REPORT: Input lag Part 7
Left: Femto embeds its valuable photo receiver in tailor made foam sections. Right: Quality is a priority – here, nothing is held back. This device is obviously made to last.
The Femto OE-200 is a photo receiver with variable amplification, which can be adjusted within a wide range of 10^3 to 10^11 V/W. The sensitivity here ranges from 190 nm to 1700 nm depending on the model.
A Femto OE-200-SI-SMA was used, which was calibrated to 830 nm, slightly higher than the visible wavelength. Since the wavelength range covered by this model stretches from 320 nm to 1060 nm, the receiver was ideal for measurements in the visible spectrum (ca. 400 nm to 800 nm).
The SMA input was used as a modified free-streaming input and placed directly in front of the display surface. The amplitude can no longer be determined precisely because of the nature of the connection, but since only the temporal resolution and possibly also a relative level is important for determining the input lag, and not the absolute brightness achieved by the monitor, this is more than sufficient for the target measurement.
Left: For signal playback, an SMA connection is built into the left hand side of the monitor. The sensitive silicon diode is visible. Right: A 3-pin LEMO port for power supply and a BNC output can be found on the right hand side of the device.
The two other inputs are a 3-Pin LEMO port for power supply and the BNC connection, via which the oscilloscope was controlled.
The PS-15-2-L power unit of the photo receiver with LEMO plug.
Naturally, the power has the necessary LEMO plug to fit the port. This is not included with the scope of supply and must be ordered separately from Femto.
With a minimum flank rise time of just 700 ns, the photo r3ceiver is more than fast enough at receiving signals for our measurements, which will primarily be carried out at millisecond level. At the amplification factor used (10^7 V/W), 900 ns can still be used for flank rising in High-Speed mode and 7 µs in Low Noise mode. These values are still around three sizes, i.e. a factor of 1000 lower than what is important for our examinations.
The total lags until the end signal is reached for a rectangular input signal are allocated to the indicated amplification factors in the table below.
Total lags for the Femto OE-200-SI-SMA.
The flank rising time is therefore just one factor in the total lag, which is much larger and thus needs to be considered. Fortunately for testers, however, the values are more than low enough. Without the knowledge that the total lag of the photo receiver is still considerably lower than the size of the hoped-for precision at a maximum of 20 µs, statements about the actual input lag would not be possible, even after measurements had been taken.
Since we do not depend on the absolute height, as already mentioned, and therefore setting times for the photo receiver become irrelevant, we can determine that the lag is even considerably lower than the total lag indicated above by the first visible response to a signal, even if this is no longer necessary at the times indicated.
For this test, various monitors were used of course in order to make it possible to make general statements and not simply cover a special case. We tested a 26-inch NEC 2690WUXi, a 24-inch Samsung 2494HM, a 20-inch ViewSonic VP2030b and a 17-inch Iiyama AS4431D TFT as well as the 21-inch CTX EX1200 CRT, which was used as a reference for all input lag measurements.
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