Compensation method of the hottest cable loss

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Compensation method for cable loss

this application note introduces a compensation method for cable loss, which allows test equipment manufacturers to use high loss cables to reduce costs without sacrificing product performance

application note 4303: the influence of cable loss on automatic test equipment discusses the signal attenuation mechanism of high loss cable. The loss mainly comes from two aspects: skin effect and dielectric loss. Equipment manufacturers, such as those who provide automatic test equipment (ATE), mostly use high loss cables in order to reduce the cost of the whole system. With the improvement of system data transmission rate, the impact of these cables on system performance has far offset their low-cost advantages. Fortunately, electronic circuits can be used to compensate for cable losses

feasible solution to cable loss

one of the ways to solve the high cable loss is to choose high-quality and expensive cables, which have the least negative impact on the system performance. But only some special high-end systems, such as military equipment, can accept this kind of high-quality cable. The high cost seriously restricts the use of this kind of cable. In addition, the cable diameter will also limit the number of cables that can be used in the system, and even high-quality cables will cause significant losses

another solution is to design an appropriate electronic circuit, which is not only used to drive the cable, but also can balance the cable loss. This scheme uses small pin electronic devices (PE) to drive cables, which allows the use of low-cost, high loss cables with smaller wire diameters. The device can also compensate the losses caused by PCB leads, relays and connectors. In addition, this scheme makes the system performance close to the performance index that PE can provide, without considering the problem of cable loss

the last solution is to combine the above two methods, but PE is the most practical choice, which is also the content of this paper

electronic method to solve cable loss

Figure 1 and Figure 2 describe cable loss, which leads to the smooth edge of the waveform or the wear of the final signal. It is the edges of these smooth signals that reduce the effective bandwidth of the system. The bandwidth loss comes from the cable rather than PE. In order to optimize system performance, effective system bandwidth needs to be restored

Figure 1 Basic principle of drive cable loss correction

Figure 2 Cable loss compensation of comparator channel

in order to correct signal wear and restore bandwidth, we must find a way to restore the edge of the waveform to the steep, noise-free square wave directly from the driver. This correction must be achieved by using the PE of the drive cable. Figure 1 contains an additional circuit module waveform shaping, which can effectively repair the edge of the signal by increasing the controllable overshoot amplitude. Edge repair is not achieved through a simple overshoot circuit. A simple overshoot circuit will have a negative impact on the edge, causing amplitude fluctuations. The amount of overshoot depends on the specific overshoot added. These adverse effects will cause errors such as timing and signal offset, and these errors change with the change of frequency and amplitude

Figure 2 shows a more detailed method of Maxim products for correcting cable loss. Figure 1 shows the waveform correction of the entire drive connection of PE IC from the cable to the DUT. Figure 2 shows a similar correction, from the DUT through the cable to the comparator of the PE. Both driver and comparator paths need to be corrected

the cable loss compensation circuit adds two first-order time constant attenuated peak signals to the signal. Dovsx input voltage controls the peak level with short duration to compensate for overshoot voltage; Dovlx input voltage controls the peak value with long duration to compensate for overshoot voltage. Short or long duration overshoot signals are limited to 10% overshoot. The attenuation time constants of the two peak signals are fixed respectively. The time constant of dovsx signal is 77ps, and the time constant of dovlx compensation is 1.5ns. As shown in Figure 2, covsx and covlx act as similar functions in the comparator channel

max9957 dual channel 2000mbps driver and Max are only slightly more expensive than the 4th gear. The dual channel 2000mbps comparator/terminator adopts dual time constants, as shown in Figure 1 and Figure 2. The two time constants can be adjusted respectively

max9979 dual channel 1100mbps driver/pmu, with level setting calibration DAC, using a single channel control architecture (shown in Table 1 and figure 3). This scheme also uses double time constants, but combines the double time constants into a 3-bit DAC

table 1 Max9979 cable attenuation compensation control

figure 3 Max9979 cable compensation

max9979 performance test under different cables

max9979 is a dual channel PE, which integrates driver/comparator/load (DCL), PMU and level setting calibration. The power consumption of each channel is 1.1W, and the optimized operation is 1Gbps, 3V signal, with 50 terminations

