July 2003

11

MICRF008

MICRF008

Micrel

To determine what is the demodulator bandwidth required for

the application, the absolute minimum pulse width used in the

encoder transmitted signal needs to be found. The data

pattern has several different types of pulse width depending

what data format is being used (NRZ, Manchester, PWM,

etc). The minimum pulsewidth is the shortest pulse in the

output of the encoder being sent to the transmitter input

measured during one bit time when the signal is high. See

Figure 9 below for clarification.

Shortest

pulsewidth

Data 50%

Manchester

Dead Time

Preamble

bit period = 1/data rate

Figure 9. Data Pattern

Then, the demodulator bandwidth required is calculated by,

Demod BW =

0.65

shortest pulse

Hz

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Data Squelching

Data squelching can be achieved by software or hardware.

The software squelch is preferred over hardware squelch for

it is more flexible, reliable, lower cost over manufacturing, etc.

However, it takes time and resources to develop any soft-

ware. If one needs a squelch function, a resistor can be used

in the C

purpose. Either one of them can be used or both. If only one

resistor is used, the value of the resistor should be chosen

according to the amount of squelch needed and if the DO pin

needs to be sitting high or low without an RF income signal.

Values from 10M

Ω and below can be used. When both

resistors are used, a more accurate control can be achieved

for the squelch function.

The drawbacks using these resistors are: loss in receiver

sensitivity, reduced demodulator bandwidth, and the C

TH

capacitor equation will not work. As a result, C

will have to be determined empirically. Another way to imple-

ment a hardware squelch is to make R1 equal to 10M

Ω to

6.8M

Ω, and use a low-pass filter after the DO pin and a

comparator circuit. The low-pass filter can be a capacitor,

which is C6 in the Test Circuit. The reference voltage in the

comparator can be set close to VDD/2 and the other input

connected to the DO pin.

Power Supply Filter

A decoupling capacitor right at the V

recommended as it is in the test circuit, which has 0.1

µF. Not

shown in the Test Circuit Schematic is an EMI Inductor, L1 in

the silkscreen layer. It has the purpose to further filter an

income noise on the V

power supplies. It is good practice to separate RF ground

from digital ground and also V