DANIELS
ELECTRONICS LTD.
TM
3-12
IM10-VR3H040-AB VHF Enhanced Extra Wideband Receiver Instruction Manual
3.3.3
29 - 71.4 MHz Analog Board Circuitry
Refer to the "OS(R/T)-3H 29 - 71.4 MHz Analog Board Schematic Diagram" in section 6.
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 C62, C63, C64, C23 (Select), variable capacitor C24 and
varactor diodes D1 and D2. Fine frequency adjustment is obtained via multi-turn trimmer
capacitor C24 in conjunction with coarse frequency jumper selections JU2, JU3 and JU4. Select
capacitor values are chosen to position the operating frequency in one of three bands:
29 - 38 MHz, 38 - 50 MHz or 50.4 - 71.4 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 varactor diodes D1 and D2. The PLL
control voltage can range between
≈
+1.0 Vdc and +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 combined with binary weighted
lumped capacitor coarse frequency jumpers (JU2, JU3, JU4). External baseband frequency
modulation is provided through connection P1 and a voltage divider network formed by R21 and
R22. A large signal division ratio, established by the resistive dividers R21 and R22, allows low
deviation (less than 5 kHz) direct frequency modulation of the VCO output signal.
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 29 - 71.4 MHz frequency range. The SELECT components
(including the loop filter) can be found in tabular format on the VHF OS-3H 29 - 71.4 MHz
Analog Board Schematic diagram.
RF output power is taken from the source of Q5 and amplified/buffered by U11. U15 provides
further amplification and isolation while delivering
≈
+10 dBm into a six-pole low-pass/notch
output filter formed by C53, C57, C58, C59, L11 and L13. The six pole output filter, with a
cutoff frequency of 50 MHz (OST-3H035, OST-3H045 TX) or 80 MHz (OSR-3H061 RX)
effectively eliminates output harmonics. SMB connector J2 provides interconnection to the
companion transmitter or receiver with an output level of
≈
+5 dBm.
