RULE OF THUMB CALCULATIONS
A Rule of Thumb is typically a quick approximation that is accurate enough to use in practice. We have developed such a "Rule of Thumb" for calculating amplifier distortion. The Cross Modulation of the first Amplifier is fairly simple to calculate. As indicated above
Cross Mod 1 = 58 + 2(108 - Actual Level)
We strongly recommend that for all practical purposes, users maintain a maximum output level from hybrid amplifiers of 100 dBU. Hence for all Hybrid Amplifiers utilizing the usual Motorola or Philips Hybrid IC, Cross Mod of the first Amplifier will be
= 58 + 2(108 - 100)
= 74 dB
The key "Rule of Thumb" for Amplifier distortion is:
Doubling the Number of Amplifiers Increase distortion by 6 dB Hence if one Amplifier generates Cross Mod distortion (for 100 dBU output level) of -74 dB, 2 Amplifiers will result in Cross Mod distortion of -68 dB, 4 Amplifiers will result in X-Mod distortion of -62 dB, 8 Amplifiers will yield -56 dB and 16 Amplifiers will yield -50 dB. However BIS specifications dictate that distortion should not be worse than -54 dB. Clearly we can therefore use between 8 and 16 amplifiers. The exact mathematical calculation yields 10 amplifiers. To calculate distortion follow these simple steps :
STEP 1
Calculate the distortion of the first Amplifier:
X-Mod = 74 + 2(Rated output - Actual output)
STEP 2
In the left hand column list down numbers from 1, that keep doubling viz. 1, 2, 4, 8, 16, etc On the right hand side column against 1, list the X-Mod of the first Amplifier that was calculated in Step 1. Keep reducing this by 6 dB as you progressively move down the column, till you reach -53 dB.
As an example, let us calculate the X-Mod distortion for Hybrid Amplifiers operating at an output level of 98 dBU.
X-Mod of First Amp = 58 + 2(108 - 98)
= 78 dB
1 78 dB 2 72 dB
4 66 dB 8 60 dB
16 54 dB 32 48 dB
This indicates that more than 16 Amplifiers can be used in cascade, if the maximum output level is maintained below 98dBU. All these calculations are of course, only considering distortion. Actually, the noise contribution should also be reviewed. The method for estimating & controlling amplifier noise has already been outlined in the previous article, in this series of articles.
DISTORTION LIMITS
The Bureau of Indian Standards vide their specification IS-14264 dictates that the customer should receive a signal which is at least 54 dB higher than the total distortion on the signal. Assuming that the headend does not generate a significant amount of distortion, it implies that all cascaded amplifiers on the network from the headend to the subscriber at the furthest point, should not generate more than -54 dB of distortion. Table 1 indicates the effect of various types of distortion on the picture quality. It is interesting to note that CTB also generates a grainy picture which can sometimes be mistaken for a noisy picture.
WIDEBAND AMPLIFIER DISTORTION
The Wideband or Broadband Amplifier, offers poorer performance than a Hybrid Amplifier and is suitable for small distribution networks carrying approximately 24 channels. Such Amplifiers typically yield higher distortion. A good working rule is to operate the Wideband Amplifier at a maximum output level of 95 dBU. Operated at this level, the Wideband Amplifier gives satisfactorily low distortion for use in delivering a quality CATV signal.
MINI HYBRID AMPLIFIERS
The Mini Hybrid Amplifier, as the name suggests, uses a "Mini" hybrid circuit. Various such hybrid circuits are available. Some of these are manufactured in India while others are of Chinese origin.
The Mini Hybrid Amplifier offers better performance than a Wideband Amplifier but is not as good as a regular Amplifier utilizing the Motorola 5342 / 6342 Hybrid IC or its Philips equivalent. To obtain good performance from a Mini Hybrid Amplifier, it is recommended that the Amplifier be operated at a maximum output level of 97 dBU.
POWER DOUBLER ICs
As we have seen earlier, increasing the output level by 3 dB increases the distortion by 6 dB i.e. distortion rises very rapidly. An increase of 3 dB in the output level may not seem much to the non technical user but actually implies double the output power level. A new type of Hybrid IC ( Philips BGD 502 or 504 ) that uses a dual output stage offers double or 3 dB higher output, for the same distortion. This family of Hybrid ICs has been called "Power Doublers". As indicated, increasing output level by 3 dB increases distortion by 6 dB. On the other hand, if the Power Doubler (Hybrid IC) Amplifier output level is not increased by 3 dB, the Power Doubler Hybrid IC will provide 6 dB lower distortions than the Hybrid IC, for the same output level.
