from the phase différence in the signais fed to it. This DC output is applied through an LPF to the
VCO, forming the phase loop. This DC voltage applied to the VCO causes it to shift frequency until
its output signal locks up with the count-down frequency provided from référencé oscillator Q3
[when two signais are in phase] at which point no DC output is produced in the phase detector, and
th VCO remains
locked
on frequency. When a new channel is selected a new
N
code is applied to the
programmable devider. The VCO is no longer locked because of the resulting phase différence in the
phase detector, and it again shifts frequency to a locked condition, in turn producing 37 MHz
output signais corresponding to the new channel programmed by the new N code. I n summary it w ill
be seen that a stable VCO frequency range w ill be produced, each specific
frequency
being deter-
mined by the N code selected by the Channel Selector switch.
Main Board Assembly (PTBM059)
The crystal oscillator Q12 is operating at 10.695 MHz in the AM/FM/USB mode, and 10.692 MHz
in the LSB mode, controlled by the crystal, X3. This signal is #1 ; in the AM and FM modes of trans
mission, fed to the IC3 to be mixed with the first TX local oscillator frequency and result in 27 MHz
transmitter frequencies, and #2; in the SSB modes of transmission, modulated through the balanced
modulator IC4 with the audio output signal from the microphone amplifier, IC5. The résultant
output of the balanced modulator is a double sideband, suppressed carrier signal. The crystal filter,
XF, pass band is restricted to 3.5 kHz so that it allowsonly one sideband to pass through its output
terminais, either USB or LSB mode, depending upon the Mode Selector switch sélection. The exact
frequency of which was determined by the Channel Selector switch sélection and the PLL circuitry,
as previously outlined, the résultant frequency, therefore, that is fed to the RF amplifier in IC3, is the
channel frequency on the channel selected [channel 1 through 120 over 26.965 to 28.305 MHz].
The 27 MHz RF amplifier output is coupled to RF predriver transistors, Q7, 8, through T4, 5. The
predrivers serve to isolate the oscillator and mixer stages from the output amplifiers, and at the same
time provide a certain amount of power gain. Q8 output is applied to the base input of Q9, the RF
driver stage and in turn to the Q10, the RF power output stage of the transmitter. These stages ampli-
fy the 27 MHz RF signal resulting in an output at L13 of 4W (or 0.5W) in AM or FM mode, and
12 watts PEP [peak envelope power] in the SSB mode.
TX Diagram
MIC
SSB MOD
TX
OUT
Modulation Circuit
AM:
The microphone feeds voice audio through Q701 on PTSW072 to the power audio IC IC5, and
finally to collectors of Q9 and final RF power amplifier Q10 through T 16, thereby amplitude modu-
lating the carrier in AM transmission.
FM:
In the FM mode, IC5 output is led to the anode of the FM modulating variable capacitor D5
(also involved for 'Tune' control) in circuit board assembly PTOS006, varying its bias to change
parallel capacitance to X2, X3, or X4, finally giving déviation to PLL output frequency.
SSB:
The IC5 output is directly fed to the balanced modulator IC4, resulting in suppressed carrier
double side band, which is in turn supplied to the crystal filter to carrier removal.
ALC:
An audio ALC [automatic level control] voltage derived from the audio signal at Q35 is fed
to IC5 to control the output of audio amplifier to prevent overmodulation. In the AM or FM
transmission, the output o f Q35 is led to Q37 and is used to control the output o f T16, whereas in
the SSB transmit mode, the output of Q35 is fed to Q38 and is connected to the primary side of T16.
This being due to the fact that the output
of
IC5 [modulation signal] is fed [to modulate the RF
signal] from the secondary side of T16 in the AM or FM mode, and from the primary side of T16 in
the SSB mode of transmission.
The transceiver is also equipped with the RF ALC circuit utilizing the RF output induced at the
input of L12 [in the SSB mode only]. The minus voltage detected through D8 is applied to the DC
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