Quadrature Modulator
CMX971
7
Direct Control Option
As an alternative to the C-BUS method of controlling the device, the CMX971 has the option of using a
direct method to control some of the device settings. This is particularly useful in systems that do not
require a microcontroller. The settings are limited to being able to reset the device, set the LO divider to
LO/2 or LO/4 and set the LO path frequency setting to 1 of 4 values to minimise quadrature error (see
section 6.3.1).
The Direct Control Option is selected by holding the CBUSOFFN pin low. This changes the function of the
C-BUS pins (CDATA, SCLK, RDATA and CSN) to those defined in Table 2 (these are FREQ1, RESETN,
DIVSET and FREQ2 respectively). Other register settings adopt a default value. Note that the FREQ1 and
FREQ2 pins have no frequency-dependent properties.
The device behaviour with CBUSOFFN = 0 is as follows:
RESETN, if asserted by taking the pin low, behaves like a C-BUS General Reset in that all registers will go
to an all zero state and the device will go into a low power mode.
When RESETN is de-asserted (taken high) the device comes out of low power mode and enters its active
state. In the active state the FREQ1, RESETN, DIVSET and FREQ2 pins have the following functionality.
Function
Pin at VDD
Active mode
Divide by 4
Pin at VSS
Low power mode, device reset
Divide by 2
RESETN
DIVSET
The FREQ1 and FREQ2 pins are decoded so as to give four values at which the quadrature accuracy is
optimised. This is important in the LO divide-by-2 mode if the input oscillator mark-space ratio is not
sufficiently close to 1. The value is a guide only – see section 6.3.1. The best intermodulation performance
is achieved at the lowest setting (‘00’) and the best wideband noise can be achieved at the highest setting
(‘11’).
FREQ1
FREQ2
0
1
0
1
0
0
1
1
best intermodulation
intermediate value
intermediate value
best wideband noise
The C-BUS registers will be automatically configured as follows:
General Control Register:
C-BUS address $1B
b7
0
b6
TXDIV
See
b5
0
b4
ENBIAS
1
b3
0
b2
0
b1
0
b0
TXEN
1
below
If DIVSET is low, b6 = 1 and the LO is divided by 2
If DIVSET is high, b6 = 0 and the LO is divided by 4
2015 CML Microsystems Plc
13
D/971/4
CMLMICRO [ CML MICROCIRCUITS ]
