MICRF221 datasheet(15/26 Pages) MICREL

15 / 26 page

Micrel Inc.

MICRF221

October 2008

M9999-100108

(408) 955-1690

REFOSC (MHz)

Carrier (MHz)

HIB Part Number

Abracon Part Number

13.54856

868.35

SA-13.548560-F-10-H-30-30-X

ABLS-13.54860MHz-10-J4Y

14.27643

915.0

SA-14.276430-F-10-H-30-30-X

ABLS-14.276430MHz-10-J4Y

14.29983

916.5

SA-14.299830-F-10-H-30-30-X

ABLS-14.299830MHz-10-J4Y

Table 4. Crystal Frequency and Vendors Part Number

Demodulation Bandwidth Calculation

JP1 and JP2 are used to select the bandwidth for the

demodulator. To set the bandwidth correctly, it is

necessary to know the shortest pulse width of the

encoded data sent in the transmitter. As shown in the

example of the data profile in the Figure 9 below, PW2

is shorter than PW1, so PW2 should be used for the

demodulator bandwidth calculation which is found by

calculating 0.65/shortest pulse width. After this value

is found, the setting should be done according to

Table 5.

For example, if the pulse period is 100µsec, 50% duty

cycle, the pulse width will be 50µsec:

(PW = (100µsec × 50%) / 100)

So, a bandwidth of 13kHz would be necessary (0.65 /

50µsec). However, if this data stream had a pulse

period with 20% duty cycle, the bandwidth required

would be 32.5kHz (0.65 / 20µsec), which exceeds the

maximum bandwidth of the demodulator circuit. If you

try to exceed the maximum bandwidth, the pulse will

appear stretched or wider.

SEL0

JP1,

SEL1

JP2,

Demod.

(hertz)

Shortest

Pulse

(µsec)

Maximum

baud rate for

50% Duty

Cycle (hertz)

Short

1712

380

1316

Open

Short

3425

190

2632

Short

Open

6850

5264

Open

13700

10528

Table 5. JP1 and JP2 setting, 915 MHz

This device is capable of higher baud rates when the

serial bit D16 is programmed high. More detail is

provided on the following pages.

CTH and CAGC Selection

Capacitors C6 (CTH) and C4 (CAGC) provide time base

reference for the data pattern received. These

capacitors are selected according to the data profile,

pulse duty cycle, dead time between two received

data packets and if the data pattern has or does not

have a preamble. See Figure 9 for an example of a

data profile.

PW1

HEADER

PW2

PW2 = NARROWEST PULSE WIDTH

t1 & t2 = DATA PERIOD

PREAMBLE

Figure 9. Example of a Data Profile

For best results C4 and C6 should be optimized for

the data pattern used. As the baud rate increases, the

capacitor values decrease. Table 6 shows suggested

values for Manchester Encoded data at a 50% duty

cycle.

SEL0

JP1

SEL1

JP2

Demod.

(hertz)

CTH

CAGC

Short

1712

100nF

4.7µF

Open

Short

3425

47nF

2.2µF

Short

Open

6850

22nF

1µF

Open

13700

10nF

0.47µF

Table 6. Suggested C6 (CTH) and C4 (CAGC) Values

JP4 (pins 5 and 6) is a jumper used to configure the

digital squelch function. When pin 6 (SQ) is held high

jumpered-to-VDD), there is no squelch applied to the

digital circuits and pin 10 (DO, data out) has a hash

signal. When pin 6 (SQ) is low, the DO pin activity is

considerably reduced. It will have more or less activity

than is shown in Figure 11 depending upon the

outside band noise. The penalty for using squelch is a

delay in getting a good signal at the DO pin, that is, it

takes longer for the data to show. The delay is

dependent upon many factors such as RF signal

intensity, data profile, data rate, CTH and CAGC

capacitor values and outside band noise. See Figures

10 and Figure 11.

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