CATV amplifier and its adjustment

CATV amplifier and its adjustment

1. Cable characteristics

　　Cable is the main component of CATV network, it has three characteristics: impedance characteristics, transmission characteristics and temperature characteristics. The main function of the amplifier in the network is to compensate the loss of the cable with its gain, so understanding the characteristics of the cable will help us understand the composition, principle and debugging of the amplifier.

　　1. Transmission characteristics of cables

　　①. Cables have different attenuation for high-frequency signals of different frequencies. The higher the frequency of the signal transmitted on a unit length (generally 100 meters) of cable, the greater the attenuation. The characteristic that the loss of the cable changes with the frequency is called the slope characteristic of the cable. The transmission attenuation of an ideal cable is proportional to the square root of the frequency of the transmitted signal. Due to this slope characteristic of the cable, slope compensation or equalization processing is required in the CATV system. The following is a table of transmission characteristics of several commonly used cables: 

　　Usually we design the line based on the attenuation of the cable at the highest operating frequency of the transmitted signal. Here we introduce a concept called electrical length. In CATV systems, the length of the cable is often expressed by the decibel loss of the cable at the highest operating frequency. We call it the electrical length of the cable.

　　The device that compensates for the negative slope of the cable in the network is the equalizer. There are generally two ways to express the equalization value: one is to directly mark the loss decibel difference between the high and low frequency reference points; the other is to mark the electrical length, which is called the equivalent equalization value. The slope of a certain section of cable is equal to its electrical length divided by the coefficient μ=1/1-(fL/fH)1/2, where fL is the low-end frequency and fH is the high-end frequency. According to this formula, it is calculated that when the frequency range is 50-750MHZ, μ=1.35, and when it is 50-550MHZ, μ=1.43. This relationship is very useful in network design and debugging.

　　The cable slope we discussed above is linear and idealized, as shown by the black line in Figure 1. In fact, the slope curve of the cable is arc-shaped and nonlinear, as shown by the red line in Figure 1. The apex of this arc is near 400MHZ, which means that the cable loss in the mid-frequency band is actually smaller than the ideal attenuation curve value, resulting in a bulge in the level near the mid-frequency band in the entire channel when the line is long.

　　②. The attenuation of high-frequency signals by the cable is proportional to the length of the cable.

　　2. Temperature characteristics

　　The slope and loss of the cable are also related to the temperature of the environment. We use a temperature coefficient parameter to describe the temperature characteristics of the cable. The temperature coefficient of the general cable is 0.2%/C0, that is, if the temperature increases by one degree, the loss will increase by 0.2%. In most parts of my country, the loss change caused by the temperature on the cable is ±5%. When the cable network is long, the impact of the temperature characteristics of the cable cannot be ignored.

　　3. Impedance characteristics

　　The nominal characteristic impedance of commonly used CATV cables is 75Ω. When the mechanical characteristics of the cable deteriorate due to long-term effects such as its own weight and wind pressure load, the characteristic impedance of the cable will change, resulting in a decrease in the reflection loss of the network, and in severe cases, ghosting of the image. In the laying of the network, we often have certain requirements for the bending degree and binding process of the cable, the purpose of which is to prevent the mechanical properties of the cable from deteriorating due to improper construction, causing the characteristic impedance of the cable to change, and thus deteriorating the reflection loss index of the network. 

　　2. Amplifier Overview

　　1. Amplifier gain 

　　In order to ensure that the CTB index is normal, the output level of the amplifier must be reduced. Generally speaking, if the level drops by 1db, the CTB index can be increased by 2db. The input level of the amplifier is determined by C/N. These indicators are related to the number of amplifiers N used in the network. Plotting the relationship between the input level, the output level and the number of amplifiers N into a curve will form a V-shaped curve, as shown in Figure 2. The difference between the upper and lower straight lines in the figure is called the limit gain of the amplifier. As can be seen from Figure 2, as the number of units N increases, the limit gain of the amplifier will also decrease, that is, the gain of the amplifier cannot be higher than the limit gain, otherwise it will not meet the requirements of the index. For a certain N, there is a corresponding limit gain, so it is very important to correctly select the gain of the amplifier.

