CMX994 Datasheet, PDF (49/61 Pages) CML Microcircuits – Local Oscillator

Direct Conversion Receivers
CMX994/CMX994A/CMX994E
close-in interferers at the same frequency band as the wanted modulation. This is especially helpful if one
considers that the adjacent channel interferer power may be far greater than the wanted signal’s. The zero IF
approach also minimises the bandwidth of I/Q output signals, which reduces the cost, power and complexity
of ADCs used to sample them. Lastly, it enables a low pass filter on each of I and Q signal paths to provide
selectivity, which simplifies design by reducing filter complexity and the required dynamic range of the ADCs.
While it is true that second order mixer intermodulation products also lie at 0Hz (DC), the
CMX994/CMX994A/CMX994E feature best in class, (+79dBm) mixer IIP2 performance to attenuate such DC
intermod products.
For specific radio systems one may wish to avoid mixing the wanted signal down to 0Hz. The
CMX994/CMX994A/CMX994E readily supports this by allowing the user to select LO frequencies that do not
exactly match the wanted RF signal’s carrier.
Near-zero IF I/Q architectures often set the LO frequency to between half to two times the required channel
bandwidth, which provides frequency separation between the wanted signal and any DC components in the
output I and Q signals. Such architectures then sample (ADC convert) the near-zero I/Q IF signal pair and then
use digital baseband processing to both implement a simple high pass filter that removes DC and perform a
final frequency mix down to 0Hz. Operating in such a near-zero IF mode requires the CMX994 baseband low
pass filters to be scaled wider appropriately to pass the chosen IF frequency; the filter components external
to the CMX994 will need to be revised; see section 6.2.3. Sampling ADC bandwidth must also be somewhat
higher performance and therefore higher power than ADCs used in the zero IF architecture. Another and
perhaps more significant trade-off of near-zero IF is that the adjacent channel image generated by
unavoidable I/Q gain and phase errors typically now falls on the wanted signal to create a significant
interference component if the adjacent channel signal level is high.
The low IF I/Q architecture is a conceptual extension of near-zero IF; it sets the mixer LO frequency to
something different than the wanted RF carrier frequency but with even more frequency separation than
near-zero IF uses. The result is that the wanted signal is mixed to an even higher frequency at the I/Q
outputs. This approach can permit receiver I/Q outputs to be AC coupled to ADCs. Low IF I/Q trade-offs are of
the same type as those presented by near-zero IF but they are more challenging, i.e. wider bandwidth ADCs
must be used.
The CMX994/CMX994A/CMX994E supports all of zero IF, near-zero IF and low IF I/Q architectures, with the
benefits of LNA to I/Q output integration with amplification, low pass filtering, and operating modes selected
via serial host control.
 2015 CML Microsystems Plc
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