carried out to establish the modulation linearity
of a typical SSB (Single Side Band) transceiver.
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.
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.
basic oscilloscope such as the aging dwd 826 pictured and an audio
signal source is the final pieces of equipment required.
this example the audio signal generator was a software application
known as Audio SweepGen – Version 22.214.171.124 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 126.96.36.199 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.