Trunk Amplifier Equalization

GAIN & SLOPE
The signal partially amplified by the pre amplifier section now passes to the main gain and slope controls. As the name suggests, the gain control allows user setting of the output signal level. The slope control permits setting of the overall slope required by the system. While some products maintain fixed pads for specific, plug-in gain and slope control values, others offer a variable potentiometer for more versatile, on site control. The advantages and disadvantages of fixed pads versus potentiometers is elaborated a little later.

POST AMPLIFIER
After the signal has been corrected for its level and slope, it is fed through the post amplifier. The post amplifier is designed to provide maximum output with the least possible distortion. Since the post amplifier already receives an input signal of fairly high signal levels, it is not necessary to optimise the post amplifier for low noise performance. Ideally, the post amplifier should be a "Power Doubler" or "Power Quadrupler". The power doubler configuration offers double the output level (3 dB higher), with equivalent distortion compared to an ordinary push-pull Hybrid IC. The power quadrupler offers 4 times the output level (6 dB higher) than an ordinary push-pull Hybrid IC. The output of the post amplifier is fed to the output diplex filter of the amplifier.

AGC / ALC
Most trunk amplifiers offer the option of providing an automatic gain control or an automatic level control facility. The AGC / ALC module is usually offered as an optional extra. A detailed explanation on the function of the AGC / ALC circuit has been provided in earlier articles and therefore not included in this write up.
It would however suffice to inform readers that the AGC / ALC module automatically compensates for 6 to 12 dB of variation in the input signal. These variations could occur not only due to changes in cable temperature but also some times due to permanent changes such as the insertion of a one way tap off at some point in the trunk line, before the amplifier. An AGC specification of +/- 4 dB would imply a total AGC range of 8 dB.

FIXED PADS & VARIABLE ATTENUATORS
Most of the budget priced line extenders offer a variable control for adjusting the gain and slope of an amplifier. The continuous variable controls offer tremendous flexibility and ease of set up. Their operation is however, relatively unstable. The setting value tends to drift over a period of time. The settings can also be inadvertently disturbed by a mechanical movement of the spindle. Fixed pads on the other hand, offer extremely high stability as well as reliability. The fixed pads are offered in a range of values and are easily plugged in or removed at site. Gold plated contacts are used to ensure that the contacts do not tarnish or deteriorate due to the adverse outdoor ambient conditions.
The disadvantage for fixed pads ofcourse is that a large inventory needs to be maintained to cater to the specific requirement of each amplifier in the network. A good system design would typically dictate that each trunk amplifier should receive an input signal of 70 dBU to 75 dBU and provide an output signal of 95 dBU to 100 dBU. The specific figures would depend on the number of amplifiers in cascade, the distance between each amplifier, the gain of each amplifier and of course whether the output stage uses power doubling technology. However an output signal level of 95
dBU to 100 dBU will generally be ideal. Once such a logical and consistent system design has been implemented, most amplifiers in the system will require a similar set of fixed value equaliser and attenuation pads. A bulk of the inventory needs to accommodate values of only about 3 dB above and below the typical value. It must be noted that fixed pads are available not only for gain control but also for equalisation. Often even the signal path between the pre and post amplifiers incorporate both, fixed and variable control. The fixed pads provide a bulk of the compensation while the variable controls are used to simply "trim" or optimise to the exact required characteristic.

THE REVERSE PATH AMP
While the above discussions have been restricted to the forward path amplification section, the reverse path amplifier can also benefit from a similar circuit topology. The reverse path typically caters to signals from 5 MHz to 50 MHz. Manufacturers often provide a variety of diplexer options to cater to different forward and reverse path frequency splits. While a range of 5 to 50 MHz may
not seem large, it must be noted that these frequencies represent a 1:10 ratio i.e. a 1 decade frequency range. Slope compensation over this one decade is often desirable particularly if the entire reverse path is to be utilised. However, most Indian networks use less than adequately shielded cable. (e.g. 60% braid RG11). Also connectoring practice is shabby. As a result, noise ingress is very substantial over the 5 MHz to 20 MHz frequency range. Due to this, most CATV networks do not transmit signals in the 5 to 20 MHz range. Hence utilised reverse path is just 20 to 50 MHz. In this case, reverse path slope compensation is often not necessary.

THE TRUNK STATION
A trunk amplifier unit often houses several amplifiers and modules. As an example, the unit may house 1 Forward Path Amplifier Module + 1 Bridger Amplifier Module + 1 Reverse Path Amplifier Module and ofcourse a built in power supply. Other modules such as AGC, status monitoring, etc may also be incorporated. A unit consisting of 2 or more amplifier modules in a single housing is often referred to as a "Trunk Station".