MICRF218 datasheet(10/23 Pages) MICREL

10 / 23 page

Micrel

MICRF218

November 2011

M9999-111111

(408) 944-0800

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:

MHz

)

2.178

(32

Freq

Operating

REFOSC





(1)

The crystal frequency for Narrow Bandwidth IF

operation is given by:

MHz

)

1.198

(32

Freq

Operating

REFOSC





(2)

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:















)

433.92(MHz

(MHz)

Freq

Operating

IF_Paramet

MHz

433.92

(3)

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 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

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

bandwidth,

and

433.92MHz

with

550kHz

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 operation at 315MHz with 400kHz IF

bandwidth,

and

433.92MHz

with

550kHz

bandwidth.

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