CMX994 Datasheet, PDF (22/61 Pages) CML Microcircuits – Local Oscillator

Direct Conversion Receivers
CMX994/CMX994A/CMX994E
6.3.1
PLL
The CMX994/CMX994A/CMX994E provides an integer-N PLL that can be used to create the local oscillator;
see Figure 15. The CMX994/CMX994A/CMX994E provides a VCO negative resistance amplifier, so only a tank
circuit needs to be implemented externally. Alternatively, this amplifier can be bypassed and an external VCO
can be used.
CMX994
VCO
NR
Amplifier
NR
Control
VCO
Output
Buffer
LO Output to CMX994
Circuits
M Divider
(Feedback)
2 - 262143
R Divider
(Reference)
2 - 32767
FREF
Phase
Detector
Lock
Detect
VCOP1
VCON1
LON LOP
DO
VCOP2
VCON2
VCO
Tank &
Varactors
Optional
External
VCO
Figure 15 Local Oscillator
The integer-N PLL has programmable M and R dividers as shown in Figure 15. The phase detector provides a
charge pump output which requires a suitable loop filter to convert this signal into a control voltage for a
VCO. The phase detector can be turned off (high impedance mode) and the PLL section disabled if an external
LO is to be used; see section 6.3.2 and 7.9 for control details.
The output frequency of the PLL is set by the following calculation:
f_out = f_ref x ( M / R )
where:
f_out = The desired output frequency in MHz
f_ref = The reference frequency supplied to the PLL on pin FREF in MHz
M = Divider value programmed in the M divider register (see section 7.9.1)
R = Divider value programmed in the R divider register (see section 7.10.1)
The PLL only supports VCOs with a positive tuning slope, i.e. a higher tuning voltage from DO results in a
higher oscillation frequency from the VCO.
The PLL has a lock-detect function that can be evaluated using register $D2, b6 (section 7.9.2).
The VCO amplifier is a negative resistance amplifier requiring an external tank circuit (see section ). The
amplifier has two control bits available in the VCO control register (section 7.11.1 register $25, b2 – b3).
These bits can be used to optimise performance for a particular tank circuit depending on its Q value.
When using the CMX994/CMX994A/CMX994E PLL, spurious products (spurs) in the receiver I/Q output may
be observed. The frequency of the spurs is linked to the PLL M divider value and which of the divide-by-2, 4 or
6 modes is selected for the receiver LO circuits. Operation in divide-by-2 and divide-by-6 modes is most
predictable: all even division ratios are problem free and all odd division ratios will give a spurious product at:
f_spur = f_lo / ( M * 2 )
 2015 CML Microsystems Plc
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