　　When the gains of two amplifiers are different (such as one is 35db and the other is 27db) but their maximum output level and noise figure are equal, if a CATV system uses 10 amplifiers with a gain of 35db, then the same system uses 13 amplifiers with a gain of 27db. If the CTB of the two systems is equal, the C/N of the latter will be improved by 35-27-20 (lg13-lg10) = 5.8db. If the C/N is equal, the CTB index can be improved by 5.8*2 = 11.6db. From the above analysis , it can be seen that the use of low-gain amplifiers has a certain effect on the improvement of general system characteristics. So is it better to have a lower amplifier gain? The answer is no. When the gain of the trunk amplifier drops below 8db, both C/N and CTB will deteriorate, because the number of amplifiers connected in series increases, and CTB will deteriorate due to the increase of 20lgN; C/N will also deteriorate due to the increase of 10lgN. In addition, if the gain is low, for the same output level, the signal value input at the input end is required to be higher, and the amplifiers at all levels are prone to nonlinear distortion. For this reason, when the line is long, it is more appropriate to select the trunk amplifier gain at around 27db.

　　2. How the amplifier works

　　In a CATV network, the amplitude-frequency characteristics of the amplifier must be related to the cable transmission characteristics. For this reason, the amplifier has the following three main working modes.

　　① Make the input signal level of the trunk amplifier independent of frequency (i.e. the input signal is flat), and the output signal level compensates for the attenuation change of the cable, that is, the positive slope of the output signal (the difference between the high and low end output levels is positive) just compensates for the negative slope generated by the cable (the difference between the high and low end output levels is negative). This method is called the output full tilt method. As shown in Figure 3a

　　② Make the output signal level of the trunk amplifier independent of frequency (that is, the output signal is flat), and the amplifier gain compensates for the attenuation change of the cable (that is, the positive slope generated by the amplifier just compensates for the negative slope generated by the cable). This method is called flat output mode. As shown in Figure 3b

　　③. The method between the above two is called the semi-tilt output method. As shown in Figure 3c, 　　the amplifier working in the full-tilt mode appeared in the early days. This amplifier divides the entire transmission frequency range into two channels, high and low, and amplifies them separately. The high-end channel gain is higher than the low-end channel. It is rarely used now. 
　 
 

　　An amplifier that works in a flat output mode uses a combination of equalization and an amplifier with flat characteristics. This mode of operation is beneficial to improving non-linear distortion because the signal input to the amplifier module is flat, but an equalizer with a large amount of equalization must be used, so it is mostly used in CATV systems of 450MHZ and below.

　　Since the highest working frequency of current amplifiers is 750MHZ or even 860MHZ, both trunk amplifiers and extension amplifiers basically adopt semi-tilted output mode. This type of amplifier is usually composed of more than two amplifier modules, and it has two equalizers set inside. One is the input equalizer, which ensures that the signal input to the first amplifier module is flat; the other is the inter-stage equalizer, which makes the output signal produce the slope we need. The choice of amplifier working mode is not random. For example, the amplifier is determined to be in flat output working mode during design. In actual application, the amplifier is not placed in a flat output state, but in a semi-tilted output mode. In this way, when debugging, it is necessary to increase the equalization amount of the equalizer at the input end of the amplifier to achieve the semi-tilted output mode, which will cause the C/N of the low-end signal to deteriorate seriously.

　　3. Amplifier gain control function

　　The amplifier has the following control methods for level fluctuations: manual control (MGC), automatic gain control (AGC), automatic level control (ALC), and automatic slope control (ASC). Manual control consists of manual control gain and equalization. The control unit can be mechanical or electrically adjustable. Amplifiers with this control method are mostly used in networks with shorter networks. When the network is longer, due to the influence of the temperature characteristics of the cable, the signal level at the user end will change greatly, which is not allowed. For this reason, an amplifier with AGC control must be used. This type of amplifier uses the channel carrier near the midpoint within the working frequency band as a reference pilot to control the gain of the amplifier, thereby stabilizing the output level of the amplifier. However, from the temperature characteristics of the cable, it can be seen that when the temperature changes, not only the signal level will change, but the slope of the signal will also change. For this reason, automatic slope control (ASC) is introduced. Usually, the amplifier with both AGC and ASC functions is called automatic level control (ALC). ALC often uses the following two methods:

　　① Use a temperature detector, such as a thermistor or a thermistor semiconductor, to measure the temperature change to control the slope and gain of the amplifier.

