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Micrel
MICRF218
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
SEL0
JP1
Short
Open
Short
Open
SEL1
JP2
Short
Short
Open
Open
Demod.
BW
(hertz)
1625
3250
6500
13000
Shortest
Pulse
(μsec)
400
200
100
50
Maximum
baud rate for
50% Duty
Cycle (hertz)
1250
2500
5000
10000
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
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.
local oscillator signal will yield the IF frequency, which
will be demodulated by the detector of the device.
Image Desired
Frequency Signal
SEL0
JP1
Short
Open
Short
Open
SEL1
JP2
Short
Short
Open
Open
Demod.
BW
(hertz)
1565
3130
6261
12523
Shortest
Pulse
(μsec)
416
208
104
52
Maximum
baud rate for
50% Duty
Cycle (hertz)
1204
2408
4816
9633
-f LO
f (MHz)
Table 7 . JP1 and JP2 setting, 418.0 MHz
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
SEL0
JP1
Short
Open
Short
Open
SEL1
JP2
Short
Short
Open
Open
Demod.
BW
(hertz)
1460
2921
5842
11684
Shortest
Pulse
(μsec)
445
223
111
56
Maximum
baud rate for
50% Duty
Cycle (Hertz)
1123
2246
4493
8987
the demodulator bandwidth calculation which is found
by 0.65/shortest pulse width. After this value is found,
Table 8. JP1 and JP2 setting, 390.0 MHz
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
SEL0
JP1
Short
Open
Short
Open
SEL1
JP2
Short
Short
Open
Open
Demod.
BW
(hertz)
1180
2360
4720
9400
Shortest
Pulse
(μsec)
551
275
138
69
Maximum
baud rate for
50% Duty
Cycle (Hertz)
908
1815
3631
7230
stretched or wider.
Table 9. JP1 and JP2 setting, 315.0 MHz.
November 2011
17
M9999-111111
(408) 944-0800
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