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Micrel, Inc.
Functional Description
The simplified block diagram (Figure 1) illustrates the
basic structure of the MICRF219A receiver. It is made up
of four sub-blocks:
MICRF219A
Therefore, the reference frequency f REF needed for a
given desired RF frequency (f RF ) is approximately:
?
?
UHF Down-Converter
ASK/OOK Demodulator
f REF = f RF / (32 +
87
1000
)
Eq. 3
?
?
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
BW IF = BW IF@433.92 MHz × ?
?
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 (f LO ) is set to 32x the crystal
reference frequency (f REF ) via a phase-locked loop
synthesizer with a fully-integrated loop filter:
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:
? Operating Freq (MHz) ?
? 433.92 ?
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
f LO = 32 x f REF
Eq. 1
The demodulator section is comprised of detector,
programmable low pass filter, slicer, and AGC
MICRF219A uses an IF frequency scheme that scales
the IF frequency (f IF ) with f REF according to:
comparator.
f IF = f REF x
87
1000
Eq. 2
July 2011
9
M9999-071811-A
RadioTech@micrel.com or (408) 944-0800
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