Vishay Telefunken
Circuit Description of the IR Receiving Modules
Basically all IR receivers of Vishay Telefunken have
the same circuit strategy. The function of the Vishay
Telefunken TSOP IR receiver modules can be
described using the block diagram in Figure 19. The
incident infrared radiation bursts generate an
equivalent photo current in the photo PIN diode. The
DC part is separated in the bias block, and the AC part
is passed to a transimpedance amplifier followed by an
automatic gain–control amplifier and an integrated
bandpass filter.
The final evaluation is performed by a comparator, an
integrator and a Schmitt Trigger stage.
The blocks ”Automatic Gain Control” and ”Automatic
Threshold Control” are responsible for the dynamic
control of the working points as well as the threshold
levels to suppress the influences of all kinds of
disturbing sources which are discussed above.
The digital output signal has active low polarity. It is an
envelope signal of the incoming optical burst without
the carrier frequency.
.
Transimpedance
Amplifier
Bias
.
.
Controlled
Gain
Amplifier
.
.
+VS
Automatic
Threshold
Control
.
Bandpassfilter
Out
Automatic
Gain
Control
Integrator & Schmitt Trigger
.
GND
Figure 19. Block Diagram of TSOP IR Receivers
Transimpedance Amplifier
Controlled Gain Amplifier
The block bias provides the necessary bias voltage for
the detector diode. It additionally separates DC and
low–frequency parts from the useful signal of the photo
current. The low frequency including DC current work
on a low impedance load in the block ”bias”. The AC
signals are fed to the transimpedance amplifier.
Controlled Gain Amplifier:
The controlled gain amplifier generates most of the
voltage gain of the whole circuitry whereby the
amplification is controlled by the Automatic Gain
Control (AGC) block. The gain variation of this
amplifier is about 50 dB.
The block bias reacts to the photodiode as a variable
frequency–dependent load resistance similar to an LC
resonant circuit with a low equivalent resistance for
low–frequency signals and a high resistance of some
100 kΩ at the operating frequency. The currents at the
operating frequency are converted by the
transimpedance amplifier to a voltage at the input of
the Controlled Gain Amplifier.
Band Pass Filter
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The band pass filter is an important part of the circuit
to get a good performance in disturbed ambient. It filters out noise coming from all kind of disturbances. As
the burst duration is relatively short the figure of merit
can not be more than 10. A narrower filter would need
a longer time to become oscillating.
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Vishay Telefunken
Table 5: Band pass filter in the TSOP Receiver: Figure of Merit
TSOP11xx
TSOP21xx
TSOP12xx
TSOP15xx
TSOP22xx
TSOP48xx
TSOP13xx
TSOP17xx
TSOP18xx
TSOP28xx
TFMM
5xx0
7
10
10
10
7
10
Figure of Merit (Band Pass Filter)
The band pass filter is tuned during production
process. The following bandpass center frequencies
are available: 30.3kHz, 33kHz, 36kHz, 36.7kHz,
38kHz, 40kHz, 44kHz and 56kHz. These are the
carrier frequencies for the most common data formats
of IR remote controls.
The duty cycle of the carrier frequency can be between
50% and 5%. A remote control system with Vishay
Telefunken IR receiver is more efficient regarding
battery power consumption if the carrier duty cycle is
low. This is shown in the following example:
Automatic Gain Control (AGC)
The AGC stage ensures that the receiver module is
immune to disturbances. It adapts to the existing noise
or disturbance level by changing the gain of the
amplifier. Also in dark ambient the AGC sets the gain
to the most sensitive value so that there are no random
output pulses. The time constant of this AGC is chosen
to be sufficiently large in order to avoid a considerable
sensitivity decrease during transmission. The AGC
does not react to the useful signal but reduces the
sensitivity in case of disturbances. The AGC has to
distinguish between useful and disturbance signals.
Hence the AGC has certain distinguishing marks
between these good and bad signals. These
distinguishing marks are different for the various IR
receiver series.
The distinguishing marks are burst length and
envelope duty cycle for most of the Vishay
Telefunkens photomodules. The values are set out in
Table 6.
