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NOISE.
Noise measurements should be made first, since high noise levels will influence distortion readings. Also ground
loops in the audio oscillator to transmitter connections and distortion analyzer to receiver connections must be
resolved before testing begins. The influence of high RF fields upon the test equipment must be determined and
corrected before accurate measurements can be made. NOTE: NOISE AND DISTORTION MEASUREMENTS
ARE MADE WITH SUBCARRIER AND REMOTE CONTROL INPUT SIGNALS REMOVED. System signal to
noise ratio is determined while modulating the transmitter 100% at 400 Hz. A level of +8 dBm across the balanced
audio input terminals of TB–1 will produce a reading of 100% modulation on the "PEAK MODULATION"
indicator. Set Receiver "PGM LEVEL ADJ" pot for +10 dBm output into the distortion analyzer. If the distortion
analyzer has a high impedance input, add a 600 ohm load resistor to match the receiver. Establish +10 dBm on the
audio voltmeter of the analyzer as the reference level for 100% modulation. Next, remove the audio signal from the
transmitter input and measure noise level below reference (100% modulation). This reading should compare with
that published under SYSTEM SPECIFICATIONS in this manual.
DISTORTION.
Harmonic distortion is usually measured at 100% modulation and at several frequencies. If pre–emphasis processing
is used in the transmitter with corresponding de–emphasis in the receiver, it is normal for available audio level at the
receiver to drop with increasing frequency according to the de–emphasis curve selected. At 15 KHz, there is
sufficient level to operate most modern distortion analyzers. Distortion levels should be within specifications. If
distortion is out of specs, check system noise, check for test equipment ground loops, RFI, and transmitter/receiver
operating frequency. If either unit is off frequency, the FM modulation sidebands are not centered within the IF filter
bandpass, which can cause audio distortion.
FREQUENCY RESPONSE.
If the STL–10A System is switched to flat processing, frequency response can be measured as if the signal were be-
ing sent over straight wires. If pre–emphasis processing is used (especially 75 micro–seconds) allowance must be
made in the transmitter audio input level to prevent over–modulation at test frequencies above 400 Hz. The simplest
and fastest method is to set the transmitter audio input level for 100% modulation at 400 Hz, then attenuate this level
20 dB. Set receiver output level to –10 dBm as the reference, then sweep the audio band for response. Response
should be within limits published in the SYSTEM SPECIFICATIONS.
STL–10A THEORY OF OPERATION.
Refer to the STL–10A block diagram in the DRAWINGS AND PARTS LIST section for the following discussion.
POWER SUPPLY 800–168.
Power supply circuit board 800–168 contains filter capacitor C5, and reverse polarity protection diodes D5, D6 and
D7. The unregulated output voltage of this supply is 17–20 volts. The voltage is supplied from bridge rectifier
D1–4 on circuit board 800–168.
INPUT/OUTPUT FILTER 800–193A.
Radio–frequency filters are installed on all input/output circuits connected to ACCESSORY connector J4, as well as
the AC line input.
CONTROL BOARD 800–197.
Applies an operating voltage to the unit.
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