CMX994 Datasheet, PDF (19/61 Pages) CML Microcircuits – Local Oscillator

Direct Conversion Receivers
CMX994/CMX994A/CMX994E
Source
Vdc1
Vdc2
Vdc3
Vdc4
Vdc5
Vdc6
Voltage Correction at Output
for Maximum Gain in
Baseband Amplifiers
25mV
50mV
100mV
25mV
50mV
100mV
Correction Polarity
Positive terminal increase,
Negative terminal decrease
Positive terminal increase,
Negative terminal decrease
Positive terminal increase,
Negative terminal decrease
Negative terminal increase,
Positive terminal decrease
Negative terminal increase,
Positive terminal decrease
Negative terminal increase,
Positive terminal decrease
Table 14 DC Offset Correction Adjustments
6.2.2
Receiver Filters and Bandwidth Options
The I and Q channels incorporate two stages of filtering to reduce blocking signals and to attenuate nearby
channels. This allows the wanted signal to be maximised without significant distortion being introduced as a
result of unwanted larger signals saturating the later amplification stages.
The CMX994/CMX994A/CMX994E supports multiple channel bandwidths, providing scalable filtering in the
baseband (I/Q) chain. Two filter stages are provided. The post mixer filter provides rejection of large off-
channel signals such as those typically used in blocking tests. With this protection in place some gain is
provided before narrower filters that provide rejection of the adjacent channel. Following this filter, the
remainder of the receiver gain is provided. Both filter stages have single-pole characteristics, having -3dB
frequency points set by separate external capacitors.
LO Input
I Channel
Q Channel
Post
Mixer
Filters
Adjacent
Channel
Rejection
Filters
Figure 13 Baseband I/Q Filtering
After setting the second filter stage bandwidth via external capacitors, the bandwidth can then be scaled
under serial control to allow multiple channel bandwidths to be supported by the
CMX994/CMX994A/CMX994E. A typical requirement is to support 6.25kHz, 12.5kHz and 25kHz channels, so
the scaling of the Adjacent Channel Rejection (ACR) filter is 1 : 2 : 4 via a bandwidth mode control. Using the
recommended external capacitors for a 6.25kHz design (see 5.2.2), the ACR filter bandwidth (-3dB) is 2kHz.
This provides 9dB rejection of the adjacent channel and 15dB rejection at 12.5kHz. Using the bandwidth
scaling control, the 2kHz filter bandwidth can then be changed to 4kHz or 8kHz (see Rx Control Register,
section 7.3), without changing external components. See also section 8.1.6.
 2015 CML Microsystems Plc
Page 19 of 70
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