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MICRF501
Gyrator Filter
The main channel filter is a gyrator capacitor implementation
of a seven-pole elliptic lowpass filter. The elliptic filter mini-
mizes the total capacitance required for a given selectivity
and dynamic range. An external resistor can adjust the cut-
off frequency of the gyrator filter. The table below shows how
the cut-off frequency varies with bias resistor:
Micrel
Limiter
The limiter serves as a zero crossing detector, thus removing
amplitude variations in the IF signal, while retaining only the
phase variations. The limiter outputs are ideally suited to
measure the I-Q phase difference, since its outputs are
square waves with sharp edges.
Demodulator
Bias Resistor (k ? )
6.8
8.2
15
30
47
Cut-Off Frequency (kHz)
70
55
30
14
8
The demodulator demodulates the I and Q channel outputs
and produces a digital data output. It detects the relative
phase difference between the I and the Q channel signals.
For every edge (positive and negative) of the I channel limiter
output, the amplitude of the Q channel limiter output is
sampled, and vice versa. The output of the demodulator is
available on the DATAIXO pin. The data output is therefore
updated 4 times per cycle of the IF signal. This also means
The gyrator filter cut-off frequency should be chosen to be
approximately the same as the cut-off frequency of the
Sallen-Key filter.
Cut-Off Frequency Setting
The cut-off frequency must be high enough to pass the
received signal (frequency deviation + modulation). The
minimum cut-off frequency is given by:
f C(min) = f DEV + Baudrate/2
For a frequency deviation of f DEV = 30kHz and a baudrate of
20k baud, the minimum cut-off frequency is 40kHz. Bit setting
Fc1 = 1 and Fc0 = 0, which gives a cut-off of (60 ± 15) kHz,
would be the best choice. The gyrator filter bias resistor
should therefore be 7.5k ? or 8.2 k ? , to set the gyrator filter
cut-off frequency to approximately 60kHz.
The crystal tolerance must also be taken into account when
selecting the receiver bandwidth. If the crystal has a tempera-
ture tolerance of say ± 10ppm over the total temperature
range, the incoming RF signal and the LO signal can theoreti-
cally be 20ppm away from each other.
The frequency deviation must always be larger than the
maximum frequency drift for the demodulator to be able to
demodulate the signal. The minimum frequency deviation
(f DEVmin ) is equal to the baudrate, according to the electrical
characteristic's. This means that the frequency deviation
has to be at least equal to the baudrate plus the maximum
frequency drift.
The frequency deviation may therefore vary from the mini-
mum frequency deviation to the minimum frequency devia-
tion plus two times the maximum frequency drift. The mini-
mum cut-off frequency when crystal tolerances are consid-
ered is therefore given by:
f Cmin = ? f × 2 f DEVmin + Baudrate/2
where ? f is the maximum frequency drift between the LO
signal and the incoming RF signal due to crystal tolerances.
A frequency drift of 20ppm is 8680Hz at 434MHz. The
frequency deviation must be higher than 28.68kHz for a
baudrate of 20k baud. The frequency deviation may then vary
from 20kHz, when the RF signal is 20ppm lower than the LO
signal; to 37.36kHz when the RF signal is 20ppm higher than
the LO signal. The minimum cut-off frequency is therefore
47.36kHz.
that the maximum jitter of the data output is 1/(4 ×? f) (valid
only for zero frequency offsets). If the I channel signal lags the
Q channel, the FSK tone frequency lies above the LO
frequency (data ‘ 1 ’ ). If the I channel leads the Q channel, the
FSK tone lies below the LO frequency (data ‘ 0 ’ ).
The inputs and the output of the demodulator are filtered by
first order RC lowpass filters and then amplified by Schmitt
triggers to produce clean square waves.
It is recommended for low bitrates (<10kbps) that an addi-
tional capacitor is connected to Pin 18 (DataC) to decrease
the bandwidth of the Rx data signal filter. The bandwidth of
the filter must be adjusted for the bitrate. This functionality is
controlled by bit RxFilt.
Received Signal Strength Indicator (RSSI)
The RSSI provides a DC output voltage proportional to the
strength of the RF input signal. A graph of a typical RSSI
response is shown in Figure 9 (f DEV = 30kHz, Gc = 1).
2.2
2
1.8
1.6
1.4
1.2
1
0.8
0.6
PIN (dBm)
Figure 9. Typical RSSI Characteristics
This graph shows a range of 0.7V to 2.05V over a RF input
range of 70dB.
The RSSI can be used as a signal presence indicator. When
a RF signal is received, the RSSI output increases. This could
be used to wake up circuitry that is normally in a sleep mode
configuration to conserve battery life.
Another application for which the RSSI could be used is to
determine if transmit power can be reduced in a system. If the
RSSI detects a strong signal, it could tell the transmitter to
reduce the transmit power to reduce current consumption.
March 2003
13
MICRF501
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