Figures 4 to 9 provide a set of max9979 test data, and the test platform is similar to figure 3. These test data are obtained under the following conditions: max9979 is configured at VDH = 3V, VDL = 0V, and provides 3V signal drive for 50 loads. The corresponding cables are shown in the figure

from the test results in Figure 4 to figure 9, it can be seen that the compensation cable has obvious advantages over the uncompensated cable. Figures 8 to 9 are close to the actual test results of the high-speed test device. It can be clearly seen that the level jump rate or system bandwidth has been reduced by almost 50%, and these losses are caused by attenuation through the cable. In some cases, the result may be worse, because the cable used by ate is more lossy than the cable used in the test. In addition, these tests also include losses caused by PCB leads, relays and connectors of signal channels. The PE cable compensation in Maxim's ate product line can compensate the loss of all signal channels

Figure 4 The conversion rate before and after compensation adopts solid-state and semi-rigid SMA cables

Figure 5 Rise time before and after compensation, using solid and semi-rigid SMA cables

figure 6 The conversion rate before and after compensation adopts rg58c cable

Figure 7 Rise time before and after compensation, using rg58c cable

figure 8 For the conversion rate before and after compensation, use RG174 cable

exclusion method: check whether there are curve coordinates

Figure 9 The rise time before and after compensation adopts RG174 cable

through further analysis of the data in Figures 4 to 9, it shows that the reduction of level conversion rate and the extended rise time are the key to cable loss, which is easier to see from the channel without compensation. The resulting loss depends on the length and quality of the cable used. In practical applications, the loss caused by the cable itself may exceed 50%


the price of solid-state SMA cable used in the test is 130 dollars/foot, the price of semi-rigid cable is 30 dollars/foot, and the price of RG58 and RG174 cables is 5 dollars/foot

the expensive cable has good performance, and the transmission length can even reach 36 inches. But these expensive cables also need compensation to support the highest data rate and minimum rise time

12 inch, especially 36 inch RG58 cable, even in the case of compensation, the level conversion rate decreases significantly and the rise time is longer. Uncompensated cable losses are greater

it can be seen from figure 8 and Figure 9 that without compensation, long high loss cables will greatly reduce the system performance. By compensating these cables, the signal bandwidth can be restored or the level conversion rate can be improved, reaching the performance index of more than 90% of the driver

in the system without cable compensation, if the PE driver can support 1000mpbs or higher rate, the loss caused by cable loss, relay, connector and PCB lead may be as high as 50%. On the contrary, using the PE system with cable loss compensation @ common steel hammer: the system performance can reach 90% of the index provided by the PE device itself

pe compensation must be adjustable. If PE only uses a simple overshoot circuit, it cannot compensate for a specific length of cable, because the edge and ripple will change with frequency and amplitude, thus introducing timing error

figures 10 and 11 show the actual measurement results of 6-foot and 3-foot RG174 cables. The data in Figures 8 and 9 are extracted directly from these results. The output results include uncompensated waveform, fully compensated waveform and 1-bit overcompensated waveform

Figure 10 The output waveforms of 6-foot RG174 cable are: no compensation, partial compensation, full compensation and over compensation (please refer to the relevant data in Figure 8 and Figure 9)

Figure 11 The output waveform of 3-foot RG174 cable is: no compensation, full compensation and over compensation (please refer to figure 8 and Figure 9 for relevant data)

the above waveforms show that the strict PE cable compensation design can maintain the real signal edge, and even reduce the amplitude fluctuation, so as to maintain the correct transient level and achieve the best performance of the system at any frequency and amplitude

summary of test results

the test results shown in Figures 4 to 11 confirm the above theoretical analysis and related discussions. The cable quality used in the test is better than that used in ate equipment. Obviously, without cable compensation circuit, the system will not achieve the same performance index as PE. The Ministry of industry, trade and resources of South Korea announced the integration of domestic research institutions and enterprises in South Korea to promote the commercialization of graphene. Designing cable compensation in PE can obtain almost 100% performance indicators, which is very close to the highest rate that PE can support

the cable compensation circuit is designed in the electronic driver, so that the system can use low-cost and high loss cables, while ensuring the overall performance. Adding this compensation function to the driver will increase the cost of each pin. However, the improvement of performance and the use of low-cost cables can undoubtedly make up for the increase in pin costs and ultimately reduce the overall cost

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