DE
DANIELS
ELECTRONICS
2-2
Enhanced AM/FM Synthesizer Instruction Manual OS(R/T)-3(A/H) 29 - 470 MHz
(Analog Board) and jumper JU2 (AM Analog Board only) installed. The external reference source
may be 9.6 MHz or 10.0 MHz through selection of jumper JU2 (Digital Board); install jumper
JU2 (Digital Board) for 10.0 MHz reference selection. If an external signal is used for the
reference source, it must be of low phase noise, high stability, with a level of 0 dBm ± 3 dB. A
sinusoidal signal shape is required for an external reference source. 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. 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 provided through the baseband input pin P1-1 on 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 approxi5 dBm
into a 50
Ω
load.
An optional modulation input is provided through connection P1-18 (Digital Board) and routed to
the Analog board via connection P3. This connection must be coupled to a low impedance, dc
coupled source and provides a phase modulated bandwidth from 0 (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 the
control of the synthesizer steady state frequency setting to the external modulating source.
Frequency control potentiometer RV1 is 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. When used in a transmitter, the loss of lock will prevent PTT from keying the power
amplifier module. This prevents transmission of a spurious, unlocked output signal. Adjustment of
tuning capacitor C24 will normally reestablish frequency lock within the synthesizer design
frequency range. Optical transmitter U5 is additionally activated in unlocked conditions and
enables the micro-controller (digital board) to respond to the unlocked PLL state. Note that the
118 – 159.4 MHz and the 406 - 470 MHz Analog Board does not incorporate a VCO tuning
capacitor; the VCO covers the full frequency range without tuning. An unlocked condition in this
case would indicate an attempt to synthesize an invalid channel frequency outside the installed
VCO frequency range.