EV0181 Datasheet, PDF (14/18 Pages) CML Microcircuits – User Manual

Evaluation Kit User Manual for CMX018
EV0181
1.6.1.5 First IF Stage
The voltage controlled local oscillator, obtained from the tank resonator circuitry, down-converts the
RF input signal, such that the first IF is typically centred at 90MHz.
The first IF is filtered using a 3-pole Chebyshev LC narrow bandpass filter (capacitively coupled). The
filter has a 3dB pass bandwidth of approximately 3.5MHz and insertion loss of approximately 10dB.
The ‘Mini Spring Air Core’ Coilcraft inductors (A03T-2) used in the filter have a tolerance of 2%.
Multiple capacitor footprints have been employed in the filter layout to achieve optimum performance
using preferred component values.
The filter has been designed to have the same I/O matching impedance as the CMX018 I/O pins,
MIX1OUT and MIX2IN, which have an impedance of 100Ω.
If the user wishes to modify the IF frequencies or filter bandwidth, refer to an RF designer’s guide for
information on RF filter design.
1.6.1.6 Second IF Stage
A 79.3MHz overtone colpitts crystal oscillator circuit is used to down-convert the first IF, such that the
second IF is centred at 10.7MHz.
This oscillator stage contains a Euroquartz low profile 79.3MHz crystal in a UM-5 package and has a
temperature stability of ± 20ppm. This AT third overtone mode crystal has been designed to operate
with a load capacitance of 20pF.
The second IF is filtered using a standard Murata 10.7MHz ceramic chip filter, Y1. The filter has a 3dB
pass bandwidth of 280 ± 50kHz and 20dB attenuation bandwidth of 650kHz maximum. The maximum
insertion loss specified for this filter is 6dB.
The I/O matching impedance of the filter is 330Ω, therefore, a simple narrow band LC matching
network has been designed around the filter for matching to the CMX018 I/O pins, MIX2OUT and
LIMIN, which have an impedance of 430Ω.
Refer to an RF designer’s guide to obtain information on matching networks and overtone colpitts
crystal oscillators.
If the user wishes to modify the IF frequencies a different crystal frequency is required.
1.6.1.7 RSSI and Demodulated Output
In low gain mode an RF signal level of -100dBm at RFIN, J1, will produce an RSSI voltage at TP4 of
typically 0.2V. The RSSI voltage will increase with increasing RF input level at a rate of approximately
20mV/dB up to a typical voltage of 0.7V (low gain mode) at an RF input level of -60dBm.
An operational amplifier U1:A is configured as a comparator to provide a carrier detect output, CDN, at
TP1. This output will go to a logic ‘0’ when the RSSI voltage level is greater than typically 300mV.
The threshold level can be adjusted by modifying the potential divider resistors R5 and R6.
A 10.7MHz quadrature tank circuit (CV1, C11, C13, C14, L4 and R11) is used to finally extract the
baseband signal. The trimmer capacitor CV1 is used to tune the circuit whilst R11 determines the Q
and thus the amplitude and distortion of the demodulated output.
© 1999 Consumer Microcircuits Limited
14
UM0181/1