　　②, Use two pilot signals of different frequencies, one for AGC control and the other for ASC control. Usually, the low pilot is used for ASC (the carrier frequency of my country's standard channel 1 or 3 can be used); the high pilot signal is used for AGC (the carrier frequency of standard channel 42 can be selected), so that the high-end level is firmly clamped and unchanged. ASC uses this as the reference level to keep the relative level of the low pilot point and the high pilot point at the optimal value through its control.

　　The first method has low control accuracy, but the circuit is simple; the second method has high control accuracy, but the circuit is more complicated. Usually high-end amplifiers use the second method.

　　4. Power supply of amplifier

　　There are generally two types of power supply voltages for amplifiers: one is AC 220V power supply, which belongs to the mains power supply mode; the other is AC 60V power supply, which belongs to the line power supply mode. The power supply circuit structure of the amplifier powered by the mains power supply mode is: transformer + bridge rectifier + simple voltage stabilizing circuit. It has poor adaptability to the change of the mains voltage. Within the range of ±10%, when the mains voltage fluctuates greatly, the AC noise modulation index will decrease, causing 50HZ or 100HZ interference, which is reflected on the TV screen as a rolling black band (50HZ) interference or two black bands (100HZ) interference.

　　The power supply circuit of the line-powered amplifier generally adopts a switching voltage-stabilized power supply. Its circuit structure uses an oscillator to generate an oscillation signal of tens of kHz. After amplification, voltage stabilization, and rectification, it generates the working voltage required by the amplifier. This power supply circuit has a wide voltage stabilization range. When the external power supply changes from 35V to 90V, it can output a stable working voltage. Therefore, most trunk amplifiers or extension amplifiers now use this power supply circuit.

　　In the line power supply mode, we also need to install a power supply and a power inserter on the line. The power supply is a device that supplies power to the amplifier. This device is actually a ferromagnetic AC voltage stabilizer. After inputting the AC voltage of 220V from the mains, it will output a stable 60V AC voltage at its output end. The power inserter is an interface device between the power supply and the line.

　　5. Several important parameters of the amplifier

　　①. Maximum output level of the amplifier: This parameter refers to the upper limit level of the amplifier's output under a certain distortion index when the amplifier is at full load (78 PAL channels for a 750MHZ system).

　　②. Amplifier noise factor: Since the amplifier is an active device, it will also generate noise. When the amplifier amplifies the signal, it will also add noise to the output end. In this way, the output signal's carrier-to-noise ratio must be lower than the input signal's carrier-to-noise ratio. The noise factor is the ratio of the input carrier-to-noise ratio to the output carrier-to-noise ratio. ③. CTB and CSO: These two parameters are amplifier distortion parameters. CTB is called combined third-order distortion, and CSO is called combined second-order distortion. It reflects the distortion condition of the amplifier at the maximum output level under full load.

　　④. Gain: The amplifier’s ability to amplify signals.

　　The above parameters are essential when designing and debugging CATV networks.

　　6. Amplifier module

　　Nowadays, CATV amplifiers all use amplifier modules inside. Generally, there are three types of amplifier modules: ordinary amplifier modules, power doubler output amplifier modules, and quadruple power output amplifier modules. The amplifier circuits inside these modules all use push-pull amplifier circuits, which can reduce harmonic distortion, especially secondary distortion. Therefore, when designing the system, we only need to consider the CTB index. As long as the CTB index is achieved, the CSO index is also achieved. The power doubler type is a parallel connection of two or four push-pull circuits working simultaneously. The amplifier using this module can increase the output level by 3db or 6db under the same distortion index.

　　7. Reverse amplification channel

　　Due to the need for multifunctional business development, current amplifiers are equipped with reverse channels, which are often composed of manual gain control, manual slope control circuit and amplifier module, and can be selected according to whether the system needs reverse function. The reverse amplifier bandwidth is divided into three types according to the two-way split frequency: low split (5~40MHZ), medium split (5~112MHZ) and high split (5~174MHZ).

