Introduction to Amplifier Operation

composite second order (CSO)

A clustering of beats 1.25 MHz above the visual carriers in a CATV network

Composite Triple Beat (CTB)

A form of interference or noise resulting from the mixing of all of the various carriers in a CATV network.

What have fiber nodes done to amplifiers in the RF network?

It has reduced the number of amplifiers between the headend and the customer, from 20 -30 to 3 and to 0 in fiber deep networks.

multimedia over coax alliance (moca)

Multimedia over Coax Alliance (MoCA) is a home networking technology that provides a coax backbone for next-generation home entertainment networking technology. MoCA RF signals have a frequency allocation between 1150 and 1500 MHz. Cable modems or CMs in the home may also be MoCA compliant. MoCA versions include 1.0, 1.1, 2.0 and 2.5.

What is the correct definition of noise figure NF, in dB, for cable amplifiers in terms of the input and output SNRs in dB?

SNR in = NF + SNR out

why do cable systems need amplifiers?

Two reasons: - To make up for signal loss that happens after propagation over a long run of coax cable - To compensate for splitting losses that occur when the signal is divided into multiple outputs.

an amplifier takes an input signal and increases it by the gain (G) of the amplifier. in linear (non-dB) terms, this means you multiply the input voltage by the gain to calculate the output voltage.

Volts out = G voltage x voltage input ex input 1 mv thru an amp w 100 gain Vo = .001 V x 100 Voltage out = 0.1

What type of amplifier is the most power efficient?

class c

list valid examples of nonlinear distortion.

combined triple beat, ctb composite second order, cso composite intermodulation noise, cin common path distortion, cpd

line extenders, distribution amplifiers

designed for high rf levels to provide adequate signals for multiple taps and sub drops; typically contribute more distortion and less broadband noise than mainstations

mainstations, trunk stations, trunk line stations, trunk amps

designed for moving HFC signals over sections of the network with few or no directional taps; may contain bridger modules that provide up to 4 additional outputs from the amp

Intermodulation distortion in amplifiers is produced by what?

high input signals that drive the amp output into nonlinear operation

what do QAM donuts come from?

inermodulation distortion in RF amps, ingress of ham or broadcast high freq radio on teh upstream

Common Path Distortion (CPD)

intermodulaion distortion of down signals due to nonlinearities found at metallic junctions. manifest as a series of beats or a band of noise, most noticeably in the upstream path.

why is tilt equalization necessary?

it compensates for the high attenuation that high frequencies face to ensure proper RF levels throughout the network

After traveling long distances over coaxial cable, what causes tilt in cable networks?

lower freqs are attenuated less that higher ones

dBm/decibel milliwatt

measure of signal strength used in optics, wifi and microwave networks. referenced to 1 milliwatt (mW). 0 dBm = 1 mW

decibel millivolt (dBmV)

measure of signal strength used in wired/cabled networks. decibels referenced to 1 millivolt (mV). in a 75 ohm system, 0 dBmV = 1 mV

signal to noise ratio (SNR)

measurement of the ratio of signal power to noise power.

is the polarity of the input signal an important aspect of an rf amplifier?

no

decibels (dB)

relative power measurement using logarithmic units to express the ratio of two values. provides a means of representing large power ratios as manageable numbers and allows overall gains and losses in a module or network to be calculate by addition and subtraction. decibel is one-tenth of a Bel.

equalization

the amp boosts the higher frequencies by a greater amount than the lower ones; an 'uptilt' in the output of the amps to account for the 'downtilt' of coax attenuation

What happens to the amplifier when the input level exceeds the maxium safe input level?

the amplifier goes into compression

impedance

the effective resistance of an electric circuit or component to alternating current, arising from the combined effects of ohmic resistance and reactance.

In a three amplifier cascade in the cable plant, which amplifier's noise figure will have the biggest impact on the overall signal?

the first amp will have the biggest impact

The total noise factor of a multistage amplifier depends mostly on which stage?

the noise factor of the first

A reason for using uptilt equalization in RF amplifiers, in cable networks, is that the amplifier's transistor noise and distortion performance worsens with increasing frequency.

true

Class A amplifiers have the characteristic that all of the input signal is used, but it is very inefficient energy wise.

true

Class C amplifiers use less than half of the input signal is used, which turns out to be very power efficient.

true

In Class AB or Hybrid class A and B amplifiers, the two active elements conduct more than half of the time.

true

In Class B amplifiers, the active element carries current half of each cycle, and is turned off for the other half in a pure Class B, while the push pull Class B design combines two units so that both cycles of the sine wave are amplified.

true

Modern cable RF amplifiers use a cascode design.

true

TRUE or FALSE: If you limit the range of input frquencies, you can get more gain from amplifiers.

true

The reality of all practical RF amplifiers is that they add additional noise to the signal as they are amplifying the signal along with any noise or interference at the input to the amplifier. The amount of additional noise added by an amplifier is given by its noise factor F sub N in linear terms, and equivalently by its noise figure NF in decibel terms. Since the amp adds noise, the output SNR is lower than the input SNR. This defines the noise factor or noise figure - the output SNR is reduced by an amoun equal to the noise figure of the amplifier.

true

True or False: High order distortions, such as third order happen when an amplifier goes non-linear.

true

True or False: It is normal and acceptable to have some distortions in a cable network.

true

amplifiers have no impact on the signal to noise ratio.

true

diplex filter

used to combine or separate two different bands of frequencies, ie up and down streams