RF Quadrature Transceiver / RF Quadrature Receiver
CMX991/CMX992
5.3.2 Direct I/Q IF Output Tx Mode
As shown in Figure 16, the filtered TxIF output from the I/Q modulator can be made available on the
TXIFOUT pin. This can then be translated up to RF frequency via user-supplied external circuits or in
some cases used directly for VHF operation. When this mode is selected the image-reject up-converter
should be powersaved (register $14, b6 – see section 6.5.1): this disables unused circuits and saves
power.
5.4
Local Oscillators
5.4.1 IF Local Oscillator
The CMX991/CMX992 provides an integer-N PLL that can be used to create the IF Local oscillator, see
Figure 17. The CMX991/CMX992 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 in the range 40 to 600 MHz (NB: see note at end of section 4.5).
LO to I/Q Rx mixers
CMX991
(and Tx IF section in
CMX991)
CMX992
VCO
NR
Amplifier
N Divider
(Feedback)
80 - 32767
M Divider
(Reference)
2 - 8191
FREF
NR
Control
Enable
VCO
Output
Buffer
Phase
Detector
Lock
Detect
Enable
VCOP
VCON
DOIF
VCO
Tank &
Varactors
Figure 17 CMX991/CMX992 IF Local Oscillator
The integer-n PLL has programmable M and N dividers as shown in Figure 17. 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 6.8 for control details. In the case of an external LO it
is necessary for the VCO Output buffer to remain enabled (section 6.2, register $11, b1) however the VCO
amplifier must be disabled (Register $11, b0).
The output frequency of the PLL is set by the following calculation:
2012 CML Microsystems Plc
22
D/991_992/18
CMLMICRO [ CML MICROCIRCUITS ]