　　3. Amplifier debugging

　　The adjustment of the amplifier mainly includes two aspects, one is the adjustment of the level, the other is the adjustment of the slope. Generally, the slope is adjusted first and then the level is adjusted.

　　1. Dry storage debugging

　　Here we take GI's MB-75SH trunk amplifier as an example. It is a four-module amplifier with one input port and three output ports (one main output port and two auxiliary output ports). It is equipped with a plug-in input equalizer and attenuator, a frequency response correction board and a Porter board (ALC board), line power supply, and semi-tilted output mode. The working gain of the amplifier is: 30±4db.

　　Frequency response correction board (MDR): It is mainly designed to correct the amplitude-frequency characteristics of the amplifier within the entire working bandwidth. It divides the entire passband of the amplifier into six points and segments for compensation correction, making the entire passband characteristics of the amplifier flat and uniform. This board must be adjusted with special equipment. This board is also equipped with an equalizer with an equalization amount (750MHZ) of 10db.

　　BODE: This board is mainly used with ADU board or TDU board (also called automatic gain control board or temperature compensation control board) to achieve the purpose of ALC. ADU board or TDU board are automatic control plug-ins. It generates control voltage by sampling and comparing high and low pilot frequencies or by sampling and comparing ambient temperature to control the BODE board, thereby achieving the purpose of AGC and ASC. The control amount of this board is ±4db.

　　Select jumper for bridge port output mode (SPLIT): If jumpers 1 and 2 are connected, bridge port 4 will have signal output and bridge port 3 will be empty; if jumpers 2 and 3 are connected, bridge port 3 will have signal output and bridge port 4 will be empty; if two distributors or one splitter are inserted at this position, both bridge ports 3 and 4 will have signal output.

　　Assume that the adjustment requirement of dry placement is: the output level of the main output port and the output bridge port is 100db. It compensates for the cable loss of 29db (750MHZ) in electrical length, and the slope generated by the cable is: -29/1.35=-21db, then the output slope should be 21/2=10.5db.

　　First, measure the input level, compare the level difference between the high and low end channels (select frequency 3 for the low end reference channel and frequency 42 for the high end channel), select an equalizer with an equalization value equal to the level difference, and insert it into the amplifier. The purpose of this is to make the signal input to the first amplifier module a flat signal. Since the frequency response characteristic correction board in the amplifier has a preset slope of +10db, the slope of the output level should be 10db at this time. Then make the following adjustments:

　　①. Adjustment of automatic gain control (ADU board):

　　a. Select the pilot signal for control, connect the signal level tester to the amplifier test point, and measure the high-end channel signal, for example: 743.25MHZ. Plug in the ADU board (the required pilot signal on the ADU board must be consistent with the selected pilot signal frequency) and connect the jumper of the DRIVE UNIT to the "auto" position.

　　b. Rotate the level control knob marked as ADU on the mainboard to make its attenuation reach the maximum value. Calculate the control margin value required at the current temperature. Calculation process: Assume that the temperature range is -21C0--49C0, the control margin at the highest temperature is set to 2db, and the additional loss caused by each degree of temperature increase is 29*0.2%=0.058 (29 is the electrical length of the cable). Take 14C0 as a temperature change order of magnitude to deduce the required control margin: 0.058*14=0.82db, take 1db, we divide the temperature range into six levels and list their corresponding control margins in Table 2. The level measured by the instrument should be equal to the required output level plus the required margin at the current temperature. If the output level is too large, an input attenuator with appropriate attenuation should be inserted to make the output level within 0.5db of the required level. The output level required by the amplifier is 100db. If the measured level is 105db and the ambient temperature is 21 degrees C. Look up Table 2 and find that the required margin at this temperature is 4 dB. The attenuation of the attenuator to be inserted is 105-(100+4)=1 dB.

Table 2 

　　c. Adjust the ADU level control knob to make the output level equal to the output level we need (100db). In this way, through the control of the ADU board, the pilot signal output changes within the range of 95db--105db, and the output of the amplifier can be maintained at the output level we need.

