micrf220 Micrel Semiconductor, micrf220 Datasheet - Page 8

• Manufacturer Part Number: micrf220
• Description: 300mhz To 450mhz, 3.3v Ask/ook Receiver With Rssi And Squelch
• Manufacturer: Micrel Semiconductor

Micrel, Inc.
Functional Description
The simplified block diagram (Figure 1) illustrates the basic
structure of the MICRF220 receiver. It is made up of four
sub-blocks:
•
•
•
•
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.
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 two times
the IF frequency. The local oscillator frequency (f
to 32 times the crystal reference frequency (f
phase-locked loop synthesizer with a fully-integrated loop
filter:
MICRF220 uses an IF frequency scheme that scales the IF
frequency (f
August 2010
UHF Down-Converter
ASK/OOK Demodulator
Reference and Control logic
Squelch Control
f
f
LO
IF
= f
= 32 x f
IF
) with f
REF
x
REF
1000
REF
87
according to:
REF
The local
LO
) via a
) is set
Eq. 1
Eq. 2
Therefore, the reference frequency f
desired RF frequency (f
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:
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,
programmable low pass filter, slicer, and AGC comparator.
8
Figure 2. Low-Side Injection Local Oscillator
f
BW
REF
IF
= f
= BW
RF
⎛
⎜
⎝
/ (32 +
Operating
IF@433.92 MHz
RF
1000
) is approximately:
433.92
87
Freq
)
×
(MHz)
REF
needed for a given
⎞
⎟
⎠
M9999-082610-
MICRF220
Eq. 3
Eq. 4
A