U2538B_05 Datasheet, PDF (5/9 Pages) ATMEL Corporation – IR Receiver for Data Communication

U2538B
3.4 Automatic Threshold Control (ATC)
During the reception of an incoming telegram, the ATC reduces the sensitivity of the demodula-
tor to establish the highest possible signal-to-noise ratio according to the signal 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.
3.5 Automatic Gain Control (AGC)
The automatic gain control 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 gradually reduces the sensitiv-
ity, but only if the duty cycle exceeds a specific value (see Figure 3-2). When using telegrams
with higher duty cycles than this value, the capacitor, 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
=
-----------------^N--------------------
14.2 + 1.1 × N
Figure 3-2. Duty Cycle
Transmitted Burst (N cycles)
t _pl
=
N
f
T
DC = ^{t pl}
T
N ≥ 6; f = 20 kHz to 60 kHz
3.6 Detector
The output signal of the bandpass filter is compared to a fixed reference (Comp 1) and to a ref-
erence 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 output free of short-time
interference.
The integrator drives the output stage after being processed through a Schmitt trigger. The inter-
nal pull-up resistor can replace an external resistor in some applications.
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4717B–IRRC–09/05