Back To Basics
From the Canadian Basic Question BankBack To Basics is a regular column in the SARC Communicator Newsletter, available on this blogsite.
In designing an HF station, which component would you use to reduce the effects of harmonic radiation?
A. Dummy load
B. Antenna switch
C. SWR bridge
D. Low pass filter
The term harmonic is employed in various disciplines, including music and acoustics, electronic power transmission, radio technology, etc. It is typically applied to repeating signals, such as sinusoidal waves. A harmonic of such a wave is a wave with a frequency that is a multiple of the frequency of the original wave, known as the fundamental frequency. The original wave is also called the 1st harmonic, the following harmonics are known as higher harmonics. For example, if the fundamental frequency is 60 Hz, a common AC power supply frequency, the frequencies of the first three higher harmonics are 120 Hz (2nd harmonic), 180 Hz (3rd harmonic), 240 Hz (4th harmonic) and any addition of waves with these frequencies is periodic at 60 Hz.
Early in the development of radio technology it was recognized that the signals emitted by transmitters had to be 'pure'. Spark-gap transmitters were outlawed once better technology was available as they give an output which is very wide in terms of frequency. The term spurious emissions refers to any signal which comes out of a transmitter other than the wanted signal. In modern equipment there are three main types of spurious emissions: harmonics, out of band mixer products which are not fully suppressed and leakage from the local oscillator and other systems within the transmitter.
These are multiples of the operation frequency of the transmitter, they can be generated in any stage of the transmitter which is not perfectly linear and must be removed by filtering.
Avoiding harmonic generation
The difficulty of removing harmonics from an amplifier will depend on the design. A push-pull amplifier will have fewer harmonics than a single ended circuit. A class A amplifier will have very few harmonics, class AB or B more, and class C the most. In the typical class C amplifier, the resonant tank circuit will remove most of the harmonics, but in either of these examples, a low pass filter will likely be needed following the amplifier.
Removal of harmonics with filters
In addition to the good design of the amplifier stages, the transmitter's output should be filtered with a low pass filter to reduce the level of the harmonics. Typically the input and output are interchangeable and match to 50 ohms. Inductance and capacity values will vary with frequency. Many transmitters switch in a suitable filter for the frequency band being used. The filter will pass the desired frequency and reduce all harmonics to acceptable levels.
The harmonic output of a transmitter is best checked using an RF spectrum analyzer or by tuning a second receiver to the various harmonics. If a harmonic falls on a frequency being used by another communications service then this spurious emission can prevent an important signal from being received. Sometimes additional filtering is used to protect a sensitive range of frequencies, for example, frequencies used by aircraft or services involved with protection of life and property. Even if a harmonic is within the legally allowed limits, the harmonic should be further reduced.
Oscillators and mix products
When mixing signals to produce a desired output frequency, the choice of Intermediate frequency and local oscillator is important. If poorly chosen, a spurious output can be generated. For example if 50 MHz is mixed with 94 MHz to produce an output on 144 MHz, the third harmonic of the 50 MHz may appear in the output. This problem is similar to the Image response problem which exists in receivers.
Looking at the possible answers above, A. a dummy load is not applicable as it takes the place of an antenna for tuning and servicing a transmitter. B. An Antenna switch merely switches antennas… and has nothing to do with harmonics. C. An SWR bridge measures reflected power, not harmonics, so D. A low pass filter remains.
A low-pass filter is a filter that passes signals with a frequency lower than a certain cutoff frequency and attenuates signals with frequencies higher than the cutoff frequency. The exact frequency response of the filter depends on the filter design. The filter is sometimes called a high-cut filter, or treble cut filter in audio applications. By attenuating (reducing) signals above the fundamental frequency we can effectively reduce harmonics.
The correct answer to our question therefore is D. Low pass filter.
~ John VE7TI