Using 1 GHz GainMaker Amplifiers in an 870 MHz System Application Note

Transcription

1 Using 1 GHz GainMaker Amplifiers in an 870 MHz System Application Note Overview Introduction The move to a 1 GHz cable telecommunications system infrastructure is being driven by the increasing need for additional bandwidth. Cable system operators are exploring the options available to add channels to their systems. One of these options is an expansion of the current operating bandwidth. An easy way to facilitate this bandwidth expansion is a simple drop-in of higher bandwidth amplifiers into the system. These amplifiers are positioned in the same locations as the previous lower bandwidth amplifiers. This drop-in scenario minimizes the cost of labor involved since minimal or no re-splicing of equipment is required. In locations where the base portion of the housing is compatible with the new amplifier module, the old module is simply exchanged with a new higher bandwidth amplifier module. In these cases no re-splicing of housings is required. In situations where the housing base is not compatible with the new higher bandwidth amplifier module, the old amplifier station is removed and a new higher bandwidth amplifier station is spliced in. Labor costs are again minimized since the new higher bandwidth station is replacing the old station in the same location. There is no need for re-routing of cables or addition of new cable; both of which can be costly. Discussion Cisco has developed 1 GHz GainMaker amplifiers that exhibit the specifications required to drop-in to the locations of lower bandwidth amplifiers. Numerous design studies have taken place proving this capability. In existing 870 MHz GainMaker systems where additional bandwidth is not required, the GainMaker 1 GHz amplifiers are backwards compatible with the GainMaker 870 MHz amplifiers. They can be used in place of the 870 MHz amplifiers along with the existing 870 MHz accessories.

2 Overview Moving to higher bandwidth amplifiers in an existing system raises an interesting question among some operators. How will this 1 GHz amplifier work in my 870 MHz system? This paper addresses this concern and shows that a seamless integration can easily take place. Issues such as cable loss, amplifier gains, gain tilt, equalizing, and padding are addressed Rev C

3 EQ, tilt, and gain EQ, tilt, and gain Equalizer value versus tilt Equalizers are designated by stating the loss of the coaxial cable for which they must compensate. 1 The value of the equalizer is based on the amount of cable that produces a loss at the upper most frequency that is equal to the given value. An equalizer is designed to compensate for that amount of cable and it will have characteristics inverse to that of the cable. It will exhibit its highest loss at the lowest frequency and its lowest loss at the highest frequency. The tilt produced by an equalizer is simply the difference in loss between the lowest and highest frequency (or the two frequencies of interest). It is important to note that the equalizer tilt and equalizer value are two distinctly different terms and should not be interchanged. An EQ value X db will produce a tilt of Y db. Gain Amplifier gains are developed to compensate for the loss created by coaxial cable and passive components used in the system. In architectures where the desired effect is to increase the bandwidth of the system without moving amplifier locations, the gain must be such that it will match the loss of the existing spacing at the new higher frequency. The gain of the new higher bandwidth amplifier must be higher than the lower bandwidth amplifier it replaces. Higher gain may also translate to longer spacing between amplifiers. The higher gain can be fully utilized in new or rebuild designs. In an existing system, where amplifier locations are already set, any additional or excess gain is attenuated away. If this additional gain is not too excessive, it can be attenuated at the amplifier inputs without causing any significant impact to the system carrier-to-noise. Gain in an amplifier also exhibits a slope (or tilt). This gain tilt or internal tilt is the difference in gain between the highest and lowest frequency in the downstream passband of the amplifier. Plotting this tilt on a graph provides the ability to determine the gain at other frequencies Rev C 3

4 Gain and cable loss Gain and cable loss 870 MHz Gain in a 1 GHz Amplifier Figures 1 and 2 demonstrate the internal tilts of the GainMaker 1 GHz amplifiers. Figure 1 shows the plot of a 1 GHz station with 15.5 db internal tilt. It can be seen that the gain at 870 MHz will be 1.3 db less than the 1 GHz gain. This is typical of the High Gain Dual (HGD), High Gain Balanced Triple (HGBT), and Unbalanced Triple (UBT) amplifiers. Figure 1. 1 GHz station with 15.5 db internal tilt As seen in Figure 2, a 1 GHz station with 9.5 db of internal tilt will have 0.8 db less gain at 870 MHz. This is seen in the Low Gain Dual (LGD) and Line Extender (LE) amplifiers Rev C

5 Gain and cable loss Figure 2. 1 GHz station with 9.5 db of internal tilt In a 1 GHz GainMaker amplifier, the gain at 870 MHz will be somewhat higher than the 870 MHz version of the same model amplifier. This is brought about by the use of equalizer circuits to create the internal tilt and the increases in gain and tilt designed into the amplifier. Table 1 shows the gains of the various amplifiers in both their 1 GHz and 870 MHz versions. The column to the right shows the difference in 870 MHz gain between the 1 GHz and 870 MHz amplifiers. Table 1. Gains of the various 1 GHz and 870 MHz amplifiers Rev C 5

