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REFOSC ?
MHz
(1)
(32 ?
)
REFOSC ?
MHz
(2)
(32 ?
)
Micrel
the image reject filter where they are combined to
reject the image frequencies. The IF signal then
passes through a third order band pass filter. The IF
Band-Pass filters are fully integrated inside the
MICRF218. After filtering, four active gain controlled
amplifier stages enhance the IF signal to proper level
for demodulation.
IF Bandwidth General Description
The MICRF218 has IF filters which may be configured
for operation in a narrow band or wide band mode
using the IF_BW pin. This pin must not be left floating;
it must be tied to VDD or VSS. With the use of a
13.4835MHz crystal and the IF_BW = VDD (wide
mode) the IF frequency is set to 2.4MHz with a
bandwidth of 1500kHz. With the use of a 13.5178MHz
crystal and the IF_BW = VSS (narrow mode) the IF
frequency is set to 1.4MHz with a bandwidth of
550kHz at 433.92MHz.
The crystal frequency for Wide Bandwidth IF
operation is given by:
Operating Freq
2.178
12
The crystal frequency for Narrow Bandwidth IF
operation is given by:
Operating Freq
1.198
12
Note: The IF frequency, IF bandwidth, and IF
separation between IF_BW modes using a single
crystal will scale linearly and can be calculated as
follows:
MICRF218
frequency may use the wider IF bandwidth by utilizing
the appropriate equation (1) or (2) for each crystal
frequency.
The following circuit, Figure 4, is an example of
switched crystal operation. The IF Bandwidth Control
and REF-OSC Control allow switching between two
operating frequencies with either a narrow bandwidth
or a wide bandwidth. In this case, the logic control
switches between 390MHz in Wide Band Mode and
315MHz in Narrow Bandwidth Mode. The advantage
of this circuit is when a RF interferer is at one
frequency, the receiver can go to another frequency to
get clear reception.
Figure 5 shows PCB layout for MICRF218 with
switched crystal operation. Please contact the Micrel
RF Application Group for detailed document.
Dual Frequency Configuration Examples:
Scenario 1:
? Frequency 1 - 315MHz Narrow Bandwidth
? Frequency 2 - 433.92MHz Wide Bandwidth
A 9.81314MHz crystal switched in circuit during
narrow IF mode, combined with a 13.48352MHz
crystal, allows operation at 315MHz with 400kHz IF
bandwidth, and at 433.92MHz with 1500kHz
bandwidth.
Scenario 2:
? Frequency 1 - 315MHz Wide Bandwidth
? Frequency 2 - 433.92MHz Narrow Bandwidth
A 9.78823MHz crystal switched in circuit during Wide
IF mode, combined with a 13.51783MHz crystal,
allows operation at 315MHz with 1000kHz IF
* ? ?
? ?
IF_Paramet er ? IF_Paramet er @ 433.92 MHz
? Operating Freq (MHz) ?
? 433.92(MHz ) ?
(3)
bandwidth, and 433.92MHz with 550kHz IF
bandwidth.
Scenario 3:
? Frequency 1 - 315MHz Narrow Bandwidth
Switched Crystal Application
Operation
Appropriate choice of two crystal frequencies and
IF_BW mode switching allows operation at two
different frequencies; one with low bandwidth
operation and the other with high bandwidth
operation. Either the lower or higher reception
? Frequency 2 - 433.92MHz Narrow Bandwidth
A 9.8131MHz crystal switched in circuit, combined
with a 13.51783MHz crystal during narrow IF mode,
allows operation at 315MHz with 400kHz IF
bandwidth, and at 433.92MHz with 550kHz
bandwidth.
November 2011
10
M9999-111111
(408) 944-0800
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