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HFBR-4663 Datasheet(PDF) 11 Page - Agilent(Hewlett-Packard) |
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HFBR-4663 Datasheet(HTML) 11 Page - Agilent(Hewlett-Packard) |
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11 / 15 page  87 The comparator is a high-speed differential zero crossing detector that slices and accurately digitizes the receive signal. The output of the comparator is fed in parallel into both the receive squelch circuit and the loopback MUX. Link Detect Circuitry and Low Light The link detect circuit monitors the input signal and determines when the input falls below a preset voltage level. When the input falls below a preset voltage, the HFBR-4663 goes into the Low Light state. In the Low Light state the transmitter is disabled, but continues sending the I MHz idle signal, the loopback is disabled, the receiver is disabled and the LMON LED pin goes to high shutting off the LMON LED. To return to the Link Pass state, the optical receiver power must be 20% higher than the shut-off state. This built-in hysteresis adds stability to the Link Monitor circuit. Once the receiver power threshold is exceeded, the HFBR- 4663 waits 250 ms to 750 ms, then checks to see that Tx+, Tx- is idle and no data is being received before re-enabling the transmitter, receiver, loopback circuit, and lighting up the LMON LED. The VTHADJ pin is used to adjust the sensitivity of the receiver. The HFBR-4663 is capable of exceeding the 10BASE-FL specifications for sensitivity. The sensitivity is dependent on the layout of the PC board. A good low noise layout will exceed the 10BASE-FL specifications, while a poor layout will fail to meet the sensitivity and BER spec. state the link can be switched on if the input exceeds the set threshold for a time given by: CTIMER x 0.7 V T= –––––––––––– 700 µA To switch the link from on to off, the above time will be doubled. A value of 0.05 µF will meet the 10BASE-FL specifications. Differences between 10BASE-FL and FOIRL 10BASE-FL is an improved version of the original FOIRL standard. The 10BASE-FL standard allows backward compatibility of a 10BASE-FL transceiver with a FOIRL transceiver. The main improve- ments incorporated into 10BASE- FL are that it can attach to a DTE by adding the SQE test, and the distance has been increased from 1 km to 2 km. The other differences are much more subtle. The threshold generator shifts the reference voltage at VTHADJ through a circuit which has a temperature coefficient matching that of the limiting amplifier. The relationship between the VTHADJ and the VTH (the peak to peak input threshold) is: VTHADJ = 408 VTH (2) In a 10BASE-FL receiver, there must be less than 1 x 10-9 bit errors at a receive power level of - 32.5 dBm average. One procedure to determine the sensitivity of a receiver is to start at the lowest optical power level and gradually increase the optical power until the BER is met. In this case the Link Detect circuit must not disable the receiver (i.e. VTHADJ should be tied to Ground). Once the sensitivity of the receiver is determined, VTHADJ can be set just above the power level that meets the BER specification. This way the receiver will shut-off before the BER is exceeded. For 10-BASE-FL, VTHADJ can be tied directly to VREF. However if greater sensitivity is required the circuit in Figure 5 can be used to adjust the VTHADJ voltage. Even if VREF is tied to VTHADJ, it is a good idea to layout a board with these two resistors available. This will allow potential future adjustments without board revisions. The response time of the Link Detect circuit is set by the CTIMER pin. Starting from the link off Figure 5. THRESH GEN REF VREF VTHADJ R1 R2 |
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Similar Description - HFBR-4663 |
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