CMX994 Datasheet, PDF (43/61 Pages) CML Microcircuits – Local Oscillator

Direct Conversion Receivers
CMX994/CMX994A/CMX994E
8.1.7
Baseband Intermodulation
The intermodulation (IMD) performance of the CMX994 I/Q baseband amplifiers is designed to be good
enough to ensure that the overall IMD performance of the down-converter section is dominated by the
performance of the mixers. Typical in-band linearity with a very large output signal is demonstrated in Figure
23. Note the absence of IMD products. Care should be taken in the CMX994 receiver system design to ensure
that the baseband sections do not clip in the presence of intermodulation test signals because, if clipping
occurs, the overall intermodulation performance of the receiver will be degraded. The CMX994 provides two
I/Q baseband filter stages which can be used to provide selectivity in order to keep IMD test signals within
the receiver dynamic range.
8.1.8
Figure 23 Baseband Intermodulation Test at circa 6Vp-p Differential Output
LO Pulling
A VCO that uses the on-chip negative resistance amplifier can be pulled in frequency if a high level signal is
applied to the mixer input, as a result of which the VCO becomes modulated resulting in distortion of the
output I/Q signals. The level of signal required before the onset of this effect is typically between around
45dBm and -20dBm (at the LNA input), depending on the settings of I/Q VGA gain, LNA gain and the PLL.
The LNA gain control provided by the CMX994 mitigates this effect; at large input signal levels the LNA gain
should be set to minimum. Designs using the CMX994 with an external LNA should consider this issue and
ensure the external LNA provides sufficient gain control range.
Designs that use the CMX994 with an external VCO / PLL, in particular a Fractional-N type, are less susceptible
to such frequency pulling but should still employ suitable isolation between the CMX994 LO input and an
external VCO, for example by using a common-base buffer stage.
Following these guidelines a design can readily achieve good receiver system operation with LNA input levels
of >+10dBm.
 2015 CML Microsystems Plc
Page 43 of 70
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