Using Highpass Filters For TVI - And
When Not To
Highpass filters used with modern television receivers are passive devices intended
to block the reception of frequencies below 54 Mhz and allow to pass signals above
that frequency. The television range used today extends from 54 Mhz to 806 Mhz,
not inclusive. Cable television frequencies extend from 54 Mhz to 300 Mhz in most
systems but as high as 500 Mhz in some of the larger cities with 70 or more channels.
When interference occurs
to TV reception it's important to try to recognize first what the nature
of the specific case is and from where it comes. If voice and
video distortion both occur and it is believed that a
strong local transmitting source such as a CR or Amateur Radio station
is involved then it's generally pretty easy to determine what to do next.
If the interference occurs
to only one TV channel or perhaps two channels spread aways apart then
the most likely cause is harmonic signal generation from the transmitter
source. This type of interference can only be solved at the
transmitter by filtration and it's not the most common type of malady.
A more frequent type of interference is when the local transmitter interrupts
the reception of many or all channels, inducing wavy lines or audio noise
into the system. This specific case is called fundamental overload
and is caused by large signal voltages present in the immediate area.
There are two primary ports
of entry that locally generated radio signals can reach and disrupt TV
circuitry. The first is through the TV's antenna or cable line and
the second is through the AC power line. Here's how to tell which
case you have. Disconnect the antenna or cable line from the back
of the set and drop it to the floor. Have the station owner transmit
again and observe the screen. If interference disappears then you know
that the offending signal was entering the TV through the antenna line
and a highpass filter installation is the next step. If interference
persists then the AC line is part of the problem and an AC line filter
may also have to be installed. Either way, a combination of simple
disconnection tests can provide a wealth of data from which a solution
can be drawn.
If a highpass filter
is part of the program here's how to choose an appropriate unit.
Be sure that the filter is designed to attenuate BOTH common mode and differential
mode interference. Common mode is where the shield and center conductor
of the TV's antenna coaxial line are both electrified by a locally generated
signal. Differential mode is where the center conductor alone is
electrified and the shield maintains its neutral (ground) integrity.
Common mode is the most common of modern cases, and a good highpass filter
should have a common mode loss of 20db or more. If manufacturers
do not publish their loss figures, shop elsewhere.
Always mount the filter
as close as possible to the input (antenna) connector of the TV set or
VCR. Generally it's best to place the filter between the incoming antenna
or cable lead and the first item to which the cable lead is connected.
But it may be more effective in some cases to connect the incoming line
to the VCR and install the highpass filter to the input connector on the
TV receiver. It's important to keep leads short and connections tight.
But keep in mind that all
cases of interference differ somewhat, and that experiemnting with different
combinations of protective devices is normal in the pursuit of good results.
Make notes as you go and don't be discouraged if early results are not
satisfying. Most interference cases can be solved without a lot of
investment or effort.