Vco And Phase Lock Loop - Codan NGT 2010 RF Technical & Service Manual

NGT—Technical description (2011/2012)
Phase lock loop reference
Drawing
NOTE
IC403 divides the reference oscillator down to approximately 5 MHz. The signal is
buffered by V411 and is then fed as the reference frequency to IC401.
The reference frequency is varied in small step sizes to produce the 10 Hz steps at the
output of the phase lock loop.
The frequency information is programmed serially from the microprocessor IC408,
based on the channel information.

VCO and phase lock loop

Drawing
NOTE
The oscillator VCO is a Colpitts type comprising V401 and associated components. The
frequency of oscillation is determined by the tuned circuit L403, C401 and the
capacitance of the varicaps D401 to D404.
The oscillator output at the tap of the inductor L403 is coupled to the buffer stage made
from V402, V403 and associated components. This output is used to drive mixers
RxMIX1 and TxMIX2.
Synthesiser IC401 generates a DC control voltage via the phase/frequency detector and
control amplifier. This is applied to the varicaps to lock VCO to the nominated
frequency.
The VCO input frequency at F IN (pin 4) on IC401 is divided down to approximately
100 kHz and compared with the reference frequency that is also divided down to
approximately 100 kHz. Any error between these two signals will produce an output
from IC401 pin 14 or 15.
The microprocessor IC408 loads serial data into IC401 depending on the channel
frequency required. This varies the divide ratio on F IN, causing the VCO frequency to
change.
The output pulses from IC401 pin 14 or 15 are applied to the charge pump amplifier
comprising V406 to V410. Pin 14 provides a 'go up' pulse, while pin 15 provides a
'go down' pulse. When locked, these pins go high impedance except for a very narrow
pulse that occurs simultaneously at both outputs, thus holding the DC output at a fixed
level.
The DC control voltage is filtered by C419, R422 and C418 to remove any reference
frequency. The signal is then applied to the varicaps to control the frequency of
oscillation.
122
04-03378 (sheet 3)
If the PCB issue status is -06 or earlier, see drawing
04-03378 (sheet 3)
If the PCB issue status is -06 or earlier, see drawing
04-03378 (sheet
04-03378 (sheet
NGT Transceiver System Technical Service Manual
3).
3).