MICRF229 datasheet(10/23 Pages) MICREL

10 / 23 page

Micrel, Inc.

MICRF229

April 15, 2015

Revision 1.0

Functional Description

The simplified Functional Diagram illustrates the basic

structure of the MICRF229 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 MICRF229 receiver requires just

a few components to operate: a capacitor from AGC 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 downconverter 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 cascaded

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 1). The product of

the incoming RF signal and local oscillator signal will

yield the IF frequency, which will be demodulated by the

detector

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 (Equation 1):

fLO = 32 x fREF

Eq. 1

MICRF229 uses an IF frequency scheme that scales the

IF frequency (fIF) with fREF according to Equation 2:

fIF = fREF x

1000

Eq. 2

Therefore, the reference frequency fREF needed for a

given desired RF frequency (fRF) is approximated in

Equation 3:

fREF = fRF / (32 +

1000

)

Eq. 3

Figure 1. 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

operating

frequency according to Equation 4:

BWIF = BWIF@433.92 MHz ×













433.92

(MHz)

Frequency

Operating

Eq. 4

These filters are fully integrated inside the MICRF229.

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.

Detector and Programmable Low-Pass Filter

The demodulation starts with the detector removing the

carrier from the IF signal. Post detection, the signal

becomes baseband information. The low-pass filter

further enhances the baseband signal.

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