Daniels VT-3/140-SWA2, Instruction Manual

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2-2 Enhanced AM/FM Synthesizer Instruction Manual OS(R/T)-3(A/H) 29 - 470 MHz
10.0 MHz and is selected via jumper JU2 on the Digital Board (9.6 MHz with JU2 not installed,
10.0 MHz with JU2 installed). If an external signal is used as the reference source, it must be a
sinusoidal, low phase noise, high stability signal of 0 dBm ±3 dB level. A poor quality reference
source will degrade receiver /transmitter performance to unacceptable levels. Transistor Q2 forms a
buffer amplifier having 50
Ω
input impedance at 10.0 MHz. The internal 9.6 MHz TCXO provides
better than ±1 ppm frequency stability from -30°C to +60°C (-40°C to +60°C optional). Fine
frequency adjustment is made through frequency control potentiometer RV1, which is accessible
through the synthesizer top cover.
The 9.6 MHz reference source is divided down to establish a channel selection step size of 5.0/6.25,
12.5, or 25.0 kHz depending on the particular synthesizer model type. A third order passive loop
filter comprised of C37, C38, C39, C45, C49, R36 and R32 is employed to achieve the required
noise performance, modulation and worst case switching time of 50 ms. A small sample of RF
energy is coupled from the VCO output buffer U16 on the FM analog board or from Q6 on the
AM analog board to the synthesizer IC U10 prescaler input (pin 11). FM modulation of the VCO
from
≈
100 Hz to 3 kHz is achieved through the baseband input pin P1-1 on the Digital Board. A
1 kHz sine wave with a level of approximately 400 mVrms at P1-1 provides FM deviation of
3.0 kHz. SMB connector J2 provides an RF output level of approximately +5 dBm into a 50
Ω
load.
An optional modulation input is provided through connector P1-18 (Digital Board) and routed to
the Analog board via connector P3. This connection must be coupled to a low impedance, dc
coupled source and provides a phase modulated bandwidth from 0 Hz (DC) to
≈
50 Hz (PLL loop
filter bandwidth) allowing for specialized applications such as paging or trunking where a separate
low frequency digital/analog modulation channel is required. Phase modulation input pin P1-18 is
routed to the transmitter audio processor spare pin P4-2 via JA4-2 on the MT-3 transmitter main
board. It should be noted that any application of the direct TCXO modulation port transfers control
of the synthesizer steady state frequency setting to the external modulating source. Frequency
control potentiometer RV1 is then effectively removed from the frequency adjust circuitry.
A lock detect LED (LED1) indicates an unlocked PLL condition. An unlocked PLL condition
normally indicates that the VCO is not tuned within the lock-in range of the desired channel
frequency. In a transmitter, the loss of lock will prevent PTT from keying the power amplifier
module, thus preventing transmission of a spurious output signal. Adjustment of tuning capacitor
C24 will normally reestablish frequency lock within the synthesizer’s frequency range. Optical
transmitter U5 is additionally activated in unlocked conditions and enables the micro-controller
(Digital Board) to respond to the unlocked PLL condition. Note that the 118 - 159.4 MHz and the
406 - 470 MHz Analog Boards do not incorporate a VCO tuning capacitor; the VCO covers the full
frequency range without tuning. An unlocked condition in either of these synthesizers would
indicate an attempt to synthesize an invalid channel frequency outside the installed VCO frequency
range.