EV0181 Datasheet, PDF (13/18 Pages) CML Microcircuits – User Manual

Evaluation Kit User Manual for CMX018
EV0181
Typical SAW filters which may be of interest are the wide band B4637 (902 - 928 MHz), B4682 (869 -
894 MHz) and equivalent narrow band devices, manufactured by Siemens Matsushita Components.
These SAW filters have typically 2dB insertion loss and I/O matching impedance of 50Ω.
The LNA switched gain function which is used to increase the dynamic range of the receiver, is
controlled by gain select switch SW1:2. In the ‘off’ position the LNA will be in high gain mode, and low
gain selected when in the ‘on’ position (default).
High gain mode is used where maximum sensitivity is required for low level input signals. Where high
level signals are present at the receiver input, which causes difficulties due to inter-modulation, the
gain of the LNA can be reduced by typically 22dB, from approximately +16dB to -6dB.
The output of the LNA is fed to the first on-chip mixer (MIXIN, pin 5) via an ac coupling capacitor, C3.
With a good front end band pass SAW filter no further filtering should be required at the LNA output.
However, customer requirements may mean an additional SAW filter is necessary at this point, for
further spurious rejection.
Please refer to the CMX018 device data sheet for specifications on LNA input levels (LNAIN, pin 1).
1.6.1.4 VCO Tank Circuitry
The local oscillator circuitry is formed by two capacitors, C22 and C26, a coaxial resonator, PR1, and
varactor diode, D1, which is driven by the frequency control potentiometer, RV1. The dc voltage
derived from the potentiometer, RV1, can be measured at TP14. Alternatively the inductor, L8, can be
removed and dc control voltage applied to the VCO tank circuitry via TP14.
The user may also drive the VCO externally from a stable source to overcome frequency drift due to
the frequency control potentiometer, RV1. This setup is recommended if the user wishes to perform
detailed measurements with the EV0181 evaluation board. The capacitor, C33, should be fitted
(100pF), R23 replaced with a 0Ω resistor and inductor, L8, removed before applying an external local
oscillator signal via OSCIN, J2 (not fitted). The external local oscillator signal connected to OSCIN
should be at a typical input level of -10dBm.
The IF filters and second IF crystal have been designed so the local oscillator frequency must be set
90MHz higher than the RF input signal, for the receiver to work correctly.
For evaluation purposes space is provided for the user to fit an LL1608 Toko inductor at L11. This
inductor is an alternative to the coaxial resonator, PR1. The inductor will have a typical value of 3.3nH
for these operating frequencies. Capacitors C22 and C26 may need to be modified with this
alternative resonator.
Customers concerned about the manufacture of a coaxial resonator, PR1, may wish to consider the
use of a PCB track resonator. Refer to an RF/Microwave designer’s guide to obtain more information
on microstrip design.
© 1999 Consumer Microcircuits Limited
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UM0181/1