CMX972 datasheet(17/41 Pages) CMLMICRO

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Quadrature Demodulator with IF PLL/VCO

CMX972

 2015 CML Microsystems Plc

D/972/2

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

$1B, b3

PLLEN

$1B, b2

RXEN

$1B, b1

Function

All features disabled for low power

Use of demodulator with signal from RXLO pin

Use of demodulator with external VCO connected to

RXLO using on-chip PLL

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:

fout = fref x ( M / R )

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