DANIELS
ELECTRONICS LTD.
TM
VHF Enhanced Extra Wideband Receiver Instruction Manual IM10-VR3H040-AB
3-11
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 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 approxi5 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.
