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Low Power 315/433 MHz OOK/ASK Superheterodyne
Receiver with SAW-based Oscillator
PT4316
The LNA incorporates gain control circuitry. When the RSSI voltage exceeds a threshold reference
value corresponding to an RF input level of approximately -60dBm, the AGC switches on the LNA gain
reduction resistor. The loading resistor reduces the LNA gain by 20dB, thereby reducing the RSSI
output by approximately 280mV. The threshold reference voltage which is compared with the RSSI
voltage to determine the gain state of the LNA is also reduced. The LNA resumes high-gain mode
when the RSSI voltage drops below this lower threshold voltage corresponding to approximately
-
66dBm RF input. The AGC has a hysteresis of around 6dB and the time constant of the AGC response
is determined by the value of the capacitor connected to pin 2 (CAGC). The capacitor value should be
chosen with consideration of the data rate.
MIXER
The doubly-balanced mixer down-converts the input frequency (RF) in the range of 250MHz to
500MHz to the intermediate frequency (IF) at 1.2MHz with a voltage gain of approximately 20dB by
utilizing either high- or low-side injection of the local oscillator signal. In the case that the RF input to
the mixer is single-ended, the unused mixer input must be tied to ground via a capacitor. The mixer is
followed by a low pass filter with a corner frequency of 5MHz in order to suppress RF signals at the IF
output.
Both the desired signal band and image signal band are converted to the IF band. If the environment,
especially the image channel, is noisy, it is recommended that an external SAW filter be added to
remove the unwanted signals. The channel selectivity indicates the ability of the noise rejection. The
measurement of channel selectivity of PT4316 without the external SAW filter is shown in the figure as
the following.
Sensitivity and Selectivity @434 MHz Band
-50
-60
RF Input Power (dBm)
Selectivity
Sensitivity
-70
-80
-90
-100
-110
430.92
432.42
433.92
435.42
436.92
438.42
439.92
RF Input Frequency (MHz)
PT4316 V1.6
-6-
August, 2007