CMX972 Datasheet, PDF (29/41 Pages) CML Microcircuits – Small 32-lead VQFN Package

Quadrature Demodulator with IF PLL/VCO
CMX972
Input Condition Noise Figure / dB Gain / dB
50R shunt resistor
16.3
50.5
matched network
7.8
64
straight in
10
56
Table 16 Typical Noise Figure and Gain of IF Amp, VGA and I/Q Mixer
The gain in the ‘straight in’ case is based on direct conversion of the signal generator power to a voltage
and calculating the gain based on the output voltage. The output signal is the differential signal at RXIN
and RXIP (or RXQN and RXQP) so if the voltage is measured at a single pin the signal level must be
doubled to get the appropriate differential signal level. Also it should be noted that making a simple
conversion of the power in the ‘straight in’ case is erroneous as the voltage calculated will be a potential
difference. As the circuit is unmatched an e.m.f. would be more appropriate (i.e. twice the potential
difference value).
7.2 Demodulator Intermodulation and Output Drive Capability
The intermodulation performance of the demodulator may be optimised by use of the output drive bit
(register $1C b7, see section 6.3.1). Performance can be further optimised by setting the IMD bits in the
VCO register (register $2F b2 to b7) to ‘111111’ = 63 decimal.
IMD bits setting
(register $2F b2
to b7) decimal
value
0
63
$1C, b7=’0’
50kHz and
100kHz tones
-23 dBm
-19 dBm
$1C, b7=’1’
50kHz and
100kHz tones
-12 dBm
-11 dBm
$1C, b7=’0’
500kHz and
1MHz tones
-24 dBm
-24 dBm
$1C, b7=’1’
500kHz and
1MHz tones
-12 dBm
-11 dBm
Table 17 Typical Third Order Intercept Performance of demodulator at 45MHz (straight-in case)
7.3 Variation with Temperature
The CMX972 demodulator exhibits excellent temperature stability. Typical variation of the receive path
gain is shown in Figure 15. The I/Q gain/phase balance (see Table 10), dc level and attenuator steps also
show only small variations with temperature.
 2015 CML Microsystems Plc
29
D/972/2