Evaluation Kit User Manual for CMX017

EV0171

© 1999 Consumer Microcircuits Limited

14

UM0171/2

1.6.2.3 Powersave and Zero-Power Modes

The evaluation device enable lines are controlled by switch SW1.

This switch allows the user to

configure the CMX017 transmitter for powersave or zero-power.

The Modulator and Buffered Oscillator stages of the evaluation device can be powered down

independently of the PA Driver and Power Amplifier stages.

In SLEEP mode (SW1:1 and SW1:2 are Off) all sections of the device are powered down and the

device current consumption is reduced to typically 10

µA. This is the zero-power mode.

In TRANSMIT-STANDBY mode (SW1:1 is Off and SW1:2 is On) only the PA Driver and Power

Amplifier stages are powered down, whilst the Modulator and Buffered Oscillator remain active.

In TRANSMIT mode (SW1:1 and SW1:2 are On) both the Modulator and Power Amplifier sections of

the device are fully operational.

1.6.2.4 VCO Tank Circuitry

The coaxial resonator, PR1, consists of a length of RG405 semi-rigid cable. The outer is cut to

approximately 0.370” (9.4mm). One end of the inner is soldered to the outer whilst the other is

soldered to the tank PCB Pad. The resonator is laid down and outer soldered to PCB ground points

using tinned wire. The overall length of the resonator is approximately 0.430” (11mm).

Reducing the length of the coaxial resonator will increase the resonant frequency and vice versa.

Increasing C16 and C19 will reduce resonant frequency.

The capacitance ratio of C19 to varactor diode, D1, will determine the achievable operating band for

the 0-3V control voltage. Therefore to reduce the bandwidth, reduce C19 and/or insert a small

picofarad capacitor, C22. The introduction of C22 will also reduce resonant frequency.

1.6.3

PCB Design

The EV0171 PCB is a 4-layer design, with an overall thickness of 0.062”, including top and bottom

layer copper.

The top layer comprises the regulation circuitry connections, RF signal tracks and

CMX017 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 screening purposes.

CMX017 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 PAOUT and PAIN/MODOUT tracks on the 4-layer PCB have been

designed to provide line impedances of 50

Ω. 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”. Therefore, for a dielectric constant between 4-5 and track widths of

0.032”, approximately 50

Ω line impedances are obtained.

Refer to an RF/Microwave designer’s guide to obtain more information on microstrip line impedance

design.