Parts List

2-4 Enhanced AM/FM Synthesizer Instruction Manual OS(R/T)-3(A/H) 29 - 470 MHz
and D2. Coarse frequency adjustment is provided by varactor diodes D6, D7. SELECT capacitor
values C40 and/or C23 are chosen to position the operating frequency in one of two bands;
118 - 138 MHz or 139.4 - 159.4.MHz. Varactor diodes D1 and D2 provide fine oscillator
frequency control. PLL feedback control voltage, at the output of the low-pass loop filter, controls
the VCO frequency through the reverse biasing of diodes D1 and D2. The PLL control voltage can
range between ≈ +1.0 Vdc and +4.7 Vdc and is nominally set to ≈ +3.3 Vdc for all synthesizer
frequencies. Setting of the PLL control voltage set point (TP4) is achieved by adjusting the
capacitance of varactor diodes D6 and D7, which are controlled by microcontroller U18 through
digital-to-analog converter U17. Microcontroller U18 controls the output of the PLL Low-pass
Filter and adjusts the output voltage of the D/A converter so that PLL control voltage set point
(TP4) is approximately 3.3Vdc at all synthesizer frequencies. The microcontroller then goes to
sleep in a low-power mode.
The PLL low-pass filter is formed by SELECT components C37, C38, C39, C45, R32 and R36.
The loop filter response is optimized for switching time, noise and modulation requirements
specific to each sub-band within the 118 - 159.4 MHz frequency range. SELECT components
(including the loop filter) can be found in tabular format on the VHF OS(R/T)-3(A/H)
118 - 159.4 MHz Analog Board Schematic diagram.
RF output power is taken from the source of Q5 and amplified/buffered by Q6. U11 provides
further amplification and isolation while d elivering ≈ +10 dBm into a five-pole low-pass output
filter formed by C53, C58, C59, L11 and L12. The five-pole low-pass output filter, with a cutoff
frequency of 190 MHz, effectively eliminates output harmonics. SMB connector J2 provides
interconnection to the companion transmitter or receiver with an output level of ≈ +5 dBm.
2.2.4 128 - 174 MHz Analog Board Circuitry
Refer to the "OS(R/T)-3(A/H) 128 - 174 MHz Analog Board Schematic Diagram" on page 4-11 of
this manual.
Field effect transistor Q5 forms part of the negative resistance VHF amplifier oscillator that is tuned
on-frequency by the combination of resonator L5 and the total capacitive reactance presented across
L5 through capacitors C40 and/or C23 (SELECT), variable capacitor C24 and varactor diodes D1
and D2. Fine frequency adjustment is obtained via multi-turn trimmer capacitor C24. SELECT
capacitor values C40 and/or C23 are chosen to position the operating frequency in one of two
bands; 128 - 152.6 MHz or 150 - 174 MHz. Varactor diodes D1 and D2 provide oscillator
frequency control. PLL feedback control voltage, at the output of the low-pass loop filter, controls
the VCO frequency through the reverse biasing of diodes D1 and D2. The PLL control voltage can
range between ≈ +1.0 Vdc to +7.0 Vdc and is nominally set to ≈ +4.5 Vdc at the synthesizer centre
frequency. Setting of the PLL control voltage set point (TP4) is achieved by adjusting fine
frequency variable capacitor C24. External baseband frequency modulation is p rovided through
connector P1 and a voltage divider network formed by R21 and R22. A large signal division ratio,