• carrier duty cycle 50%, peak current of emitter
IF=200mA, the resulting distance is 20m
• carrier duty cycle 10%, peak current of emitter
IF=800mA, the resulting distance is 22m
Table 6. Conditions of the AGC for Data Signal
Gap time needed for each burst which is
longer than 1.8ms
Maximum number of short bursts in 1 second
TSOP11xx
TSOP21xx
TSOP12xx
TSOP15xx
TSOP22xx
TSOP48xx
TSOP13xx
TSOP17xx TFMM5xx0
1*
burstlength
4*
burstlength
5.5 *
burstlength
1*
burstlength
1.3 *
burstlength
2200
800
1000
1400
1000
The AGC of the TSOP18xx and TSOP28xx has a
completely different concept to detect a disturbance
signal. It evaluates whether there is a gap time in the
signal which is 15ms or longer (for TSOP18xxSS3V:
25ms or longer). This gap time should be repeated at
least each 90 ms. Almost all data formats fulfill this
condition. An example is shown in Figure 20.
But all disturbance signals which are repetitive with
50 Hz, 60 Hz, 100 Hz or 120 Hz do not have such a
gap time. The disturbance signals, coming from
fluorescent lamps, switching power supplies or picture
tubes in a TV are detected as disturbance correctly by
this AGC. Of course, this AGC also evaluates the
white noise caused by DC light sources.
Signal Gap Time
0
20
40
60
80
100
120
140
160
180
time [s]
Figure 20. Example of a Signal Gap Time which is needed for the AGC of TSOP18../28..
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06.00
Vishay Telefunken
Based on these AGC parameters each series of IR
receivers has its significant characteristic.
The most important questions are:
• What kind of disturbance is suppressed ?
• What data formats can be received continuously ?
Automatic Threshold Control (ATC):
The signal, coming from the band pass filter is
evaluated by a comparator. Noise should be below the
threshold where data signal will be detected by the
comparator. In quiescent mode the comparator
threshold is just as sensitive as necessary to avoid
unexpected pulses at the output. When a signal is
received the comparator level is shifted up to a higher
value. This gives additional security in the data
transmission and prevents random pulses during a
data message.
A further benefit of the ATC is the stable output pulse
width. Without the ATC the output pulses would be too
long at strong IR input signals.
The threshold slides back to the initial value after a
time of approximately 10 ms if it is not retriggered in
the meantime.
This time constant will ensure a stable signal
evaluation during the data message for the most
common transmission codes. Using this method, disturbance pulses being detected as falsely transmitted
signals during the transmission of an information block
can be efficiently avoided.
Integrator and Schmitt Trigger
The integrator is triggered when the signal reaches the
above mentioned comparator threshold. It needs
several cycles from the comparator output in series
until the integrator is loaded and the output is triggered.
The integration time necessary to control the output
via the Schmitt Trigger is given in Table 7 for each
series of IR receiver modules. Due to the integrator
there is a minimum time for the burst length (integrator
ramp up time) and a minimum time between the bursts
(integrator ramp down time).
The integrator prevents the feed–through of short
disturbances or spikes to the output. A long integrator
can improve the signal to noise ratio significantly. The
design of integrator and Schmitt Trigger is carried out
so that the output pulse width is close to the optical
burst length at the input. The integrator and Schmitt
Trigger guarantee a certain minimum output pulse
width. It is not possible to get an output pulse which is
shorter than the Integrator time mentioned in Table 7.
Table 7. Integrator Data of the Vishay Telefunken IR receiver
TSOP11xx
TSOP21xx
TSOP12xx
TSOP15xx
TSOP22xx
TSOP48xx
TSOP13xx
TSOP17xx
TSOP18xx
TSOP28xx
TFMM
5xx0
Integrator time
4 cycles
7 cycles
4 cycles
7 cycles
4 cycles
250 µs
Minimum Burst length
6 cycles
10 cycles
6 cycles
10 cycles
6 cycles
300 µs
Minimum Gap between the bursts
10 cycles
14 cycles
10 cycles
14 cycles
10 cycles
400 µs
Output stage
As shown in Figure 19, the digital output of the TSOP
IR receiver modules is an open collector transistor with
an internal pull up resistor. An additional external pull
up resistor can be used if more current is needed to
drive the input of the decoding device or if a faster
switching time is required. The low level will be below
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0.2 V even at a sink current of 2 mA. The capacitance
at the output can be up to 500 pF without risk of
damaging the output stage at continuous operation.
It is not recommended to operate the TSOP IR
receiver modules with an external pull down resistor
which is shifting the High Level to a voltage below 3V.
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