Basic Fm Transmitter Test: Power, Frequency And Deviation - Motorola R-2200 Operator's Manual

3. Place the pick-up loop in close proximity to the
receiver so that the local oscillator signal is coupled
to the monitor input. Using the frequency error
function, adjust the local oscillator for the desired
frequency.
4. Change the "Function" switch to "Hi Gen" and
enter the proper carrier frequency. Introduce suffi-
cient signal level into the RF input to fully quiet the
receiver and adjust the discriminator to zero volts
DC for receivers requiring such an adjustment. For
receivers with quadrature or more critical detectors,
adjust detector tuning control or local oscillator in-
jection frequency for minimum distortion or best
SINAD performance.
5. For pagers with the battery saver feature, check the
local oscillator for pulse operation without remov-
ing the unit from it's housing.
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TRANSMITTER
FIGURE 5.6- TRANSMITTER TESTS SETUP
5.10.0 BASIC FM TRANSMITTER TEST:
POWER, FREQUENCY, AND DEVIATION
A. Basic Terminated Measurements
1. Connect the service monitor's "RF In/Out"
port to the RF output of the unit under test
and make the other connections as shown in
figure 5.6.
CAUTION: For power output measurements, connect a
transmitter only to the service monitor's "RF In/Out"
port. Do not connect it to the service monitor's "An-
tenna" port.
The built-in RF load dissipates up to 50 W for three
minutes and up to 125 W for one minute. If a technician
keys a high-power transmitter into the monitor for a
time long enough to threaten overheating the power-
measuring circuitry, the system's audible alarm sounds,
warning to un-key.
5-5
2. Set "Function" switch to "Pwr Mon" (Or "Pwr
Mon/ Anlzr On"), "FM", and "Squelch" control
to threshold. Enter the correct transmitter carrier
frequency.
3. Set the step attenuator to -30 dB. For low-power
transmitters, it may be necessary to reduce the at-
tenuator setting to unsquelch the monitor. Too
high an attenuator setting or too tight a squelch
setting inhibits the frequency error display. Ac-
curate measurements require sufficient signal
level from the radio to fully quiet the service
monitor's receiver.
4. Use good quality cables of minimum length be-
cause cable-loss can be a significant factor in RF
power measurement, especially at UHF and
above. Key the transmitter and read the power
indicated on the display.
5. Read the transmitter's frequency error indicated
on the display.
6. Refer to the radio's service manual to determine
if power and frequency are within spec limits and
determine if any adjustments are required.
7. Proceed to check modulation as follows:
a. Turn off all modulation sources at the "Mod
Out" of the service monitor in order to check
the level of PL or DPL deviation from the
transmitter. Switch "Function" to "Pwr Mon"
to observe recovered audio.
b. Key up transmitter with appropriate PL or
DPL enabled and read "Dev" from the left
LCD. Refer to radio service manual to deter-
mine if adjustment is necessary.
c. With PL or DPL modulation properly set, in-
troduce I KHz modulation to transmitter
audio input by adjusting the "1 KHz Lvl" con-
trol. Check service manual to determine the
minimum level required for proper MIC sen-
sitivity as well as the maximum level required
to insure proper IDC function. Read "Mod
Out" levels on the DVM by temporarily
switching the left LCD to "DVM", and
"Source" to "AC".
d. At the maximum audio input level, read
"Dev" on the display and verify that it falls
within the maximum rated system deviation
specified in the radio's service manual. Make
adjustments as required.
e. Verify proper MIC sensitivity by setting "Mod
Out" to the minimum threshold level per the
service manual and reading corresponding
"Dev" on the display.
B. Off-The-Air Measurements
1. Connect the TEKA-24A pick up antenna to the
service monitor's "Antenna" connector. Change
the "Function" switch to "Sens Mon" (or "Sens
Mon/ Anlzr On")
2. Operate the transmitter under test either into its
own antenna or into a dummy RF load, as shown
in figure 5.6.
3. Set the service monitor's "Squelch" control to
threshold, and enter the correct carrier frequency.