TTI Freequency TCB-880, Руководство по обслуживанию

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4-5. Description of other Circuits
4-5-1. Transmitter
A. RF Amplification
RF carrier frequencies are obtained at the output of doublers amplifier Q701 and turning IFT coil
L701, L702. The input of VCO frequencies 13MHz is selected at the doublers output, which creates the
necessary 27MHz. The output of doublers amplifier Q701 is fed through turning IFT coil L701, L702 to
the base of pre driver amplifier Q702. It’s output; 27MHz is coupled to RF driver amplifier Q703, a low-
level class C power amplifier. Driver Q703 supplies the necessary power gain to operate RF final Q704 at
the maximum efficient. The output of Q704 is supplied to the antenna through L-C turning circuit.
B. Circuit for Suppression of Spurious Radiation
The turning circuit between the output of final amplifier Q704 and antenna, 4-stage “
π
”
network C722, C723, C724, L711, C725, C726, L712, C729, C731, L732, C732, C733, C734, L714,
C735 serves as a spurious radiation suppressor. This network also servers to match the impedance
between TX power amplifier Q704 and the antenna.
C. Circuits for Limiting Power
After finished all alignment, the constant voltage supply circuit limits the available power 4W or
slightly less. RV911 and corresponding three-transistor control supply voltage of RF power amplifier.
When power low switch function Q914 changed the supply voltage. Tune all the trimming parts for
maximum indication of RF power meter and adjust RV571 to make 4W indication of RF power meter.
After finishing the above adjustments check the RF power meter reading is changed 1W under “LOW”
state.
The turning is adjustment so that the actual power is from 3.8W to 4.0W. There are no other
additional controls for adjusting the TX output power.
D.
Modulation Control
Modulation of the RF is a process that begins with the audio picked up by the microphone.
<FM>
The microphone input is fed to mic audio amplifier IC401 that drives modulation vricap diode
D302 in the VCO circuit RV401 limits the incoming modulation audio levels to inhibit over modulation.
While reading the modulation factor on the modulation analyzing equipment, adjustment RV401 shall not
exceed 2.0KHz deviation.
<AM>
Modulation signals are filtered with RC network and goes to the audio power amplifier IC201 in
to make normal signal level to achieve wanted modulation. IC201 drives T201, which is a combination
AF output/modulation transformer. During transmit, one of the secondary windings of T201 is tied
between the 13.2V DC supply and the collectors of RF driver Q703 and RF power amplifier Q704. As the
audio passes through T201 it causes the collector supplies of Q703 and Q704 to vary with audio,
producing an AM signal at carrier frequency.
To avoid over modulation of the carrier, a protection of the modulating signal is fed back from
the T201 through the AMC circuit, Q233 and Q234, to control the gain of IC201 and sets the maximum
level of modulation. Form the center tab of RV231 and Q233 the feedback signal is rectified by diode
D233, filtered by C233, and supplied the Q234. The collector of Q234 is tied directly TX audio input to
control gain. That is, when the audio output is higher that the preset level of RV231, the information is
reflected through Q233 and Q234, reducing the gain of IC201