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Micrel, Inc.
Functional Description
The simplified block diagram (Figure 1) illustrates the basic
MICRF220
Therefore, the reference frequency f REF needed for a given
desired RF frequency (f RF ) is approximately:
structure of the MICRF220 receiver. It is made up of four
sub-blocks:
? UHF Down-Converter
f REF = f RF / (32 +
87
1000
)
Eq. 3
?
?
?
ASK/OOK Demodulator
Reference and Control logic
Squelch Control
Outside the device, the MICRF220 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
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:
BW IF = BW IF@433.92 MHz ×
?
?
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
? Operating Freq (MHz) ?
? 433.92 ?
Eq. 4
the device. The image reject mixer suppresses the image
frequency which is below the wanted signal by two times
the IF frequency. The local oscillator frequency (f LO ) is set
to 32 times the crystal reference frequency (f REF ) via a
phase-locked loop synthesizer with a fully-integrated loop
filter:
These filters are fully integrated inside the MICRF220.
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,
f LO = 32 x f REF
Eq. 1
programmable low pass filter, slicer, and AGC comparator.
MICRF220 uses an IF frequency scheme that scales the IF
frequency (f IF ) with f REF according to:
f IF = f REF x
87
1000
Eq. 2
August 2010
8
M9999-082610- A
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