CMX994 datasheet(21/61 Pages) CMLMICRO

21 / 61 page

Direct Conversion Receivers

CMX994/CMX994A/CMX994E

 2015 CML Microsystems Plc

D/994_A_E/1

Page 21 of 70

The second stage filter capacitor should be selected for the smallest bandwidth requirement. The calculation

for the capacitor value is as follows:

000





Where



filter pole frequency (-3dB point).

So a capacitor of 4nF would yield a frequency pole of 1989Hz. This may be a typical figure when using a

channel spacing of 6.25kHz. To maintain the accuracy of the compensation the capacitor must have a low

temperature coefficient and tolerance better than or equal to 2%.

A compensation filter would need to be applied in the digital domain having the inverse characteristic.

This would be:



)

(





This would normally be implemented as a FIR filter. It should be followed with another non-critical FIR that

rolls off the signal when out of band. This second filter may be part of a required channel filter.

Selecting 2x or 4x bandwidth will require the compensation filter to be adjusted in proportion.

The first stage filter capacitor may be calculated in a similar way:

1200





Where



filter pole frequency (-3dB point).

So a capacitor of 1.5nF would yield a frequency pole of 88.4kHz. This may be a typical figure if the maximum

channel spacing required were 25kHz. There is a wider tolerance on this, as the internal resistors are not

trimmed. Consequently it is not required to have a low tolerance value on the first stage filter capacitor.

Should it be required to have this closer to the passband then a correction filter may be required. This would

have the same form as for the correction filter for the second stage. In general a margin of at least 50%

between the filter cut-off and wanted channel is recommended to allow for the resistor tolerances and

bandwidth change with gain settings noted in section 6.2.2.

Because both filter stages are handling large dynamic signals, the linearity of the external capacitors is

important. Use of good dielectric materials is recommended; poor linearity could result in a degradation of

the on-channel signal in the presence of large off-channel interferers.

6.2.4

Operation at Wider Bandwidths

It is possible to use a much wider channel bandwidth than those used as examples elsewhere in this

document. For maximum I/Q bandwidth (1.6MHz) capacitors C1, C2, C3 and C4 in Figure 9 should be

removed. In this case the filter calibration circuit should be disabled using b7 of the VCO Control Register

($25); see section 7.11.1.

For systems that require greater bandwidths the output should be taken before the baseband amplifiers and

filters; see section 8.1.9.

6.3

Local Oscillator Operation

The CMX994/CMX994A/CMX994E can use either an external Local Oscillator (LO) source or the on-chip VCO

and PLL. The on-chip integer-N PLL can also be used with an external VCO connected to the LO input.

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