CMX972 Datasheet, PDF (32/41 Pages) CML Microcircuits – Small 32-lead VQFN Package

Quadrature Demodulator with IF PLL/VCO
CMX972
7.5 Measurement of CMX972 Demodulator Intermodulation Performance
The measurement of the intermodulation (IMD) performance of the CMX972 demodulator requires great
care because generally the IMD products are at a very low level. As a result, it is important to ensure
products being measured are generated by the CMX972, not the measurement instrument or the test
system.
It is also important to ensure that measurements are taken before the onset of clipping in the I/Q output
stages – the effect is shown in Figure 19. Considering the graph, at signal levels below –51dBm per tone
(two tone signal, tones of equal amplitude) the measured IMD product rises at the classical rate of 2dB for
every 1dB increase in tone level. For input levels above –51dBm the rate of increase rises dramatically due
to the onset of clipping. The effect can be seen in the plots of the composite signal: the calculated line is
based on a calculation of the peak-to-peak swing of the output I/Q voltage from measured tone level at the
output of the CMX972, however the actual output level is also plotted and the two lines deviate at the on-
set of clipping.
It will be apparent that any calculation of IMD parameters, e.g. input third order intercept point, from
measurements taken after the onset of clipping will give erroneous results if trying to characterise receiver
operation at normal signal levels.
10
3.5
0
3
-10
-20
2.5
Signal
-30
2
IMD Product
-40
Calculated level of
1.5
composite signal
-50
Composite Signal
1
(Measured)
-60
0.5
-70
-80
-53
-52
-51
-50
-49
Input Signal Level Per Tone / dBm
0
-48
(Note: the two curves ‘Signal’ and ‘IMD Product’ are levels in dBm so should be referenced to the left hand
Y-axis; the other curves are output voltages and use the right hand Y-axis.)
Figure 19 Variations in Signal and IMD Product Levels
Typical IMD measurements for the CMX972 demodulator usually involve IMD products at least 75dB
below the wanted signal.
The input level where compression commences will vary somewhat from device to device, the value of
-44.5dBm¹ (Figure 19) is typical but should only be used as an initial guide.
1 _{Note: -50.5 dBm per tone = -44.5 dBm PEP,}
 2015 CML Microsystems Plc
32
D/972/2