RF Quadrature Transceiver / RF Quadrature Receiver
CMX991/CMX992
7.9
Receiver Spurious Rejection Performance
The immunity of a receiver to signals away from the wanted channel is an important factor in the overall
quality of a radio receiver. Receivers may suffer interference due to off-channel signals due to a variety of
mechanisms including intermodulation, blocking and receiver spurious responses.
7.9.1 Blocking
The CMX991/CMX992 blocking performance can be evaluated by injecting an unwanted signal at a 1MHz
offset and observing the IF output response with the unwanted signal present or not present, as shown in
Figure 26. The channel filtering was used at the IF output of the CMX991 1st mixer to ensure the spectrum
analyser was capable of measuring the low levels and a LNA was before the spectrum analyser to ensure
the measurement was above the analyser noise floor. The results were taken with the LO path in /2 mode,
LO at 820MHz. The wanted signal was at 455MHz @-110dBm and the unwanted was –18dBm at
455.9MHz. The plot in Figure 26 shows the output response with (upper trace) and without (lower trace)
the unwanted signal present. The noise floor rises by 3.6dB when the blocker is present; note that the
scale is 5dB/div on plot.
Marker
1
[T1]
RBW
VBW
SWT
200 Hz
200 Hz
RF Att
Unit
0
dB
Ref Lvl
-80 dBm
-118.91 dBm
45.00149299 MHz
1.5
s
dBm
-80
-85
0.3 dB Offset
1
[T1]
-118.91 dBm
45.00149299 MHz
-88.49 dBm
A
2
1
[T1]
[T2]
2
45.00003006 MHz
3.56 dB
-90
7.45058060 nHz
-95
1AVG
2VIEW
1SA
2SA
-100
-105
-110
-115
-120
-125
-130
EXT
1
1
Center 45 MHz
1
kHz/
Span 10 kHz
Figure 26 IF Output Response with and without Blocking Signal Present
7.9.2 Half IF Response
The half IF response occurs at a frequency half the 1st IF frequency away from the wanted signal is a 2nd
order product. For example if receiving at 455MHz with a 1st IF of 45MHz and ‘low side’ LO at 410MHz the
half IF response is at 455MHz – (45/2)MHz = 432.5MHz.
2012 CML Microsystems Plc
41
D/991_992/18
CMLMICRO [ CML MICROCIRCUITS ]