　　②. Adjustment of installation temperature compensation control (TDU board):

　　a. Jump the electrical length selection jumper of the TDU board compensation cable to 30db (10db, 20db, 30db are available) and adjust the Thermal level potentiometer on the TDU to minimize the loss of the Porter board.

　　b. Connect the signal level tester to the amplifier test point, measure the level of the highest frequency signal at the output end and record it as V0. According to the ambient temperature, look up Table 2 to find the control margin required at this temperature, and adjust the Thermal level potentiometer on the TDU so that the highest frequency signal level at the output = V0 - margin value.

　　c. Insert the corresponding input attenuation to make the highest frequency output signal level equal to the required output level.

　　③. Choice of power supply mode

　　MBS-75SH dry-amplifier adopts line power supply, and the power supply voltage range is ~38V--~90V. The power supply mode is very flexible, including input port power supply, main output port power supply, and bridge port power supply. It has four built-in fuses, and 15 power supply modes can be set by plugging or unplugging these four fuses.

　　2. Debugging of the building

　　The working mode of the floor amplifier design is mostly flat output mode. It is generally a single module amplifier, so the various indicators are relatively low. For this reason, it is recommended that the output level adjustment should not exceed 100db. In terms of slope adjustment, since it works in a flat mode, the output signal slope cannot be ≥5db, otherwise the carrier-to-noise ratio of the low-end signal will be degraded. According to regulations, the level difference (i.e. slope) of the high and low end channels of the user output port cannot be greater than 10db. When adjusting the slope, we can make an estimate as follows: the branch line of the distribution network uses SYWV-75-9 type cable, which is generally no longer than 100 meters. The slope generated by 100 meters of SYWV-75-9 cable is -7db; the household line is SYWV-75-5 cable, which is generally no longer than 20 meters. The slope generated by 20 meters of SYWV-75-5 cable is -3db. The slope caused by the branch distributor is about -1db, so the total slope generated is: 7+3+1=11db. The maximum allowable level slope at the user end is 9db, so the slope of the output level of the building amplifier should be 11-9=2db.

　　As mentioned above, the negative slope of the cable to the RF signal is not linear. In the middle section near the working bandwidth, the cable loss is smaller than the ideal situation. The resulting slope characteristic is shown in the red curve in Figure 1, which causes the signal level to bulge in the middle. In the case of a long network, we must take some compensation measures. There are two methods:

　　①. Install an equalizer with the characteristic curve shown by the blue line in Figure 1 every two or three amplifiers on the network (based on a level bump of 3-4db). It can be seen from the figure that this equalizer can well compensate for the mid-section level bump caused by the cable.

　　② Install an adjustable notch filter in the network. The notch point frequency and notch amount of this notch filter can be adjusted. In this way, we can adjust the notch point frequency and notch amount according to the actual situation of the network level, so as to achieve better compensation.

　　3. Conclusion

　　The main function of the amplifier in the CATV network is to compensate for the loss of the cable on the transmission signal. According to the transmission characteristics and temperature characteristics of the cable, the amplifier is equipped with an automatic gain control circuit (AGC), an automatic slope control circuit (ASC) and a temperature compensation circuit. The input level of the amplifier is determined by the carrier-to-noise ratio (C/N) index, the output level of the amplifier is determined by the combined third-order distortion (CTB) index, and the gain of the amplifier is determined by the number of series-connected stages.

　　There are three working modes of the amplifier: full-tilt output mode, flat output mode, and half-tilt output mode. Now most trunk amplifiers are of half-tilt output mode, while floor amplifiers are mostly of flat output mode.

　　The adjustment of the amplifier mainly includes two aspects, one is the adjustment of the level, and the other is the adjustment of the balance. Generally, the slope is adjusted first and then the level. If the amplifier uses a pilot control module (ALC), the high and low-end pilot signals should be reasonably selected. Generally speaking, the low-end pilot signal uses the video carrier frequency of the low-end signal, and the high-end pilot signal uses the video carrier frequency of the high-end signal. If the amplifier uses a temperature compensation module, pay attention to the temperature compensation range calibrated by the module and the control amount of the module, and then set the margin value according to these two parameters for debugging.