F800 CIRCUIT DESCRIPTION
8 (25)
Uppgjord (även faktaansvarig om annan) -
Prepared (also subject responsible if other)
Nr
-
No.
KL/ECS/S/LT Magnus Lindahl
ECS/S/LT-96:5044
Dokansv/Godk -
Doc respons/Approved
Kontr
-
Checked
Datum
-
Date
Rev
File
KI/ECS/S/LTC
1996-06-06
A
965044.DOC
The 12,5 kHz VCO signal is compared with the 12,5 kHz reference oscillator signal in
two phase demodulators / comparators in the PLL circuit. The first phase
demodulator, PC1, is made with analogue components and has a high gain. This is
the main comparator. The second phase demodulator is implemented with digital
components. The second comparator, PC2, has a wider range to allow faster lock
times. When a small phase error is achieved the second phase demodulator is
disconnected to avoid generation of noise etc. The comparison results in two signals,
one from each demodulator. From the first phase demodulator, PC1, a slow ramp
signal is obtained and from the output of the second phase demodulator, PC2, fast
pulses are obtained. In locked mode only PC1 is active and the output of PC2 is in a
high impedance state.
Both of these two signals are connected to the loop filter.
Internal
control
logic
Divider
Divider
2 stages
Mod.
Phase
Comparator
Phase
Comparator
12,5 kHz
9 MHz
152,6 - 167,4 MHz
Data
Strobe
Address
From
reference
osc.
A2
A1
A0
From VCO
LOOP
Filter
DC voltage
to VCO
Selector for prescaler
D3
D2
D1
D0
Modulation Phase
Fig C: Principal diagram for PLL circuit and loop filter
A linear phase modulator is also implemented in the PLL circuit. The modulator is
implemented in the VCO signal chain between the divider and the phase comparators.
The modulation is done as minor changes of the signal from the VCO.
The loop filter is an integrating network and an LP filter. The filter is used to block the
remaining 12,5 kHz signals from the PLL comparators. The output from the loop filter
is a DC voltage with the phase modulated AF signal as a ripple.
Due to the characteristics of the loop filter, both phase and frequency modulation are
used. AF signals > 1000 Hz can be considered to be frequency modulated in the VCO
and AF signals < 1000 Hz are phase modulated in the PLL.
The AF signal to be phase modulated in the PLL, is passed through a LP filter before
it is connected to the modulation input to the PLL.
