CMX994 Datasheet, PDF (20/61 Pages) CML Microcircuits – Local Oscillator

Direct Conversion Receivers
CMX994/CMX994A/CMX994E
6.2.3
The ACR filter may introduce some deterministic distortion in the signal passband, this distortion can be
compensated by using filters external to the CMX994/CMX994A/CMX994E; see section 6.2.3 for further
information.
The scaling of the post-mixer filter is less critical. The bandwidth (-3dB), using the recommended values, is
approximately 88kHz (at maximum gain). This should be suitable for all the channel bandwidths up to 25kHz,
so no scaling is provided. Note that the bandwidth of this section will reduce by 30% (typ.) as gain is reduced
in the baseband section.
Baseband Filter Design and Required Correction
The pole frequencies of the filter stages are set by the addition of external capacitors (see also section 5.2.2);
the resistors are internal to the chip and those on the second filter stage are trimmed so as to match the
external 10kΩ reference resistor².
600
10k
CMX994
600
External
Capacitor
CMX994
10k
External
Capacitor
First stage filter.
Resistors have 20% tolerance
Second stage filter.
Resistors are matched to external
10k, 1% resistor
Figure 14 Schematic Representation of Filters used in the I and Q Paths
Filtering close to the passband distorts the signal and increases the BER, so it is necessary to provide
correction for the filter distortion in subsequent external digital filtering.
Both filter stages are single-pole filters. The first filter stage is designed to reduce blocking signals and it will
typically be set at 4 x the maximum required channel spacing. As a result, tolerance is not particularly critical
and it is usually not necessary to compensate the wanted signal for this filter. The second filter is designed to
operate close to, or within, the passband. It is therefore important that compensation is provided for the
wanted signal.
Different modulation schemes occupy different bandwidths within a channel, so optimisation of the filter
positioning will vary depending on the modulation scheme. Some experimentation may be required to get
the best results but, as a guide, the second filter stage should have its frequency set to between 30% and 40%
of the channel spacing. The aim is to reduce adjacent and close-in channels as much as possible. Provided the
filter effects are compensated for later, the lowering of the signal amplitude at the band edges is usually well
tolerated, although the extent of this will depend on the modulation scheme used.
The CMX994/CMX994A/CMX994E allows up to three different channel spacings to be selected via the C-BUS
interface. So if, for example, the three channel spacings required are 6.25kHz, 12.5kHz and 25kHz, then the
first filter should be set at a nominal desired frequency to ensure large blocking signals are rejected, typically
a cut off frequency around 100kHz would be suitable. If only two channel spacings are required, of 6.25kHz
and 12.5kHz for example, then it would be slightly beneficial to set the post mixer filter bandwidth to 50kHz.
Note that the three channel spacings are always in a 1:2:4 ratio relative to the smallest channel spacing,
which is set by external components.
2 _{The external resistor should be 1% tolerance or better.}
 2015 CML Microsystems Plc
Page 20 of 70
D/994_A_E/1