MICRF218_11 MICREL [Micrel Semiconductor], MICRF218_11 Datasheet - Page 10

• Manufacturer Part Number: MICRF218_11
• Description: 3.3V, 315/433MHz Wide-IF Bandwidth ASK Receiver
• Manufacturer: MICREL [Micrel Semiconductor]

November 2011
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:
The crystal frequency for Narrow Bandwidth IF
operation is given by:
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:
Switched Crystal Application
Operation
Appropriate choice of two crystal frequencies and
IF_BW mode switching allows operation at two
different
operation
operation. Either the lower or higher reception
REFOSC
REFOSC
*
IF_Paramet

 

Operating
frequencies;
and
433.92(MHz


Operating
Operating
er
(32
(32
the
Freq



IF_Paramet
1.198
2.178
other
12
12
(MHz)
one
Freq
Freq
)
)
)
with
MHz
MHz
with

 

er
@
high
low
433.92
MHz
bandwidth
bandwidth
(1)
(2)
(3)
10
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,
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
bandwidth,
bandwidth.
Scenario 3:
 Frequency 1 - 315MHz Narrow Bandwidth
 Frequency 2 - 433.92MHz Narrow Bandwidth
A 9.8131MHz crystal switched in circuit, combined
with a 13.51783MHz crystal during narrow IF mode,
allows
bandwidth,
bandwidth.
operation
and
and
and
at
433.92MHz
at
at
433.92MHz
315MHz
433.92MHz
with
with
with
with
[phone]
M9999-111111
550kHz
400kHz
1500kHz
550kHz
IF
IF