DE
DANIELS
ELECTRONICS
Enhanced AM/FM Synthesizer Instruction Manual OS(R/T)-3(A/H) 29 - 470 MHz
2-3
2 . 2 . 2
29 - 71.4 MHz Analog Board Circuitry
Refer to the "OS(R/T)-3H 29 - 71.4 MHz Analog Board Schematic Diagram" located in section 4
page 3 of this manual.
Field effect transistor Q5 provides a VHF negative resistance amplifier/oscillator that is tuned on
frequency by the combination of resonator L5 and a 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 provided by 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, and 50.4 -
71.4 MHz. Varactor diodes D1 and D2 provide oscillator frequency control. PLL voltage feedback
control from the output of the lowpass loop filter controls the VCO frequency through the reverse
biasing of varactor diodes D1 and D2. The PLL control voltage can range from
≈
+1.0 Vdc to
+7.0 Vdc and is normally set to
≈
+4.5 Vdc for the nominal programmed synthesizer center
frequency. Setting the PLL control voltage set point (TP4) is made 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 lowpass filter is formed from select component values 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 component values
(including the loop filter) can be found in tabular format as part of 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 to a six pole lowpass/notch output
filter formed from C53, C57, C58, C59, L11, and L13. The six pole output filter provides a 50
MHz (OST-3H035, OST-3H045 TX) or 80 MHz (OSR-3H061 RX) cutoff frequency which
reduces harmonic output to acceptable levels. SMB connector J2 provides interconnection to the
companion transmitter or receiver with an output level of
≈
+5 dBm.
2 . 2 . 3
118 - 159.4 MHz Multichannel AM Analog Board Circuitry
Refer to the "OS(R/T)-3H 118 - 159.4 MHz Analog Board Schematic Diagram" located in section
4 page 7 of this manual.
Field effect transistor Q5 provides a VHF negative resistance amplifier/oscillator that is tuned on
frequency by the combination of tapped resonator L5 and a total capacitive reactance presented
across L5 through capacitors C40 and/or C23 (Select), varactor diodes D6, D7, and varactor