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ZL20200
Data Sheet
19
Zarlink Semiconductor Inc.
Ftcxo
= Reference frequency
MOD
= Fractional Modulus
Fvco
= UHF VCO Frequency
The synthesizer provides a lock detect output. When the output pulse from the phase detector is less than half a
reference clock period an in-lock signal is generated. These in-lock signals then clock a 4 bit counter into which a
threshold value has been programmed. When the required number of successive in-lock pulses have been
generated the lock detect output is set.
The ZL20200 has a single lock detect output pin for the UHF synthesizer and VHF synthesizers. The lock detect
signal is asserted when all active synthesizers are in lock. If a synthesizer has not been enabled in the power
control registers then that synthesizer will be inactive and will have no effect on the lock detect output.
1.5 VHF Frequency Synthesizers
The ZL20200 includes two VHF synthesizers to generate the second LO for the receiver and the transmit IF. They
operate with their respective on-chip VHF VCO's and off-chip loop filters. The tank circuits and tuning components
for the VCO's are also off chip.
The two synthesizers are identical and are shown in Figure 11.
Figure 11 - VHF Frequency Synthesizer
The synthesizer uses a 2 modulus 16/17 prescaler with an 'M' counter and an 'A' swallow counter. This allows
maximum flexibility when using this synthesizer. The reference counter is a simple 14 bit counter. All counter values
are programmed via the serial bus and programming details are shown in the programming section. Both counters
operate as count down. At the start of a count the counters are loaded with their respective values. The initial
prescaler ratio is 17 assuming A > 0; when the A counter reaches zero the prescaler ratio is 16 until the M counter
reaches zero. The complete process is then repeated.
The total divide ratio (N) for this type of counter is given by
N = 16*M + A
M is always greater then A
The values of M and A can be easily calculated from the total divide ratio N.
M = INT (N/16)
A = N - 16*M
Reference Counter
14 bit
Dual Modulus
Prescaler
16/17
M Counter
13 bit
A
4 bit
Phase
Detector
Charge
Pump
Lock Detect
TCXO
VHF LO
VHF
CP
+1