As Pure As It Gets

Signal Purity

By Jonathan Imberi

All Amateur Radio operators are responsible for the quality of the signals transmitted from their station. The rules require the transmitted signal to be stable in frequency and pure in tone or modulation. An unclean signal is unpleasant to listen to, and it may also cause interference to others using the band or other services. 

Chirp is a common problem with transmitters used for CW operation. Chirp occurs when the oscillator in a transmitter shifts slightly whenever the telegraph key is closed. As a result, other stations receive the transmitted signal as a chirping sound rather than as a pure tone. 

Chirp usually happens when the oscillator power supply voltage changes transmitting occurs. A transmitter may also chirp if the load on the oscillator changes when transmitting. The voltage regulation must be improved if it is noticed that the supply voltage is fluctuating. With better regulation the voltage won't shift when the transmitter is keyed. There are always other causes besides voltage problems. Amplifier stages after the oscillator may be holding it down and pulling its frequency. A better buffer (isolation) or driver stage between the oscillator and the next stage in the transmitter may be required. Some oscillators are also sensitive to temperature changes. If there is too much current through the frequency determining components, their temperature may increase, changing the resonant frequency. 

It is always possible that other signals are causing interference to your station. It can be frustrating trying to track down the source of the interference. Computer systems are often the source of signal interference. Computers include a variety of oscillators and other noise sources. Add this to the many cables and connecting wires coming out of the back of the computer and you can see that there are many ways for unwanted signals to get to your radio. Always use shielded cables when connecting computer equipment to a radio for HF data operation, and be sure to properly ground all of your station equipment. This will help clear up any unwanted computer noise that may get into your receiver.