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PT4316 V1.6

- 6 -

August, 2007

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

-110

-100

-90

-80

-70

-60

-50

430.92

432.42

433.92

435.42

436.92

438.42

439.92

RF Input Frequency (MHz)

Selectivity

Sensitivity