MICRF218 datasheet(9/23 Pages) MICREL

9 / 23 page

Micrel

MICRF218

November 2011

M9999-111111

(408) 944-0800

Figure 1 Simplified Block Diagram

Functional Description

Figure 1 illustrates the basic structure of the

MICRF218. It is composed of three sub-blocks; Image

Rejection UHF Down-converter with Switch-able Dual

IF Bandwidths, the OOK Demodulator, and Reference

and Control Logics.

Outside the device, the MICRF218 requires only three

components to operate: two capacitors (CTH, and

CAGC) and the reference frequency device, usually a

quartz crystal.

Additional five components may be used to improve

performance. These are: low cost linear regulator

decoupling capacitor, two components for the

matching network, and two components for the pre-

selector band pass filter.

Receiver Operation

LNA

The RF input signal is AC-coupled into the gate circuit

of the grounded source LNA input stage. The LNA is

a Cascoded NMOS.

Mixers and Synthesizer

The LO ports of the Mixers are driven by quadrature

local oscillator outputs from the synthesizer block.

The local oscillator signal from the synthesizer is

placed on the low side of the desired RF signal to

allow suppression of the image frequency at twice the

IF frequency below the wanted signal. The local

oscillator is set to 32 times the crystal reference

frequency via a phase-locked loop synthesizer with a

fully integrated loop filter.

Image Reject Filter and IF Band-Pass Filter

The IF ports of the mixer produce quadrature down

converted IF signals. These IF signals are low-pass

filtered to remove higher frequency products prior to

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