Micrel, Inc.

MICRF008

August 2008

12

M9999-080108

SEL0 Pin – Setting the Demodulator Bandwidth

The SEL0 pin sets the demodulator bandwidth. When

the pin is connected to ground the demodulator

bandwidth is set to its minimum. When the pin is left

floating (internal pull-up) or connected to VDD, the

demodulator bandwidth is set to its maximum. The

demodulator bandwidth is a function of the RF frequency

used. See Table 7 for the most common frequency used

versus the demodulator bandwidth.

RF Carrier Frequency (MHz)

SEL0

315

390

418

433.92

0

1.6kHz

2.0kHz

2.2kHz

2.4kHz

1

3.6kHz

4.0kHz

4.4kHz

4.8kHz

Table 7. Demodulator Bandwidth

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:

[]

Hz

pulse

shortest

0.65

BW

Demod

=

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 software. If one needs a

R1 and R2 are in the test circuit for this 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 10MΩ 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

capacitor will have to be determined empirically. Another

way to implement a hardware squelch is to make R1

equal to 10MΩ to 6.8MΩ, 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 VDD pin is strongly

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

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 VDD line, generally

present in switching power supplies. It is good practice

to separate RF ground from digital ground and also VDD

RF from other VDD lines