More Power, Better Pictures
Welcome to Tech Talk, Pico Macom's series of technical notes, featuring useful information and equipment tips. This installment of Tech Talk discusses Pico Macom’s line of quality amplifiers and accessories.
Choosing an amplifier
It is very common to hear, “I need a little bit more signal.” This time in Tech Talk we will discuss the selection and application of amplifiers. There are different types of amplifiers with features suited to various applications:
- Headend amplifiers are rack-mount amplifiers that are used primarily to overcome the losses associated with the channel combining system and to provide a high level “launch” of signal from the headend.
- Distribution amplifiers are strand or wall-mount amplifiers that are used to overcome the cable, splitter and tap losses associated with distributing signal to multiple dwellings.
- Drop amplifiers are usually fixed-value amplifiers that are used to overcome the losses associated with splitting and distributing signal within a single residence.
When selecting the amplifier, knowledge of the signal frequencies, architecture, and spacing is required to ensure that all signals pass through and increase in level with the least amount of induced noise and distortion.
Forward Bandwidth
To know the forward bandwidth, you need to know the lowest and highest channels. In the U.S., the lowest channel carried is typically channel 2, and most amplifiers will accommodate the low frequency of 54 MHz. The highest frequency is not as easily determined. Most cable television companies have placed digital signals above the highest analog channel. These signals cannot be seen without proper test equipment. If you are creating your own analog channel lineup with a headend, the highest channel is usually 78 with a frequency of 550 MHz (CA-30/550). If amplifying a CATV channel lineup, it is best to assume the highest frequency as 1 GHz (CA-30/1000).
Reverse Bandwidth
The upstream bandwidth is used for signals that return to the headend. These signals can be from channels (such as local government or security) or services (such as cable modem or telephone). The bandwidth required usually starts at 5 MHz and extends to 36 or 42 MHz (PIDA-1000). Due to the low frequencies there is little attenuation through the cable, so the reverse path does not require as much amplification as the forward path.
Amplifier Gain
The gain of an amplifier is rated in dB. This is the amount of change that is available to the signal applied to the input. If a signal of 12dBmV were applied to the input, the resulting output of a 30dB amplifier would be 42dBmV. The exception to the rule is when the maximum input of the amplifier has been exceeded. This will cause compression of the signal and, regardless of how much you increase the input, there will be no more output level achieved.
Amplifier Placement
Where to place amplifiers is a complicated question. The goal is unity gain. Ideally, every amplifier will have the same input and output level. However, due to convenience, this is rarely possible. Achieving unity gain is done in two ways:
- The first is by running the amplifiers with a tilted output (higher frequencies at a higher output level than lower frequencies).
- The second is by using extra cable, signal attenuators (FAM-*) and equalizers (TSEQ-2150-*) to compensate for the effects of short or over-spacing of amplifiers.
By design, on a tilted distribution system the amplifier should be placed at the point where the highest and lowest frequencies are at the same amplitude level. Then an attenuator may be placed if needed to reduce the overall level to the required input level. If the spacing is shorter, additional cable can be inserted in line to simulate the proper spacing. If the spacing is greater, a signal equalizer will reduce the lower frequency levels more than the higher frequencies, compensating for the cable loss and “balancing” the input levels. With the same input levels at each amplifier, the output levels can be set consistent throughout the system.