Again, reviewing the earlier formula which dictated that distortion increased by 6 dB when the Number of Amplifiers used is double. Hence, twice as many Power Doubler Amplifiers can be used in a cascade, compared to regular Hybrid IC Amplifiers, for the same output level of say 100 dBU. Of course, the Power Doubler Hybrid IC and Amplifier are more expensive than the regular Hybrid IC equivalent.
DISTORTION & NUMBER OF CHANNELS
Just as a truck engine would strain to pull a full load compared to an empty truck, similarly a CATV Amplifier would generate higher distortion if it is used to amplify 67 channels instead of 1 channel. Actually there is a formula that links the distortion to the number of channels being amplified. However an easier method is to remember that amplifier distortion increases by 3 dB if the number of channels is doubled. Similarly a Hybrid IC Amplifier that would operate well with 69 channels and 100 dBU output level can be operated with an output level of 103 dBU if used on an 18 channel CATV network. However it is apparent that changing the number of channels from 69 to 35 permits the output level to be increased from a maximum of 100 dBU to 101.5 dBU. The difference is not much and 1.5 dB could easily be the calibration error of your signal level meter measuring a 100 dBU output signal. Hence it best to adhere to the recommended maximum output level of 100 dBU for a Hybrid IC based CATV Amplifier.
CONCLUSION
The article presents a simplified overview of distortion. It also provides an easy, non mathematical method to determine the distortion generated in a chain of CATV Amplifiers. Further, a simple practice of maintaining a maximum output level of 100 dBU for CATV Hybrid Amplifiers is recommended. This will provide good performance and sufficiently low distortion for use in almost any practical CATV network.
CROSS MODULATION DISTORTION (X-Mod) :
Diagonal bars usually running from the top right hand corner to the lower left corner of the screen. This is often termed as "Wind Shield Wiper" effect since it resembles the Wind Shield Wiper across the screen. A faint picture of another channel is superimposed. This is most prominent when the screen goes blank in between programmed or when texts such as titles are scrolled against a dark background.
COMPOSITE TRIPLE BEAT DISTORTION (CTB) :
The picture is "Wormy". Appears as if tiny worms are crawling across the screen! Grainy picture. The picture will appear with good contrast and colour but consists of grains or tiny dots all over the screen. A grainy picture due to noise will often lack colour and Contrast.
A Rule of Thumb is typically a quick approximation that is accurate enough to use in practice. We have developed such a "Rule of Thumb" for calculating amplifier distortion. The Cross Modulation of the first Amplifier is fairly simple to calculate. As indicated above
Cross Mod 1 = 58 + 2(108 - Actual Level)
We strongly recommend that for all practical purposes, users maintain a maximum output level from hybrid amplifiers of 100 dBU. Hence for all Hybrid Amplifiers utilizing the usual Motorola or Philips Hybrid IC, Cross Mod of the first Amplifier will be
= 58 + 2(108 - 100)
= 74 dB
The key "Rule of Thumb" for Amplifier distortion is:
Doubling the Number of Amplifiers Increase distortion by 6 dB Hence if one Amplifier generates Cross Mod distortion (for 100 dBU output level) of -74 dB, 2 Amplifiers will result in Cross Mod distortion of -68 dB, 4 Amplifiers will result in X-Mod distortion of -62 dB, 8 Amplifiers will yield -56 dB and 16 Amplifiers will yield -50 dB. However BIS specifications dictate that distortion should not be worse than -54 dB. Clearly we can therefore use between 8 and 16 amplifiers. The exact mathematical calculation yields 10 amplifiers. To calculate distortion follow these simple steps :
STEP 1
Calculate the distortion of the first Amplifier:
X-Mod = 74 + 2(Rated output - Actual output)
STEP 2
In the left hand column list down numbers from 1, that keep doubling viz. 1, 2, 4, 8, 16, etc On the right hand side column against 1, list the X-Mod of the first Amplifier that was calculated in Step 1. Keep reducing this by 6 dB as you progressively move down the column, till you reach -53 dB.
As an example, let us calculate the X-Mod distortion for Hybrid Amplifiers operating at an output level of 98 dBU.
X-Mod of First Amp = 58 + 2(108 - 98)
= 78 dB
1 78 dB 2 72 dB
4 66 dB 8 60 dB
16 54 dB 32 48 dB
This indicates that more than 16 Amplifiers can be used in cascade, if the maximum output level is maintained below 98dBU. All these calculations are of course, only considering distortion. Actually, the noise contribution should also be reviewed. The method for estimating & controlling amplifier noise has already been outlined in the previous article, in this series of articles.
