MICRF008(2008) Просмотр технического описания (PDF) - Micrel

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производитель

MICRF008
(Rev.:2008)

QwikRadio® Sweep-Mode Receiver

Micrel 

Micrel, Inc.

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:

C TH

=

5 × bit period × REFOSC [F]

144.55 × 103

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.

Frequency

(MHz)

315

390

418

433.93

1000

CTH

82nF

100nF

120nF

120nF

Baud Rate (Hz)

2400

CTH

39nF

47nF

47nF

47nF

4800

CTH

18nF

22nF

22nF

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 capacitor. 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.

Baud Rate (Hz)

1000

2400

4800

CAGC (µF)

4.7 to 10

2.2 to 4.7

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

MICRF008YM 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 MICRF008YM. 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.

Frequency (MHz)

315

390

418

433.92

REFOSC (MHz)

2.44

3.02

3.24

3.36

Table 6. Reference Oscillator Frequency

For a list of ceramic resonator manufactures, see

“Application Hint 35.”

August 2008

11

M9999-080108

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