Parts List

Enhanced AM/FM Synthesizer Instruction Manual OS(R/T)-3(A/H) 29 - 470 MHz 2-1
2. THEORY OF OPERATION
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2.1 Internal Power and Control (Digital Board)
Refer to "OS(R/T)-3(A/H) Digital Board Schematic Diagram" on page 4-19 of this manual . The
synthesizer operates from a +9.5 Vdc power source applied to connector pin P1-2. Total current
draw is approximately 160 mA for FM synthesizers and 65 mA for AM synthesizers. POWER
DOWN control line P2-4 controls the +5.0 Vdc microcontroller regulator U2 through power
MOSFET switch U1. For receiver applications the synthesizer is always ON, with the enable line
P2-4 directly connected to +9.5 Vdc. For transmitter applications, pin P2-4 is controlled by MT-3
Transmitter Board jumper J18 which selects the synthesizer standby mode. In Low Current
Standby Mode, less than 14 mA current is drawn (for FM, < 4 mA for AM) however, a delay of
approximately 50 ms from PTT activation to transmit ter turn is then required to allow for
synthesizer lock time. In Normal Mode, with the synthesizer ON continuously, less than 10 ms
delay is encountered. This capability comes at the expense of additional standby current (160 mA
for FM, 65 mA for AM).
2.2 Synthesizer Analog Circuitry (Analog Board)
There are four separate analog circuit boards covering the four frequency bands of 29 - 50 MHz,
118 - 159.4 MHz, 128 - 174 MHz and 406 - 470 MHz respectively. All four analog boards are
similar in operation and circuit topology with the VCO, Loop Filter and Output Low-pass Filter
sections being band specific.
2.2.1 Common Analog Board Circuitry
This section describes the circuitry common to all versions of the FM Synthesizer Analog Board.
The Analog Board utilizes four optical receivers (U1 - U4) and one optical transmitter (U5) to
provide an isolated data interface to the digital board. In normal operating modes, +9.5 Vdc
Regulator IC U8 provides continuous +5.0 Vdc to the internal TCXO and power control optical
receiver U1. This results in a standby current level of ≈ 14 mA for FM and 4 mA for AM.
Primary power is controlled through activation of optical receiver U1 under control of the digital
board micro controller U4. Regulator U6 provides switched +8.0 Vdc with regulator U7 providing
switched +5.0 Vdc to all analog supply points. Power MOSFET IC U9 works as a clamping
circuit to quickly discharge VCO filter capacitors C32 and C33 when powered down, resulting in
immediate suppression of RF output from the VCO.
The heart of the OS-3A/H Enhanced Synthesizer is U10 - a low power, single chip synthesizer IC.
A 9.6 MHz reference signal is provided either from the internal TCXO (JU1-B Analog Board) or
from an external source via SMB connector J1 with jumper JU1-A (Analog Board) and jumper
JU2 (AM Analog Board only) installed. The external reference source may be 9.6 MHz or