J. B. Wood wrote:
In article ,
wrote:
For sure. I was on the verge of returning my last analog TV because it
wouldn't get one channel at all, and several others were very poor. But
then I got an idea and added an attenuator at the antenna input. Problem
solved -- got a great picture on all channels. The new HDTV has a better
dynamic range and can put up with the strong signals, so it doesn't need
the attenuator. I'm about 10 - 15 miles line of sight from urban
broadcast towers. I use a commercial TV antenna in the attic.
Roy Lewallen, W7EL
Hello, Roy, and all. The trade-off here is that while putting a pad
(attenuator) in front of the amplifier certainly results in a lower signal
level at the amplifier output being presented to the down-stream
components, the noise figure of the pad+amp cascade is increased by the
attenuator loss (assuming a matched (e.g. 50 or 75 ohms)) pad. If the pad
is placed at the amplifier output the noise figure is preserved but the
dynamic range (usually specified in terms of a third-order intercept
point) of the cascade is degraded. Noise figure and intercept points of
RF distribution system amplifiers used in a shipboard environment have
always been of vital interest to the USN. Sincerely, and 73s from N4GGO,
John Wood (Code 5550) e-mail:
Naval Research Laboratory
4555 Overlook Avenue, SW
Washington, DC 20375-5337
The attenuation of an attenuator at the input of an amplifier adds
directly to the noise figure (e.g., a 20 dB pad increases the noise
figure by 20 dB), but the dynamic range remains the same. The dynamic
range is the ratio of the largest to smallest signal a receiver can
handle with acceptable performance, often defined as the ratio of the
third order intercept to the noise floor, which doesn't change when you
add an attenuator. For example, just picking some numbers, suppose the
noise floor is -100 dBm and the intercept 0 dBm, for a dynamic range of
100 dB. Add a 20 dB attenuator to the input. Now a signal (at the input
of the attenuator) of -80 dBm will be at the noise floor, and +20 dBm at
the third order intercept. The dynamic range is still 100 dB, just
shifted 20 dB higher. Many of the finest receivers have switchable
attenuators at the front end to increase the maximum signal handling
capability.
In any case, there's no real alternative to an attenuator if the signal
to a TV set is too large for it to handle.
Roy Lewallen, W7EL