Extracted from LBI-38379C
Maintenance Manual for the M-PAtm
403-512 MHz Portable FM Radio
Field RF Retuning Procedures
These field RF retuning procedures will be necessary if the
factory tuned 20 - 22 MHz operating band does not meet needed requirements. For example,
the factory tuned 450 - 470 MHz band can be retuned to cover the 440 - 460 MHz band.
Factory tuned RF bands are listed in Table 1 of this
manual and on the specification page. Synthesizer and receiver circuits must be retuned
when the operating band is moved. Note that if the radio is retuned, the frequency range
limits of the RF board cannot be exceeded. Perform the procedures in the order presented.
SET-UP PROCEDURE
- Program the radio with the low, middle and high-side retune
channels in the desired operating frequency band. The low and high-side channels should be
20 - 22 MHz apart and the middle channel should be centered between the two. Do not exceed
the frequency range limits for the RF board.
- Separate the Front and Rear Cover Assemblies and connect the
RF/Logic Extender cable between the RF Board and the Control Board.
SYNTHESIZER RETUNING
- Set the radio to the middle retune channel. Monitor the VCO
tuning voltage at TP1 (A5 pin 3) with a dc volt-meter. Adjust the RX VCO coil
for a reading of 2.5 Vdc. See figure 2.
- Set the radio to the lowest retune channel. The monitored
voltage should be greated than 1.0 Vdc.
- Set the radio to the highest retune channel. The monitored
voltage should be less than 4.0 Vdc.
- If the lowest and highest retune channels puch the VCO tuning
voltage mesaured at TP1 outside of the specified limits, slightly readjust the RX VCO
coil to center the low and high retune channel tuning voltages within or around this
1.0 - 4.0 Vdc window.
- Repeat steps 1-4 for the TX VCO coil with the
following exceptions: The radio must be keyed when adjustments are made to the TX VCO
coil. Also, connect and appropriate RF load to the antenna before keying.
- Turn the radio off and connect the PC Programmer to the
radio. Power the radio back up. Using the PC Programmer, set the modulation tracking Data
value to OD (hex).
- Set the radio to the middle retune channel.
- Couple a modulation analyzer (with a monitoring oscilloscope)
to the RF output of the radio. Set the modulation analyzers filters as follows: No
high-pass filters and a 20 kHz low-pass filter. The modulation analyzer should have an
almost dc response. If it does not, an alternative is to use a second M-PA monitoring the
transmit frequency and monitor the discriminator output from the RF Board with a dc
coupled oscilloscope. The monitoring point is J101 pin 4.
- Apply a 110 mV rms, 1 kHz tone to the TQ-3370s
modulation input. Key the radio from the TQ-3370 and adjust R19 on the RF Board for
a deviation of +/- 4.3 kHz.
- Modify the RF/Logic Extender cable so a low-frequency signal
can be injected directly into the RF Board. This modulation procedure is outlined in the TRANSMITTER
ALIGNMENT of this manual.
- Using a 100uF (or greater) capacitor, couple 20Hz, 1.0 Vp-p
square wave signal into the TX AUDIO (J102 pin 1).
- Key the radio and monitor the demodulated output from the
modulation analyzer. Adjust R18 (modulation balance pot) for a good square wave
response.
RECEIVER RETUNING
Network Analyzer Method
- Remove the solder mask on the solder side of the RF Board at
the thin run between the output of FL4 and the input of A6 (A6 pin 1). Cut this run.
- Solder the Center conductor of a small 50-ohm coax cable to
FL4s output at the cut run, Solder the coax shield near FL4.
- Terminate the radios antenna with a 50-ohm load.
- Connect the other end of the coax to the network analyzer and
tune FL4 (all three sections) for best return loss of the desired receive freqneucy range.
- Un-solder the coax cable from the output of FL4 and solder it
to the input of A6. Ground the shield near A6.
- With the network analyzers output at -30 dBm or lower,
adjust FL1 (all three sections) for best return loss over the desired receive frequency
range. The receiver must be powered-up during this adjustment.
- With a small jumper, reconnect the output of FL4 to the input
of A6. Test the radio over the new frequency range.
Spectrum Analyzer And Best Quieting Methods
This procedure involves retuning the receivers helical
filters at the low, middle and high-side frequencies. The same frequencies used to retune
the VCO can be used.
The preferred method is to connect a small 50-ohm coax to
the output of IF crystal filter FL2 to ground, and connect this cable to a spectrum
analyzer tuned to 45 MHz and set to 2 dB/division.
A less desirable alternative using the same tuning sequence
can be achieved by varying the signal generators level and tuning for the best
quieting (SINAD). No coax cable connectins are required using this procedure.
- Remove the input and output (outer) runing cores from FL1 and
FL4 (four cores total).
- Set the radio and signal generator to the low-side receive
frequency.
- Set the radio and signal generators level to -30 dBm.
If tuning by quieting, set the generators level for a high receiver noise level.
- Alternative tune the center cores of FL1 and FL4 for maximum
IF signal. If tuning by quieting, tune for sensitivity and reduce the generators
level as the receiver becomes more sensitive.
- Set the radio and signal generator to the high-side receive
frequency.
- Reinstall the input tuning cores into FL1 and FL4.
Alternately adjust these cores for maximum signal or best quieting.
- Set the radio and signal generator to the middle receive
frequency,
- Reinstall the output tuning cores into FL1 and FL4.
Alternately adjust these cores for maximum signal or best quieting.
- Under certain circumstances, best tuning may be achieved when
certan tuning cores are not reinstalled. This is acceptable. Also any excessively loose
core should be secured with a drop of adhesive.
TABLE 1 - Test Frequencies (Mhz)
RF Board
Freq. 1 Freq. 2 Freq. 3
19D902282G1 403.025
413.025 422.975
19D902282G6 450.125
460.025 469.975
19D902282G3 470.025
481.025 491.975
19D902282G8 492.025
503.025 513.975
TABLE 2 - Frequency Error
Specifications
RF Board
Frequency Split
Maximum Error
19D902282G1
403-440 MHz
+/- 248 Hz
19D902282G6
440-470 MHz +/- 276 Hz
19D902282G3
470-492 MHz
+/- 288 Hz
19D902282G8
492-514 MHz
+/- 302 Hz
TABLE 3 - Tracking Data Parameters with Factory Settings
Parameter Factory Setting
High Power 5.0 - 5.2 watts
Low Power 4.2 - 4.4 watts
Modulation 4.2 - 4.4 kHz
Squelch Opening 8 - 10 dB SINAD
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