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Micrel
Functional Description
The simplified block d iagram, shown in Figure 1,
illustrates the basic stru cture of the MICRF219 receiver.
It is made up of four s ub-blocks:
? UHF Down-converter
MICRF219
The IF BW can be calculated via direct scaling:
BW IF = BW IF@433.92 MHz ×
? Operating Freq (MHz) ?
? 433.92 ?
?
OOK Demodula tor
?
?
Reference and Control logic
Auto-poll circuitry
These filters are fully integrated in side the MICRF219.
Afte r filt ering, four active gain cont rolled amplifier stages
enhance the IF signal to i ts proper level for
B W Operating Freq = BW @433.92MHz * ?
?
Outside the device, the MICRF219 receiver re quires just
three components to operate: two capacitors ( CTH, and
CAGC) and the reference frequency device (usually a
quartz crystal). An ad ditional five components are used
to improve performa nce; a power supply d ecoupling
capacitor, two c omponents for the matching network,
and two components for the pre-selector b and-pass
filter.
Receiver Operation
UHF Downconverter
The UHF down-converter has six compone nts: LNA,
mixers, synthesizer, image reject filter, ban d pass filter
and IF amp.
LNA
The RF input signal is AC-coupled into the ga t e circuit of
the grounded source LNA input stage. The LNA is a
Cascoded NMOS amplifier. The amplified R F signal is
then fed to the RF ports of two double balanc e d mixers.
Mixers and Synthesizer
The LO ports of the Mixers are drive n by quadra tu re
local oscillator outputs from the synthesizer b lock. Th e
local oscillator signal from the synthesizer is place d on
the low side of the desired RF signa l to a ll ow
suppression of the image frequency at tw i ce the IF
frequency below the wanted signal. The loc a l o scillat or
is set to 32 times the crystal reference frequ e n cy via a
phase-locked loop synthesiz er with a fully integrated
demodulation.
OOK Demodulator
The demodulator section is co mprised 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. Pos t detection, the signal
becomes base band information . The programmable
low-pa ss filter further enhances the baseband
information. There are four programmable low-pass
filter BW se ttings: 1625Hz, 3250Hz, 6500Hz, 13000Hz
for 433.92MHz operation. Low pass filter BW will vary
with RF Operating Frequency. Filter BW values can be
easily calculated by direct scaling . See equation below
for filter BW calculation:
? Operating Freq (MHz) ?
? 433.92 ?
It is very important to choose filter setting that fits best
th e in tended data rate to minimize data distortion.
D em od BW is set at 1 3000Hz @ 433.92MHz as default
(assu ming both SEL0 a nd SEL1 pins are floating). The
low pass filter can be hardware set by external pins
SEL0 and SEL1.
loop filter.
Image-Reject Filter and Band-Pass Filter
SEL0
0
SEL1
0
Demod BW (@ 434MHz)
1625Hz
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
1
0
1
0
1
1
3250Hz
6500Hz
13000Hz
- default
image frequencies. The IF signal then passes through a
third order band pass filter. The IF center frequency is
1.2MHz. The IF BW is 330kHz @ 433.92MHz. This
varies with RF operating frequency.
Table 1. Demodulation BW Selection
June 2011
7
M9999-060811
(408) 944-0800
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