DISTORTION LIMITS
The Bureau of Indian Standards vide their specification IS-14264 dictates that the customer should receive a signal which is at least 54 dB higher than the total distortion on the signal. Assuming that the headend does not generate a significant amount of distortion, it implies that all cascaded amplifiers on the network from the headend to the subscriber at the furthest point, should not generate more than -54 dB of distortion. Table 1 indicates the effect of various types of distortion on the picture quality. It is interesting to note that CTB also generates a grainy picture which can sometimes be mistaken for a noisy picture.
WIDEBAND AMPLIFIER DISTORTION
The Wideband or Broadband Amplifier, offers poorer performance than a Hybrid Amplifier and is suitable for small distribution networks carrying approximately 24 channels. Such Amplifiers typically yield higher distortion. A good working rule is to operate the Wideband Amplifier at a maximum output level of 95 dBU. Operated at this level, the Wideband Amplifier gives satisfactorily low distortion for use in delivering a quality CATV signal.
MINI HYBRID AMPLIFIERS
The Mini Hybrid Amplifier, as the name suggests, uses a "Mini" hybrid circuit. Various such hybrid circuits are available. Some of these are manufactured in India while others are of Chinese origin.
The Mini Hybrid Amplifier offers better performance than a Wideband Amplifier but is not as good as a regular Amplifier utilizing the Motorola 5342 / 6342 Hybrid IC or its Philips equivalent. To obtain good performance from a Mini Hybrid Amplifier, it is recommended that the Amplifier be operated at a maximum output level of 97 dBU.
POWER DOUBLER ICs
As we have seen earlier, increasing the output level by 3 dB increases the distortion by 6 dB i.e. distortion rises very rapidly. An increase of 3 dB in the output level may not seem much to the non technical user but actually implies double the output power level. A new type of Hybrid IC ( Philips BGD 502 or 504 ) that uses a dual output stage offers double or 3 dB higher output, for the same distortion. This family of Hybrid ICs has been called "Power Doublers". As indicated, increasing output level by 3 dB increases distortion by 6 dB. On the other hand, if the Power Doubler (Hybrid IC) Amplifier output level is not increased by 3 dB, the Power Doubler Hybrid IC will provide 6 dB lower distortions than the Hybrid IC, for the same output level.
Again, reviewing the earlier formula which dictated that distortion increased by 6 dB when the Number of Amplifiers used is double. Hence, twice as many Power Doubler Amplifiers can be used in a cascade, compared to regular Hybrid IC Amplifiers, for the same output level of say 100 dBU. Of course, the Power Doubler Hybrid IC and Amplifier are more expensive than the regular Hybrid IC equivalent.
DISTORTION & NUMBER OF CHANNELS
Just as a truck engine would strain to pull a full load compared to an empty truck, similarly a CATV Amplifier would generate higher distortion if it is used to amplify 67 channels instead of 1 channel. Actually there is a formula that links the distortion to the number of channels being amplified. However an easier method is to remember that amplifier distortion increases by 3 dB if the number of channels is doubled. Similarly a Hybrid IC Amplifier that would operate well with 69 channels and 100 dBU output level can be operated with an output level of 103 dBU if used on an 18 channel CATV network. However it is apparent that changing the number of channels from 69 to 35 permits the output level to be increased from a maximum of 100 dBU to 101.5 dBU. The difference is not much and 1.5 dB could easily be the calibration error of your signal level meter measuring a 100 dBU output signal. Hence it best to adhere to the recommended maximum output level of 100 dBU for a Hybrid IC based CATV Amplifier.
CONCLUSION
The article presents a simplified overview of distortion. It also provides an easy, non mathematical method to determine the distortion generated in a chain of CATV Amplifiers. Further, a simple practice of maintaining a maximum output level of 100 dBU for CATV Hybrid Amplifiers is recommended. This will provide good performance and sufficiently low distortion for use in almost any practical CATV network.
CROSS MODULATION DISTORTION (X-Mod) :
Diagonal bars usually running from the top right hand corner to the lower left corner of the screen. This is often termed as "Wind Shield Wiper" effect since it resembles the Wind Shield Wiper across the screen. A faint picture of another channel is superimposed. This is most prominent when the screen goes blank in between programmed or when texts such as titles are scrolled against a dark background.
COMPOSITE TRIPLE BEAT DISTORTION (CTB) :
The picture is "Wormy". Appears as if tiny worms are crawling across the screen! Grainy picture. The picture will appear with good contrast and colour but consists of grains or tiny dots all over the screen. A grainy picture due to noise will often lack colour and Contrast.
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