U2538B Datasheet, PDF (5/9 Pages) ATMEL Corporation – IR RECEIVER FOR DATA COMMUNICATION

U2538B
Automatic Threshold
Control (ATC)
During the reception of an incoming telegram, the ATC reduces the sensitivity of the
demodulator to establish the highest possible signal-to-noise ratio according to the sig-
nal strength. This prevents interferences which may arise during transmission from
affecting the output. The advantage of the circuit is achieved if its output voltage
exceeds V_Th (Comp 1). That is the case when the input signal strength is more than
twice as much as the minimum detectable signal intensity.
Automatic Gain Control
(AGC)
The automatic gaincontrol improves the circuit's resistance to interference by adapting
the amplification of the gain-controlled amplifier to the relevant existing interference
level. In order to prevent the circuit from responding to transmitted data signals, it grad-
ually reduces the sensitivity, but only if the duty cycle exceeds a specific value
(see Figure 4). When using telegrams with higher duty cycles than this value, the capac-
itor, C_AGC, maintains the sensitivity for a certain time period. A higher capacitance
enables a longer transmission time. A capacitance of C₁ = 22 nF is adequate for most
known telegrams. A typical value for the maximum duty cycle (DC) can be calculated by
the following formula:
DC_max = ₁----₄---_.--₂-----+---^N--₁---_.--₁----_´-----_N---
Figure 4. Duty Cycle
Transmitted Burst (N cycles)
t
pl
=
N
f
T
DC = ^{t pl}
T
N ³ 6; f = 20 kHz to 60 kHz
Detector
The output signal of the bandpass filter is compared to a fixed reference (Comp 1) and
to a reference generated by the ATC circuit (Comp 2). The output of the comparator with
the higher threshold voltage controls the integrator. Using the integrator keeps the out-
put free of short-time interference.
The integrator drives the output stage after being processed through a Schmitt trigger.
The internal pull-up resistor can replace an external resistor in some applications.
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4717A–IRRC–05/03