Amplifier
Operational "Sweet Spot" Page 1 |
| If you are just beginning your quest to learn about RF signals
that transmit television channels, you will need to know about an RF amplifier and how it
works with the coaxial cable, splitters, directional coupler taps that provide just the
right amount of signal level to television sets in hotels, hospitals, schools, and
apartment houses. An RF amplifier can boost (amplify) a weak signal to be strong
enough to overcome the losses of the coaxial cable, taps, splitters, etc. These RF
amplifiers are available with a single active RF gain block inside for simple tasks.
The most common types of RF amplifiers used in the cable television industry (CATV)
have two active RF gain devices inside. The more complex amplifiers used outdoors in
cable television systems may have as many as SIX RF gain devices of specialized
configurations we will discuss later. |
| Almost every day, someone asks me: "What are the best
input and output levels for this amplifier?" That is a good question which can
be answered easily or with complex analysis. As always, the simple answer is the
best. So here goes: Always choose the correct amplifier
for the job at hand. That may not sound like the answer you expected, but it
is the right one. Now the details... |
| Rule 1. Input signal levels should
ALWAYS exceed the noise figure of the RF amplifier by 3 to 6 dB. This may
sound easy, but some folks are fooled. Here's why. For practical purposes, the
input gain device of a two-stage amplifier establishes the noise figure for the entire
amplifier. This rule can be broken by too much interstage loss
due to equalizers, pads, attenuators and response networks. And of course the
input noise figure of the amplifier is equal to the noise figure of the input gain stage
(hybrid, transistor, etc.) plus the loss of any input passive devices (equalizer, diplex
filter, test point, power chokes, etc.) that precede that first RF gain stage. Don't
forget that the noise figure of this input gain stage may have a different dB rating at 50
MHz from the value at 450/550/750 MHz. Be sure to check the RF amplifier
specifications. If the published data does not specifically say that there is a
different noise figure at a given frequency, it "should" be safe to
"assume" the published value is at the highest RF amplifier operating
frequency. Typically the noise figure for a 450 or 550 MHz
hybrid amplifier chip is 1 dB better at 50 MHz than it is at the top frequency.
It could be as much as 1.5 to 2 dB for a 750 MHz chip. (September
12, 2012 update: GaAs amplifiers have a very FLAT noise figure performance
from 50 MHz to 1 GHz.) |
| "Well now, with a noise figure like that, why can't I hit
the input gain stage harder and get a better carrier-to-noise (C/N) ratio?"
That brings us to the other battle cable TV has waged since cable began: unwanted
mixer products, lines in the pictures, composite-triple-beats (CTB), etc. This is of
course, the result of over-driving any RF amplifier device to the point where
intermodulation (unwanted mixing of signals) produces other undesired signals (CTB,
etc.). It can happen in the input stage of an amplifier if the input levels are too
high, but is usually most prevalent with the higher RF signal levels produced at the
output stage of the RF amplifier. |
| Rule 2. Match required output levels
to the correct amplifier RF hybrid technology. A "rule of thumb"
was published in a Quality RF Services full page ad in the April and June 1998 issues of
Communications Technology. Here it is in simple terms: |
| RF Output Level |
Hybrid Technology |
| 25 to 32 dBmV |
Silicon Push-Pull (PP) |
| 32 to 36 dBmV |
Silicon Power-Doubled (PD), GaAs
PP |
| 36 to 40 dBmV |
Silicon Quadra-Powered (Dual PD) |
| 40 to 45 dBmV |
Feedforward/GaAs PD |
|
| This chart is a suggested general rule for broadband signal
amplification solutions. I have updated the chart on September
12, 2012. The Gallium Arsenide (GaAs) push-pull amplifier CTB distortions are
equal to or slightly better than a silicon power-doubled RF amplifier, and it draws much
less current, producing less heat. A GaAs power-doubled amplifier chip has CTB
distortions that are about 5 dB better than a silicon power-doubled chip, and again draws
less current, producing less heat. There are exceptions to the rule based on the
number of RF carriers to be amplified which will be addressed later in this text. < CLICK HERE FOR PAGE 2 > |