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Micrel Inc.
MICRF221
Mixers and Synthesizer
The LO port of the mixers are driven by quadrature
local oscillators of the synthesizer block. The
synthesizer block produces the local oscillator signal
on the low side of the desired RF signal with
D3
SEL0
0
1
D4
SEL1
0
0
Demod BW (@ 915MHz)
1625Hz
3250Hz
suppression of the image frequency, at twice the IF
frequency below the wanted signal. The local
oscillator is set to 64 times the crystal reference
0
1
1
1
6500Hz
13000Hz
- default
frequency by way of a phase locked loop synthesizer
with a fully integrated loop filter.
Image Reject Filter and Band-Pass Filter
The IF ports of the mixer produce quadrature down-
converted IF signals. The IF signal is filtered by the
image reject filter to remove image frequency
components and then follow up with a third order
band-pass filter. The IF center frequency is
1.285MHz. The IF BW is 380kHz @ 915 MHz, and
the IF BW varies with RF operating frequency. The IF
BW can be calculated via direct scaling.
Slicer and Slicing Level
The signal prior to the slicer is still AM. The data slicer
converts the AM signal into ones and zeros based
upon the threshold voltage built up in the CTH
capacitor. After the slicer, the signal is ASK or OOK
digital data.
The slicing threshold defaults at 50%. The slicing
threshold can be set via serial programming through
register D5 and D6.
BW IF = BW IF@ 915MHz*(Operating Freq (MHz)/
915)
D5
1
D6
0
Slicing Level
Slice Level 30%
These filters are fully integrated inside the MICRF221.
After filtering, four active gain controlled amplifier
stages enhance the IF signal to proper level for
0
1
0
1
1
0
Slice Level 40%
Slice Level 50%
Slice Level 60%
- default
demodulation.
OOK Demodulator
The demodulator section is comprised of detector,
programmable low pass filter, slicer, AGC and
Desense.
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
programmable low-pass filter further enhances the
baseband information. There are four settings for
programmable low-pass filter BW options: 1625Hz,
3250Hz, 6500Hz, 13000Hz. for 915MHz operation.
Low pass filter BW will vary with RF Operating
Frequency. Filter BW values can easily calculated by
direct scaling. See equation below for filter BW
calculation:
BW Operating Freq = BW @ 915MHz*(Operating Freq
(MHz)/ 915)
It is very important to choose filter setting that fits best
for the intended data rate to minimize data distortion.
Demod BW is set at 13000Hz @ 915MHz as default
(assuming both SEL0 and SEL1 pins are floating).
The low pass filter can be hardware set by external
pins SEL0 and SEL1, or via serial programming
through register D3 and D4
AGC
AGC monitors the signal amplitude from the output of
the programmable low-pass filter. When the output
signal is less than 750mV thresh-hold, AGC increases
the gain of the mixer and the IF amplifier. When the
output signal is greater than 750mV, the AGC lowers
the gain of the mixer and the IF amplifier.
Desense
Desense is a function designed to reduce the
sensitivity of the MICRF221 receiver to a maximum of
45dB for training the MICRF221 receiver to recognize
an intended transmitter. Very often, a receiver needs
to learn how to recognize a particular transmitter. It is
important for the receiver not to learn the signal of a
stray transmitter near by. The simplest solution is to
turn down the receiver gain, so the receiver only
recognizes the transmitter at close range.
The desense function is accessible only through serial
programming.
October 2008
8
M9999-100108
(408) 955-1690
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