or silver mica. C3 and C4 can just be ceramic disc. At lower frequencies, the values of C1 and C2 might need to be increased.
To round out the collection, here's a Pierce oscillator using an FET. The version on the left is from an electronics textbook. The version on the right is from the "Grenade" shortwave pirate radio transmitter designed by "Radio Animal."
On the right is a drawing for a VFO (Variable Frequency Oscillator) that can replace a crystal oscillator. A VFO would allow you to operate across a range of frequencies, without investing in a lot of crystals.
On the drawing, C1 is a small Air-Dielectric Variable Capacitor and would be your main tuning. C2, and C3 set your main tuning range. For temperature stability, those capacitors should be Polystyrene. With the values listed in the drawing, the tuning range will be approximately 1641-1711 kHz tuning range (about 70 KHz). Changing C3 to 100 pF will change the tuning range to 1565-1625 kHz (about 60 KHz). The small calculator above can be used to find out the tuning range for a different set of capacitors.
There are 4 keys to building a stable VFO:
1. Use a regulator IC or a zener diode to regulate the voltage going to the oscillator. Voltage fluctuations produce frequency drift.
2. Build the circuit carefully. Keep component leads short, do your best soldering work, and make sure all the components are physically secure (no wiggling).
3. Choose your inductor and variable capacitor carefully. Air-core inductors are more stable across a range of temperatures than iron-core inductors. Air-dielectric variable capacitors are more stable than mica compression and other types.
4. Put a buffer amplifier between the VFO's output and the rest of the transmitter. Failure to do this makes the VFO vulnerable to being "pulled" by modulation, by changes in antenna loading, etc.
The circuit shown here is a series-tuned Clapp oscillator. Unlike the parallel LC tanks seen in many oscillators, this one has the LC components in series. Similar VFOs have been used successfully in many ham radio transmitters and receivers.
A buffer amp can be inserted between the oscillator and the rest of a transmitter circuit. The buffer not only increases the power of the signal but also prevents the oscillator frequency from being affected by modulation, changes in antenna loading, etc.
Above is the simplest buffer amp we could dig up. It is designed to work with a 9 volt power supply.
Above is a deluxe buffer amp that uses a pair of bipolar transistors. Some of the resistor values are a little different depending on whether it is built with 3904's or 2222's.
Simple AM modulators work by varying the amount of power flowing through the transistor which is serving as the RF output amplifier. By imposing an audio waveform on the power supply, amplitude modulation is achieved.
Rather than giving detailed examples, this page gives simplified schematics, followed by links to circuits that actually use the various techniques.
An external audio amp sends its output to the 8-ohm side of an 8-to-1000 ohm (or similar) audio transformer. The other winding of the transformer is inserted in the power supply going to the final RF amp transistor. The audio transformer must be rated to handle the level of power going through it. An inadequate transformer will produce a bad-sounding signal. In some cases it takes a lot of searching and experimentation to find the best transformer.
A lightly amplified audio signal is fed to the base of an NPN transistor. This transistor is inserted into the power supply going to the RF amp transistor. A choke between the two transistors keeps RF out of the power supply and audio circuitry. The RF choke must be rated to handle the level of current going through it.
A line-level audio signal is fed to an audio amplifier I.C. The output of this chip is used as a power supply for the RF amp transistor.
This circuit is physically smaller and lighter than designs that use a modulation transformer. The audio quality is good.
The RF output transistor is an FET (often an IRF510 or similar). The FET's source is grounded. The FET's drain is connected to a transformer, which has the modulation (audio) amp on one side and the RF output taken from the other side.
This is the system used in Charles Wenzel's circuit. This design has had many re-incarnations, for example in a proposed 13.5 MHz transmitter circuit. The Wenzel modulator is capable of high quality modulation at levels approaching 100%. For 100 milliwatt transmitters, suitable transistors are 2N4401 or 2N5551.
The Final (aka Power Amp, Output Amp) is the final stage of a tansmitter. This stage generally has some gain (voltage and current). Shown here are some partial schematics of finals taken from a wide variety of sources. Unless otherwise noted the input is on the left side, output on the right.
This circuit will output 1.2 watts with a 13.8 volt power supply. Value of RFC not critical, but must be able to handle some current, try 15 turns of wire on a toroid core.
This circuit will output 4 to 5 watts with an adequate power supply. The design shown is optimized for 7 MHz; the transformers may need a bit of modification to optimize for other bands. Heat-sinks hould be provided for the transistors.
A low-pass filter attentuates the energy above a specified cut-off frequency. These filters are used to reduce the intensity of harmonics so that they don't interefere with other signals. Most of this filters on this page were designed for various ham radio gear operating in the bands from 1.8 to 14 MHz.
Unless otherwise noted these filters have 50 or 52 ohms input and output impedance. Capacitances are expressed in picofarads, inductances in microhenries. Most authors recommend using silver-mica capacitors.
