EV0181 Datasheet, PDF (16/18 Pages) CML Microcircuits – User Manual

Evaluation Kit User Manual for CMX018
EV0181
1.6.2.4 VCO Tank Circuitry
The coaxial resonator consists of a length of RG405 semi-rigid cable. For operating frequencies
between 950 - 1025 MHz (i.e RF input 860 - 935 MHz) the outer shield should be cut to approximately
0.256” (6.5mm). One end of the inner wire should be soldered to the outer whilst the other end should
be soldered to the tank PCB pad. The resonator should be laid down and outer soldered to PCB
ground plane. The overall length of the resonator should be approximately 0.343” (8.7mm).
Reducing the length of the coaxial resonator, PR1, will increase the resonant frequency and vice
versa. Increasing C22 and C26 will reduce resonant frequency.
The capacitance ratio of C26 to varactor diode, D1, will determine the achievable operating band for
the 0 - 3V control voltage. Therefore to reduce the bandwidth, reduce C26 and/or insert a small
picofarad capacitor, C30. The introduction of C30 will also reduce the resonant frequency.
1.6.3
PCB Design
The EV0181 PCB is a 4-layer design, with an overall thickness of 0.062” (1.57mm), including top and
bottom layer copper. The top layer comprises the regulation circuitry connections, RF signal tracks
and ground plane. The second layer is purely a ground plane for the supply. Layer three contains the
analog power supply tracks, AV_DD which feed all the board’s sub-circuits. The bottom layer contains
another ground plane for additional screening.
CMX018 ground pins are connected directly to the top layer ground plane as close to the package as
possible. The power supply pins are bypassed to ground directly at the package using decoupling
capacitors with direct returns to ground.
Using microstrip techniques the RFIN track on the 4-layer PCB has been designed to provide a line
impedance of 50Ω. This design technique is based on the track width and separation distance
between track and ground plane for a given dielectric constant of the PCB material. The layout
assumes the board material is FR4 with a ground plane on the second layer, separated from the top
layer by 0.016” (0.41mm). Therefore, for a dielectric constant between 4-5 and track width of 0.032”
(0.81mm), an approximate 50Ω line impedance is obtained.
Refer to an RF/Microwave designer’s guide to obtain more information on microstrip line impedance
design.
© 1999 Consumer Microcircuits Limited
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