Tait TM8235, Руководство по обслуживанию

Страница: 76 / 578

76 Circuit Descriptions TM8100/TM8200 Service Manual
© Tait Electronics Limited June 2006
3.4 Frequency Control Loop
Introduction
The FCL is included in the block diagram of the frequency synthesizer
(see
Figure 3.5 and Figure 3.6 ).
The FCL forms part of the frequency-synthesizer module. The basis of the
FCL is a VCXO, which generates the reference frequency required by the
main PLL of the synthesizer.
Elements of
FCL Circuitry
The FCL is a simple frequency-locked loop. The circuitry consists of the
following elements:
■
VCXO (XL501, Q501, Q503)
■
TCXO (XL500)
■
buffer amplifier (IC500)
■
mixer (IC501)
■
low-pass filter (IC502, pins 5 to 7)
■
modulator buffer amplifier (IC502, pins 1 to 3).
The A4 band has additional circuitry:
■
2.612MHz mixer (IC506)
■
amplifiers (Q504, IC509)
■
TCXO divide by 4 (IC508)
The TCXO supplies a reference frequency of 13.0000MHz (10.4MHz for
the A4 band), which is extremely stable, regardless of the temperature.
The VCXO runs at a nominal frequency of 13.0120MHz, and is frequency-
locked to the TCXO reference frequency.
Circuit Operation
The VCXO is mixed with the TCXO output to create a nominal difference
(or offset) frequency of 12kHz
SYN CDC FCL
. In A4 band radios, there is
additional circuitry and the VCXO is mixed with the TCXO to produce
2.612MHz. This is then mixed with 2.6MHz (TCXO divided by 4) to
produce the 12kHz
SYN CDC FCL
.
The signal
SYN CDC FCL
is fed via the CODEC IC502 in the CODEC
circuitry to the FPGA on the digital board. The FPGA detects the offset
frequency, compares it with the programmed offset frequency, and outputs
a corresponding feedback signal
CDC VCXO MOD
via IC205. The feedback
signal is amplified and inverted by the modulator buffer amplifier and output
as the loop voltage for the VCXO. With this design the VCXO frequency
can be adjusted by very small precise amounts, and because the loop is
locked, the VCXO inherits the temperature stability of the TCXO.
Modulation
The FCL modulation is implemented within the FPGA and appears at the
output of IC205, and therefore on the VCXO loop voltage. Consequently,
the VCXO is frequency modulated directly by the relevant modulation
information. The latter may be the microphone audio, an audio tap-in
signal, internal modem signals, or any combination of these.