Monday, January 23, 2012

Crystal oscillators and radio transmitters

Since the very early days of ham radio, crystals have been used to provide stable and precise source of transmitter frequency control. In addition, they have also been used in filter circuits to control bandwidth and prevent the reception of unwanted signals.

A quartz crystal from my collection. It is a FT-243 style holder, suitable for use in a vacuum tube transmitter.
Early transmitters used variable capacitors and inductors to create a self resonant tuned circuit. One drawback to this simple design is that it could change frequency depending on temperature, humidity, supply voltage or the proximity of conductive objects (Including the operator!). Shielding and temperature compensated components helped to reduce frequency drift but it was still common practice to follow the transmit frequency of another operator as both your receiver and his transmitter drifted.

The QRP sized AMECO AC-1. Using a single 6V6GT as oscillator and output tube.
From 1951 holders of the Novice class license were only permitted to use crystal controlled transmitters limited to 75 Watts. They were also further restricted to a small segment (3.700 - 3.750 Mc) in the 80M band. This had the side effect of popularizing simple crystal controlled transmitters using either a single vacuum tube as both oscillator and output amplifier or transmitters using one tube for the master oscillator circuit and one for the power amplifier, also known as MOPA (Master Oscillator, Power Amplifier).

The Heathkit AT-1 transmitter. One oscillator tube 6AG7 and one power amplifier tube 6L6.
In either case a crystal was required for every frequency you wanted to transmit on and few new hams could afford more than a couple of crystals for their transmitter. Often you would call CQ and listen up and down the band for someone replying to your call as the other operator would not necessarily have the same frequency crystal as yours.

But how do crystals work?

From Wikipedia:

A crystal oscillator is an electronic oscillator circuit that uses the mechanical resonance of a vibrating crystal of piezoelectric material to create an electrical signal with a very precise frequency. This frequency is commonly used to keep track of time (as in quartz wristwatches), to provide a stable clock signal for digital integrated circuits, and to stabilize frequencies for radio transmitters and receivers. The most common type of piezoelectric resonator used is the quartz crystal, so oscillator circuits designed around them became known as "crystal oscillators."

Quartz crystals are manufactured for frequencies from a few tens of kilohertz to tens of megahertz. More than two billion (2×109) crystals are manufactured annually. Most are used for consumer devices such as wristwatches, clocks, radios, computers, and cellphones. Quartz crystals are also found inside test and measurement equipment, such as counters, signal generators, and oscilloscopes.

An electric current causes the quartz crystal to deform.

When a crystal of quartz is properly cut and mounted, it can be made to distort an electric field by applying a voltage to an electrode near or on the crystal. This property is known as piezoelectricity. When the field is removed, the quartz will generate an electric field as it returns to its previous shape, and this can generate a voltage. The result is that a quartz crystal behaves like a circuit composed of an inductor, capacitor and resistor, with a precise resonant frequency.

Deforming the quartz crystal causes an electric current to flow

Quartz has the further advantage that its elastic constants and its size change in such a way that the frequency dependence on temperature can be very low. The specific characteristics will depend on the mode of vibration and the angle at which the quartz is cut (relative to its crystallographic axes).[8] Therefore, the resonant frequency of the plate, which depends on its size, will not change much, either. This means that a quartz clock, filter or oscillator will remain accurate. For critical applications the quartz oscillator is mounted in a temperature-controlled container, called a crystal oven, and can also be mounted on shock absorbers to prevent perturbation by external mechanical vibrations.