The output power and hence the current drawn by the output stage of an SSB transmitter varies at a syllabic rate, ie in accordance with the speech waveform. Measurement of input power is therefore not possible because the conventional meter is much too slow in operation to follow the rapid variation of the input current.
The power rating of an SSB transmitter is therefore expressed as peak envelope power (PEP) as derived in Chapter 4. This is the power which exists at the peaks of the speech waveform. The maximum permitted PEP is 400W (26dBW).
The recommended method of measuring PEP is to monitor the output of the transmitter on a dummy load, by means of a cathode-ray oscilloscope, when the transmitter is modulated by the output of a 'two-tone generator' as shown in Fig 10.8. This device contains two AF oscillators which produce non harmonically related sinusoidal tones of equal amplitude which are combined.
Fig 10.8. Method of measuring PEP output in relation to mean output using a two-tone source. Power output = IČ / R; peak envelope power = 2 x mean power represented by V
Fig 10.9. Speech peaks should not exceed level determined by V in two-tone test
The transmitter operating level is set to produce a mean power output in the dummy load of 200W as measured by the current flowing through it. The pattern produced on the oscilloscope will be as in Fig 10.8 and the limits of the deflection V should now be marked by two thin lines drawn by a chinagraph pencil. The mean power as just measured (200W) is equivalent to a PEP of twice this value, ie 400W. Thus the two lines on the 'scope face represent the deflection which corresponds to an output of 400W PEP. It is not essential to set the transmitter output to 200W mean, any power will do, but 200W is a convenient level because it causes a deflection which corresponds to the maximum PEP permitted.
To avoid waveform errors, the current in the load resistor is measured by a thermocouple meter. If an RF voltmeter is used to measure the voltage across the load, waveform errors in the voltmeter must be taken into account in calculating the power output.
When high-power equipment is used, the overall accuracy of the power measurement must be known and taken into account in calculating the power in order to avoid exceeding the licensed power.
When the two-tone generator is replaced by the microphone, the oscilloscope now shows the extremely peaky speech waveform (see Fig 10.9). The maximum deflection must not be allowed to exceed that deflection which corresponds to 400W PEP.
Other power levels may be determined as follows. Suppose the deflection corresponding to 400W PEP is 5cm and the deflection resulting from peaks of speech is 2.5cm. The PEP is then:
Note that the deflections are squared. This is because the deflection on the oscilloscope is proportional to the voltage which causes it, whereas power is proportional to the square of the voltage.
It will be appreciated that the majority of SSB transmitters and transceivers have outputs of 200W or less. The necessity for PEP measurement only really arises when a high-power transmitter is in use, or where a transceiver is followed by a linear amplifier, many of which are rated at power levels greater than that permitted by the UK licence.
The procedure described above and illustrated in Fig 10.8 can be used to measure the power output of a transmitter operating in any mode. A single tone of a convenient frequency (say 1000Hz) should be used to modulate an AM transmitter. The modulation depth should be adjusted to 100% by variation of the tone input to the modulator when the transmitter is operating at the input intended. If CW or FM is in use, the transmitter should be in the 'key down' or 'transmit' state respectively.
