The licence (UK Amateur Licence, Section 7 : Equipment) requires tests to be carried out from time to time to ensure that there are no excessive spurious or harmonic emissions in addition to checking the frequency and bandwidth of the transmission.

The problem is that it is not easy to directly check that a transmitter is within its original specification because of the requirement for a receiver capable of covering up to 6x the fundamental frequency with a dynamic range of at least 60dB (see typical harmonic performance). It also requires the transmitter to be used with a dummy load of adequate dissipation and the signal taken from an rf sampler in the feed line in order to be able to search for spurious emissions as well as the harmonics.

So the question posed was: Is there an easier way in practice to check for compliance?

It was suggested that perhaps harmonics have not been such a problem after all. This may be because a component failure in the final stage of a properly designed transmitter or its low pass output filter could reduce the attenuation of the harmonics but would also most likely alter the loading reducing the output power, or result in a catastrophic failure. In either scenario you would notice something was not right! In addition the ATU could also provide some attenuation and the poor match of the harmonics to the antenna would further reduce the harmonic energy radiated.

In order to test the above hypothesis a fault analysis was carried out using a simulation of the output stages of a typical transmitter. The Kenwood TS-440 was chosen and modelled using LTspice (link to the analysis). To down load the model for the TS_440, right click on the file name ts_440.asc and choose ‘save link as …’.

It was found that out of the faults that would cause increases in the harmonics :-

  • 20% would result in failure due to over current in the output transistors.
  • 20% would cause failure due to overvoltage of the output transistors.
  • 20% would result in a very noticeable loss of output power and high losses in the transistors causing rapid thermal cut out.
  • 40% increased the level of the harmonics, but the worst case was still 54dB below the full power fundamental, and therefore only had a marginal effect. These components were ahead of the low pass filter section and were there to improve the linearity of the amplifier or to protect the transistors from the stray inductance of a real circuit.

A simple way to check the linearity of the output stage is to determine the bandwidth of the SSB signal. This is because the non-linearity produces intermodulation distortion which cause the bandwidth of the signal to spread out wider than normal, as well as generating harmonics (link to IMD theory).

Measuring the SSB bandwidth would also check the integrity of the rest of the signal path from the microphone to the final output stage. The TS-440 is typical in having the signal routed through four mixers, each one supplied by a phase locked loop oscillator (PLL). The PLL filter output voltage goes directly to its associated voltage controlled oscillator (VCO). Any degradation increasing the noise and spurious frequencies at this point will modulate the oscillator causing FM side bands. The widening of the signal in this manner will not be removed by the filters following the mixers and will show up as a broadening of the final output, as well as adding to the number of spurious signals (notes on the PLL).

The modulator for a typical FM transmitter generates a single output component of constant amplitude but varying frequency. Non linearity in the power amplifier therefore cannot cause intermodulation distortion because there is no other frequency component to intermodulate with. Hence the increased non linearity of a Class C amplifier compared to a Class B amplifier is often accepted in exchanged for increased efficiency. The extra harmonic content in the output is then dealt with by more extensive low pass filtering. Capacitor or inductor failures in the output filtering can be expected to reveal themselves by abnormal low output power or by catastrophic failures as previously argued for the TS-440. Faults in the audio section and increased noise at the input to the VCO can however lead to over deviation increasing the band width of the transmitted signal. The signal path for a typical FM transmitter is shown here.

Bandwidth Measurement Methods

1. At HF Frequencies:-

  • The easiest method is to monitor the transmission using the waterfall display on the nearest on-line SDR receiver while counting up to forty.
  • Alternatively you could ask a local amateur station to check your bandwidth. This can be done by using the upper or lower side band mode. The other station tunes up to your signal until it is just too feint to hear and notes the frequency. Then tunes across the signal until it is again too feint to hear. Your signal bandwidth is then the difference between these two frequencies minus the bandwidth of the other stations receiver.

2. At VHF and UHF Frequencies:-

  • The easiest method is to transmit into a dummy load and use an RTL-SDR dongle plugged into a laptop or desktop computer to monitor the signal using a short length of wire draped near the load as an antenna. The dongles are still available for approximately £6 on the e-bay (2018), the SDR software is free, details can be found here.

Conclusions

Component failures in the low pass output filter that could reduce the attenuation of the harmonics can be expected to reveal themselves by either causing a catastrophic failure of the transmitter or by a very noticeable reduction in output power.

Increases in harmonic distortion brought about by overdriving or component failure in the power amplifier widens the SSB signal bandwidth as well as increasing the harmonic content in the output. The output can also be broadened by degradation of components in the rest of the signal path from the microphone. This is particularly true for any parts associated with the phase locked loop oscillators in the circuit.

Similarly, faults in the audio section and increased noise at the input to the VCO in an FM transmitter can lead to over deviation increasing the band width of the transmitted signal.

Hence in practice, periodically calibrating the frequency and checking the bandwidth of the SSB, CW and FM output signals using a suitable SDR receiver is sufficient to demonstrate that the transceiver remains within its specification. This provides an easy and speedy way to meet all of the Licensing requirements on equipment emissions tests on established designs.