MICRF218 datasheet(17/23 Pages) MICREL

17 / 23 page

Micrel

MICRF218

November 2011

M9999-111111

(408) 944-0800

The oscillator of the MICRF218 is Colpitts in

configuration. It is very sensitive to stray capacitance

loads. Thus, very good care must be taken when

laying out the printed circuit board. Avoid long traces

and ground plane on the top layer close to the

REFOSC pins RO1 and RO2. When care is not taken

in the layout, and crystals from other vendors are

used, the oscillator may take longer times to start as

well as the time to good data in the DO pin to show

up. In some cases, if the stray capacitance is too high

(> 20pF), the oscillator may not start at all.

Refer to Equations 1 and 2 for crystal frequency

calculations. The local oscillator is low side injection

(32 × 13.51783MHz = 432.571MHz), that is, its

frequency is below the RF carrier frequency and the

image frequency is below the LO frequency. See

Figure 10. The product of the incoming RF signal and

local oscillator signal will yield the IF frequency, which

will be demodulated by the detector of the device.

-fLO

f (MHz)

Desired

Signal

Image

Frequency

Figure 10. Low Side Injection Local Oscillator

Narrow and Wide Band Crystal Part Numbers,

WB = IF Wide Band, NB = IF Narrow Band

JP1 and JP2 are the bandwidth selection for the

demodulator bandwidth. To set it correctly, it is

necessary to know the shortest pulse width of the

encoded data sent in the transmitter. Similar to the

example of the data profile in the Figure 11 below,

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

the demodulator bandwidth calculation which is found

by 0.65/shortest pulse width. After this value is found,

the setting should be done according to

Table 6. 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, then

the bandwidth required would be 32.5kHz (0.65 /

20µsec), which exceeds the maximum bandwidth of

the demodulator circuit. If one tries to exceed the

maximum bandwidth, the pulse would appear

stretched or wider.

SEL0

JP1

SEL1

JP2

Demod.

(hertz)

Shortest

Pulse

(µsec)

Maximum

baud rate for

50% Duty

Cycle (hertz)

Short

1625

400

1250

Open

Short

3250

200

2500

Short

Open

6500

100

5000

Open

13000

10000

Table

6. JP1 and JP2 setting, 433.92 MHz

Other frequencies will have different demodulator

bandwidth limits, which are derived from the reference

oscillator frequency.

Table 7 and 8 below shows the

limits for the other two most used frequencies.

SEL0

JP1

SEL1

JP2

Demod.

(hertz)

Shortest

Pulse

(µsec)

Maximum

baud rate for

50% Duty

Cycle (hertz)

Short

1565

416

1204

Open

Short

3130

208

2408

Short

Open

6261

104

4816

Open

12523

9633

Table

7. JP1 and JP2 setting, 418.0 MHz

SEL0

JP1

SEL1

JP2

Demod.

(hertz)

Shortest

Pulse

(µsec)

Maximum

baud rate for

50% Duty

Cycle (Hertz)

Short

1460

445

1123

Open

Short

2921

223

2246

Short

Open

5842

111

4493

Open

11684

8987

Table 8. JP1 and JP2 setting, 390.0 MHz

SEL0

JP1

SEL1

JP2

Demod.

(hertz)

Shortest

Pulse

(µsec)

Maximum

baud rate for

50% Duty

Cycle (Hertz)

Short

1180

551

908

Open

Short

2360

275

1815

Short

Open

4720

138

3631

Open

9400

7230

Table 9. JP1 and JP2 setting, 315.0 MHz.

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