MICRF219A datasheet(9/23 Pages) MICREL

9 / 23 page

Micrel, Inc.

MICRF219A

July 2011

M9999-071811-A

RadioTech@micrel.com

Functional Description

The simplified block diagram (Figure 1) illustrates the

basic structure of the MICRF219A receiver. It is made up

of four sub-blocks:

•

UHF Down-Converter

•

ASK/OOK Demodulator

•

Reference and Control logic

•

Auto-poll circuitry

Outside the device, the MICRF219A receiver requires

just a few components to operate: a capacitor from

CAGC to GND, a capacitor from CTH to GND, a

reference crystal resonator with associated loading

capacitors, LNA input matching components, and a

power-supply decoupling capacitor.

Receiver Operation

UHF Downconverter

The UHF down-converter has six sub-blocks: LNA,

mixers, synthesizer, image reject filter, band pass filter

and IF amplifier.

LNA

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

LNA input device. The LNA configuration is a cascoded

common source NMOS amplifier. The amplified RF

signal is then fed to the RF ports of two double balanced

mixers.

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 (Figure 2). The

product of the incoming RF signal and local oscillator

signal will yield the IF frequency, which will be

demodulated by the detector of the device. The image

reject mixer suppresses the image frequency which is

below the wanted signal by 2x the IF frequency. The

local oscillator frequency (fLO) is set to 32x the crystal

reference frequency (fREF) via a phase-locked loop

synthesizer with a fully-integrated loop filter:

fLO = 32 x fREF

Eq. 1

MICRF219A uses an IF frequency scheme that scales

the IF frequency (fIF) with fREF according to:

fIF = fREF x

1000

Eq. 2

Therefore, the reference frequency fREF needed for a

given desired RF frequency (fRF) is approximately:

fREF = fRF / (32 +

1000

)

Eq. 3

Figure 2. Low-Side Injection Local Oscillator

Image-Reject Filter and 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 the

image reject filter where they are combined to reject the

image frequency. The IF signal then passes through a

third order band pass filter. The IF bandwidth is 330kHz

@ 433.92MHz, and will scale with RF operating

frequency according to:

BWIF = BWIF@433.92 MHz ×

⎟

⎠

⎞

⎜

⎝

⎛

433.92

(MHz)

Freq

Operating

Eq. 4

These filters are fully integrated inside the MICRF219A.

After filtering, four active gain controlled amplifier stages

enhance the IF signal to its proper level for

demodulation.

ASK/OOK Demodulator

The demodulator section is comprised of detector,

programmable low pass filter, slicer, and AGC

comparator.

or (408) 944-0800

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