CMX972 Datasheet, PDF (17/41 Pages) CML Microcircuits – Small 32-lead VQFN Package

Quadrature Demodulator with IF PLL/VCO
CMX972
5.2 Differential Amplifiers
A pair of differential amplifiers are provided which may be used to implement filtering or buffering. These
uncommitted amplifiers may be used to implement Sallen-Key or Multiple Feedback (MFB) style filters,
buffering or configured as needed.
The amplifiers are low power and are enabled using the General Control Register (see section 6.2.1). It is
also possible for the amplifiers to be enabled with individual I and Q paths, see section 6.4.1.
5.3 Local Oscillator (LO)
The device allows for a flexible choice of routing for the LO input to the demodulator, to suit a variety of
applications.
The options available, controlled by the General Control Register (see section 6.2.1), are as follows:
 The demodulator LO may be derived from the external input (RXLO) or from the internal
VCO/PLL.
 The selected LO source is fed back to an integer-N PLL circuit which may be used to control the
on-chip (or an external) VCO from its Charge Pump output (DO).
Three bits in the General Control Register are used to define the allowed states. The permitted
combinations are shown in Table 13.
VCOEN PLLEN RXEN
Function
$1B, b3
$1B, b2 $1B, b1
0
0
0 All features disabled for low power
0
0
1 Use of demodulator with signal from RXLO pin
0
1
1 Use of demodulator with external VCO connected to
RXLO using on-chip PLL
1
1
1 Use of demodulator with LO supplied by on-chip
VCO and PLL
Note: Other combinations of control bit are illegal states and should not be used.
Table 13 LO Connections
5.3.1 Demodulator LO Input
The RXLO pin is a single-ended input for the demodulator LO signal. Internal ac coupling is provided so an
external dc blocking capacitor is not required. Note that the LO should be at twice or four times the desired
input frequency.
5.3.2 VCO and PLL
The internal VCO may be connected to the internal PLL and the demodulator. If required, an external VCO
can be connected to the PLL using the RXLO input, in this case the on-chip VCO must be disabled using
bit 3 in the General Control Register (see section 6.2.1 and Table 13).
5.3.2.1 PLL
The PLL functions are shown in Figure 12. The output frequency of the PLL is set by the following
calculation:
f_out = f_ref x ( M / R )
 2015 CML Microsystems Plc
17
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