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SINGLE SIDE BAND TWO TONE TEST
SSB two tone test to determine modulation linearity

Procedure carried out to establish the modulation linearity of a typical SSB (Single Side Band) transceiver.  

It is often necessary to measure and confirm the linearity of SSB (Single Side Band) transceiver, amplifies and transverters to ensure signal fidelity and to prevent adjacent channel interference. The standard two tone test applies two audio frequency tones to the transceiver microphone from a quality sinusoidal generator at approximately 700 and 1900Hz. The exact audio frequency is not critical other than they cannot be harmonically related and should be sufficiently separated with in the audio band pass. Both audio frequency tones need to produce a near identical independent modulation display on the oscilloscope.

The equipment required is a large dummy load capable of dissipating the RF at the power level to be tested and to correctly satisfy this test it should be completed at near full transmit power (Short duration as the transmitter is running at 100% duty cycle). In this case the dummy load also had a sample signal output however if a dummy load was used that did not have this feature it would be necessary to include a sampling circuit.

A basic oscilloscope such as the aging dwd 826 pictured and an audio signal source is the final pieces of equipment required.

In this example the audio signal generator was a software application known as Audio SweepGen Version 3.7.4.36 that generates two audio tones out puts one left and one right.  By locating the microphone in relation to the two stereo speakers to produce a near identical independent modulation display on the oscilloscope.

Photo 1 The rather long in the tooth dwd 826 oscilloscope used for the measurement.

 

Photo 2 Audio SweepGen - Version 3.7.4.36 lite software is used to generate the two audio tone frequencies.

 

Photo 3 The trace showing an ideal sinusoidal patter with clean zero cross over indicating very good modulation linearity.

References

 

SweepGen http://www.satsignal.eu/software/audio.html#SweepGen

 

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Page last revised 03 December 2014  
 

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