6 Gain and cable loss Cable loss Signal attenuation through coaxial cable increases at higher frequencies. This increase is not linear. It is a function of the square root of the frequency ratio. That is why, when plotted on a linear graph, cable loss will exhibit a bow shape. Cable equalizers, when plotted on the same linear graph, will exhibit a bow shape inverse to that of the cable. As discussed in the previous section and shown in Figures 1 and 2, this contributes to the additional gain at 870 MHz in a 1 GHz amplifier Rev C

7 Pad and equalizer adjustments Pad and equalizer adjustments Pad and equalizer adjustments when using 870 MHz equalizers Replacing an 870 MHz amplifier with a similar 1 GHz version will require some minor rebalancing of the amp. In a system where 870 MHz equalizers will continue to be used, a higher value input pad and a lower value input EQ are needed to set the station back to its original 870 MHz output levels. This is due to the slight increase in gain and internal tilt. In order to demonstrate the effects on the input pad and equalizer, the example in Figure 3 is provided. In the drawings, the levels and loss above the coax line and the gain preceding the slash (/) are representative of 870 MHz. The levels and losses below the coax line and the gain after the slash (/) are representative of 50 MHz. Figure 3. Pad and equalizer adjustments when using 870 MHz equalizers 870 MHz Amplifiers HGD HGD 10 dbmv Gain = 40/27.5 db 50 dbmv 37.5 dbmv -30 db at 870 MHz -6.9 db at 50 MHz 20 dbmv 30.6 dbmv Gain = 40/27.5 db 50 dbmv 37.5 dbmv 10 dbmv db tilt arriving at amp 0 db input tilt required Select 13.5 db 870 MHz EQ (10.6 db tilt) Select 10 db pad HGD 1 GHz Amplifiers HGD 8.3 dbmv Gain = 41.7/27.5 db 50 dbmv 37.5 dbmv -30 db at 870 MHz -6.9 db at 50 MHz 20 dbmv 30.6 dbmv Gain = 41.7/27.5 db 50 dbmv 37.5 dbmv 10 dbmv TP db tilt arriving at amp -1.7 db input tilt required Select 12.0 db 870 MHz EQ (9.4 db tilt) Select 11.5 db pad Generally, a couple rules of thumb can be applied to the input pad and equalizer selection when using the 870 MHz equalizers. These changes are based on comparison to the existing 870 MHz input pad and equalizer. For the input equalizer - decrease the equalizer one value (-1.5 db). Amplifiers spaced within 2 db of maximum gain may require a two value decrease (-3.0 db) Rev C 7

8 Pad and equalizer adjustments For the input pad - increase the pad 1.5 db for the amplifiers with a 1.7 db gain delta and increase the pad 1.0 db for the amplifiers with a 1.2 db gain delta. See Table 1. Pad and equalizer adjustments when using 1 GHz equalizers In a system where the 1 GHz equalizers will be used, the changes to the pad and EQ values will vary based on the amount of cable being equalized for between any two amplifiers. The key here is to select the 1 GHz EQ based on the tilt created between 50 and 870 MHz. Since the absolute loss changes at 870 MHz based on the EQ value, the pad selection is affected. Remember, there is 1.2 to 1.7 db of excess gain that can be taken up by the additional EQ loss at 870 MHz. After this, the input pad value will need to decrease proportionally to the increase in the 870 MHz loss of the equalizer. In order to demonstrate the effects on the input pad and equalizer, the following example is provided. In the drawings, the levels and loss above the coax line and the gain preceding the slash (/) are representative of 870 MHz. The levels and loss below the coax line and the gain after the slash (/) are representative of 50 MHz. Figure 4. Pad and equalizer adjustments when using 1 GHz equalizers HGD 870 MHz Amplifiers HGD 10 dbmv Gain = 40/27.5 db 50 dbmv 37.5 dbmv -30 db at 870 MHz -6.9 db at 50 MHz 20 dbmv 30.6 dbmv Gain = 40/27.5 db 50 dbmv 37.5 dbmv 10 dbmv db tilt arriving at amp 0 db input tilt required Select 13.5 db 870 MHz EQ (10.6 db tilt) Select 10 db pad HGD 1 GHz Amplifiers HGD 8.3 dbmv Gain = 41.7/27.5 db 50 dbmv 37.5 dbmv -30 db at 870 MHz -6.9 db at 50 MHz 20 dbmv 30.6 dbmv Gain = 41.7/27.5 db 50 dbmv 37.5 dbmv 10 dbmv TP db tilt arriving at amp -1.7 db input tilt required Select 12.0 db 1 GHz EQ (8.7 db tilt) Input padding required = 11.7 db Additional EQ loss = 0.9 db at 870 MHz Subtract additional EQ loss Select 11 db pad Rev C

