MICRF008BM Просмотр технического описания (PDF) - Micrel

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MICRF008BM

QwikRadio™ Sweep-Mode Receiver

Micrel 

MICRF008

CTH Capacitor

In order to calculate the right value for the CTH capacitor, the

data format needs to have a preamble that resembles the

data pattern, that is it has the same period and duty cycle.

(See “Application Hint 42”. ) If the data pattern has no

preamble, a large capacitor value should be used, such as

1µF to 2.2µF. However, if the data pattern has a preamble,

the CTH capacitor can be calculated and should be used

instead of a large capacitor. This will guarantee a stable and

reliable performance for the receiver. If the data pattern has

variable data rates, the CTH capacitor should be calculated

for the lowest data rate and optimized in range tests. To find

the CTH capacitor value follow the procedure below:

1. Find the data period or bit period, and the

reference oscillator frequency. The reference

oscillator frequency is the RF carrier frequency

divided by 129. The bit period is the elapsed

time from one high and one low of the data

pattern.

2. The CTH capacitor is calculated by:

CTH

=

5

×

bit period

144.55

×

×

REFOSC

103

[F]

where:

• REFOSC is the reference oscillator frequency in

MHz.

• Bit period is given in seconds and is the inverse

of the baud rate for Manchester encoding.

• The result obtained is in farads.

It follows the CTH capacitor value for the common frequencies

mentioned above in the Table 4. Again, as mentioned before,

the data pattern needs preamble.

Baud Rate (Hz)

Frequency (MHz)

315

1000

CTH

82nF

2400

CTH

39nF

4800

CTH

18nF

390

100nF

47nF

22nF

418

120nF

47nF

22nF

433.92

120nF

47nF

22nF

Table 4. Recommended CTH Capacitor Values

CAGC Capacitor

The function of the CAGC capacitor is to minimize the ripple on

the AGC control voltage by using a sufficiently large capaci-

tor. It is suggested a value between 1µF to 10µF depending

on data dead time, noise, and recovery time from strong to

low RF signals. Large capacitor values can be connected to

VDD if fast charge time is required (C2). When connected to

VDD, it will charge 10 times faster. The drawback is the ripple

noise from AGC pin being thrown into the VDD line. The signal

on this pin is current-based, with an attack current of 15µA,

and a decay current of 1.5µA. It is suggested the following

values for the CAGC capacitors in the Table 5. Values can be

further optimized during receiver range tests.

Micrel

Baud Rate (Hz)

1000

CAGC (µF)

4.7 to 10

2400

2.2 to 4.7

4800

1 to 2.2

Table 5. Recommended Values for CAGC

Reference Oscillator Frequency

A Colpitts oscillator inside the chip generates the reference

oscillator frequency. It requires a resonator of some kind

connected to the REFOSC pin. Either a ceramic resonator or

a crystal can be used. A resonator is chosen due to its lower

cost and because the MICRF008BM is running in sweep

mode, which does not require the precision of a crystal.

Resonators found in the market normally have a precision of

0.5%. This precision is sufficient for the MICRF008BM. The

reference oscillator frequency can also be generated by an

external source through connector J1 and capacitor C1. The

maximum level should not exceed 0.5VRMS. The reference

oscillator frequency is calculated by the following equation

and Table 6 shows the resonator frequency for the most

common used frequencies:

REFOSC = fc

129

where:

• REFOSC is the reference oscillator frequency in

MHz.

• fc is the RF received carrier frequency of interest

in MHz.

Freq. (MHz)

REFOSC (MHz)

315.0

2.44

390.0

3.02

418.0

3.24

433.92

3.36

Table 6. Reference Oscillator Frequency

For a list of ceramic resonator manufactures, see “Applica-

tion Hint 35.”

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.

SEL0

0

1

RF Carrier Frequency (MHz)

315

390

418

433.92

1.6kHz

2.0kHz

2.2kHz 2.4kHz

3.6kHz

4.0kHz

4.4kHz 4.8kHz

Table 7. Demodulator Bandwidth

MICRF008

10

July 2003

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