Variable frequency systems

An array of crystals – used to enable a transmitter to be used on several different frequencies; rather than being a truly variable frequency system, it is a system which is fixed to several different frequencies (a subset of the above).

• Variable-frequency oscillator (VFO)

• Phase-locked loop frequency synthesiser

• Direct digital synthesis

Text 6. Frequency multiplication

For VHF transmitters, it is often not possible to operate the oscillator at the final output frequency. In such cases, for reasons including frequency stability, it is better to multiply the frequency of the free running oscillator up to the final, required frequency.

If the output of an amplifier stage is tuned to a multiple of the frequency with which the stage is driven, the stage will give a larger harmonic output than a linear amplifier. In a push-push stage, the output will only contain even harmonics. This is because the currents which would generate the fundamental and the odd harmonics in this circuit (if one valve was removed) are canceled by the second valve. In the diagrams, bias supplies and neutralization measure have been omitted for clarity. In a real system, it is likely that tetrodes would be used, as plate-to-grid capacitance in a tetrode is lower, thereby reducing stage instability.In a push-pull stage, the output will contain only odd harmonics because of the canceling effect.

Frequency tripler A basic design for a frequency tripler (screen grids, bias supplies and other elements are not shown).

Types of am modulators

A wide range of different circuits have been used for AM. While it is perfectly possible to create good designs using solid-state electronics, valved (tube) circuits are shown here. In general, valves are able to easily yield RF powers far in excess of what can be achieved using solid state. Most high-power broadcast stations still use valves.

Frequency doubler A basic design for a frequency doubler (screen grids, bias supplies and other elements are not shown).

Anode modulation using a transformer.	An example of a series modulated amplitude modulation stage.

Plate AM modulators

In plate modulation systems the voltage delivered to the stage is changed. As the power output available is a function of the supply voltage, the output power is modulated. This can be done using a transformer to alter the anode (plate) voltage. The advantage of the transformer method is that the audio power can be supplied to the RF stage and converted into RF power. With anode modulation using a transformer, the tetrode is supplied with an anode supply (and screen grid supply) which is modulated via the transformer. The resistor R1 sets the grid bias, both the input and outputs are tuned LC circuits which are tapped into by inductive coupling. In series modulated amplitude modulation, the tetrode is supplied with an anode supply (and screen grid supply) which is modulated by the modulator valve. The resistor VR1 sets the grid bias for the modulator valve, both the RF input (tuned grid) and outputs are tuned LC circuits which are tapped into by inductive coupling.

When the valve at the top conducts more than the potential difference between the anode and cathode of the lower valve (RF valve) will increase. The two valves can be thought of as two resistors in a potentiometer.

1. Cheney, M., Uth, R., & Glenn, J. (1999). Tesla, master of lightning. New York: Barnes & Noble Books. Page 71.

2. Ronald Kitchen, RF Radiation Safety Handbook, Butterworth Heinemann 1993, ISBN 0 7506 1712 8 Chapter 10

3. Sleeper, M. B. (1922). Design data for radio transmitters and receivers. Everyday engineering series, [no.] 6. New York: Norman W. Henley Pub.