9 Pad and equalizer adjustments Here again, a couple rules of thumb can be applied to the pad and equalizer selection when using the 1 GHz equalizers. These changes are based on comparison to the existing 870 MHz input pad and equalizer. For the input equalizer - decrease the equalizer one value (-1.5 db). In some cases, the equalizer value may not need to change for amplifiers spaced within 2 db of maximum gain. For the input pad - increase the pad 1.0 db for the amplifiers with a 1.7 db gain delta and increase the pad 0.5 db for the amplifiers with a 1.2 db gain delta. See Table 1. The 1 GHz equalizer has additional loss at 870 MHz. As seen in the previous section, when 870 MHz equalizers were used, the input pad was used to offset the increased gain. Now the additional equalizer loss offsets some of that increased gain. Amplifiers spaced within 2 db of maximum gain are unlikely to require a change to the input pad Rev C 9

10 Cascade performance Cascade performance An example of noise and distortion calculations for a cascade is a clear way to show the impacts of the slight additional gain at 870 MHz. Assuming a cascade of a node plus 6 amplifiers with a preceding fiber link, the carrier to noise and distortion ratios can be calculated and compared. In this example, the node and amplifiers are operating at 50 dbmv output at 870 MHz with a 12.5 db output tilt. The node is a GainMaker High Gain Balanced Triple followed by three High Gain Duals and three Line Extenders. The 1 GHz node and amplifiers are modeled with the additional gain at 870 MHz that would be expected (see Table 1). The performance numbers in Table 2 show the end-of-line (EOL) results for each of two scenarios. The first is the original 870 MHz cascade. The second is the same cascade after replacing the node and each amplifier with a 1 GHz version of each unit. The calculations for the 1 GHz model are based on operation to 870 MHz for an even comparison. Table 2. End-of-line results As can be seen in the table above, there is a negligible degradation to the carrier to noise performance. When we examine the performance of the RF section (Node + Amps), there is a 0.3 to 0.4 db change to CTN. Once the fiber link performance is added in, this change is diluted to 0.1 db in each case since the fiber link is the dominant CTN contributor. Other nodes, cascades, levels, etc. can be considered in a comparison model like this. It is not expected to see anything greater than 0.5 db degradation to the EOL CTN. A benefit to the cascade performance can be seen in the distortion performance. The improvement in the station distortion performance of the 1 GHz products is reflected in the EOL performance Rev C

11 Summary Summary Migrating to a 1 GHz node and amplifier platform should be viewed as an easy and beneficial move. The benefits in performance and bandwidth enhancement capability far outweigh any negatives. There is no additional cost to deploy the higher bandwidth equipment when compared to deploying the same model lower bandwidth equipment. Optimizing the 1 GHz equipment for the existing 870 MHz bandwidth is a straightforward process. There are minimal changes required to the input pad and equalizer regardless of whether the 870 MHz or 1 GHz equalizers are used to maintain the existing 870 MHz system. From an operational standpoint, it is easier to make those changes at the amplifier input rather than trying to re-configure the inter-stage losses to change the gains and tilts. As shown in the performance calculations, there isn't any significant degradation to the system performance. On the other hand, setting the stage for a 1 GHz bandwidth expansion does require some pre-engineering and analysis of the existing system and levels. The only decision will be whether to limit the new equipment to the existing lower operating bandwidth or set it up initially for the capability to operate at its full bandwidth Rev C 11

12 Loss tables Loss tables Table 3. Forward Cable Equalizers MHz - Loss Table EQ Value (db) Part Number Typical Insertion Loss (db) at Various Frequencies (MHz) Rev C

13 Loss tables Table 4. Forward Cable Equalizers - 1 GHz - Loss Table EQ Value (db) Part Number Typical Insertion Loss (db) at Various Frequencies (MHz) Rev C 13

14 For Information For Information Support Telephone Numbers If you have technical questions, call Cisco Services for assistance. Follow the menu options to speak with a service engineer Rev C

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16 Cisco Systems, Inc Sugarloaf Parkway, Box Lawrenceville, GA Cisco and the Cisco logo are trademarks or registered trademarks of Cisco and/or its affiliates in the U.S. and other countries. To view a list of Cisco trademarks, go to this URL: Third party trademarks mentioned are the property of their respective owners. The use of the word partner does not imply a partnership relationship between Cisco and any other company. (1110R) Product and service availability are subject to change without notice. 2008, 2012 Cisco and/or its affiliates. All rights reserved. August 2012 Printed in USA Part Number Rev C