Crown Low Power Transmitters - Crown Broadcast

5.8 Bias Set (RF Power Amplifier)
The Bias Set trim pot is located inside the PA module on the input circuit board.
Set the trim pot to its midpoint for near-optimum bias.
5.9 Performance Verification
Measure the following parameters to receive a comprehensive characterization of
transmitter performance:
❑ Carrier frequency
❑ RF output power
❑ RF bandwidth and RF harmonics (see section 5.12)
❑ Pilot frequency, phase, and modulation percentage
❑ Audio frequency response
❑ Audio distortion
❑ Modulation percentage
❑ FM and AM noise
❑ Stereo separation between left and right
❑ Crosstalk between main channel and subcarrier
❑ 38–kHz subcarrier suppression
In addition to the above tests, which pertain to signal quality, a complete check of
the unit will include items listed in section 5.21.
5.9.1 Audio Proof-of-Performance Measurements
References to “100%” modulation assume 9% pilot and 91% for the remainder of
the composite stereo signal.
Because the audio processing threshold is at 90% modulation, it is not possible to
make audio proof-of-performance measurements at 100% modulation through the
audio processor. Instead, data is taken at a level below the audio processing
threshold at 80% modulation.
5.9.2 De-emphasis Input Network
A precision de-emphasis network, connected between the test oscillator and the
audio input of the transmitter, can be very helpful when making the audio
measurements. Note that the input impedance of the transmitter or the source
impedance of the test oscillator can affect network accuracy. With the de-emphasis
network, oscillator level adjustments need only accommodate gain errors, instead
of the whole pre-emphasis curve.
Adjustments and Tests
5–7