Maxim DS33Z41 Spezifikationen PDF herunterladen (Seite 12)

Q5. When the cable is unplugged for the network interface, why does the receive carrier loss

(RCL) bit in the status register or the pin of the LIU or SCT occasionally report an incorrect

status?

A5. It is unlikely that the device actually reports the RCL status incorrectly. The most common cause of this problem is

that a noise/signal source is coupling into the receive-side interface (the RTIP and RRING pins). The following are

suggestions to verify that noise/signal coupling is the problem and what can be done to solve the problem.

1. Set the equalizer gain limit (EGL) of the device to the lower of the two settings (i.e., if the two choices are -43dB

and -15dB, choose the -15dB setting). This will allow the LIU to remove low-level signals that may be causing the

device to report the RCL status incorrectly. If this solves the problem, then the EGL bit can be left at this setting

for future operation. However, if long-haul operation is required by the system, another suggestion should be

considered.

2. Set the transmitter power-down (TPD) of the device to turn off the transmitter. This will prevent the transmitted

signal from coupling into the receiver. If this solves the problem, then the problem is probably that the transmit

TIP/RING signals are too close in proximity to the receive TIP/RING signals. The best solution is to examine the

layout of the board and shorten the TIP/RING traces and further separate the transmit and receive signals.

3. When the line is disconnected, use an oscilloscope to probe the receiver TIP/RING pins to observe any noise that

may be present. If there is a large amplitude and noise on the line with a frequency near the nominal line rate,

this noise can be falsely recovered as a signal. The best solution is to examine the board to find the source of the

coupled noise and move or shield it from the receiver pins.

4. Use an oscilloscope to probe the power supply directly at the supply pins of the device. If there is noise with an

amplitude of 50mV or greater superimposed on the power-supply rail, then it can cause problems with the

receiver. The best solution is to use decoupling capacitors to filter the power supply to reduce the noise.

Q6. What are the differences between the DS21448 and the older DS21Q48/DS21Q348?

A6. The DS21Q48 and DS21Q348 are quad-port, 17mm x 17mm multichip modules that consist of four independent LIUs.

The DS21Q48 operates at 5.0V and is composed of DS21Q48 devices, while the DS21Q348 operates at 3.3V and is

composed of DS21348 devices. The DS21448 is a monolithic version of the DS21Q348, and is meant to be a drop-in

replacement that is package, pin, and software compatible. The DS21448 only operates at 3.3V and cannot be used as a

direct replacement for the DS21Q48.

Q7. Why is the master clock (MCLK) signal necessary for proper device operation?

A7. Because of the hybrid analog and digital nature of the line interface, the master clock signal present on the MCLK pin

is necessary to operate both the clock- and data-recovery engine and the jitter attenuator. Without the master clock, the

device would not be able to properly recover the clock and data signal present on the incoming line. The master clock

signal is also used as an alternate recovered clock on the RCLK pin whenever the device enters a receive carrier loss

state. To ensure that the device operates within specifications, the master clock should be derived from an accurate low-

jitter source such as a crystal oscillator. T1/E1 devices usually use a source with a frequency tolerance rating of ±50ppm

(or better) and a period jitter rating measured in picoseconds.

Q8. Is the center-tap connection of the transformer secondary side necessary for better analog

performance of the DS3151, DS3152, DS3153, and DS3154 devices?

A8. Connecting the center tap of the transformer secondary side to the ground is optional. If used, the center tap if used

would serve to reduce the common-mode noise. However, the DS315x devices are all designed to use a coaxial cable

connection, which has little if any common-mode noise. In addition, if the traces between the transformer and the

DS315x device pins are short in length, the effects of inductance and common-mode noise will be negligible.

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