Snowy picture on Sky

Hi All,
We have a peculiar problem with one of our customers Sky installs. The
output ( via RF) of the Sky box becomes extremely snowy when the aerial
cable is attached. On checking the usual suspects, i.e. the RF output
channel interfering with another and so on, everything is perfect.
Irrespective on the Sky RF channel used the snowy picture on the Sky channel
remains the same, while the terrestrial channels are perfect. The analog
aerial levels were also checked for overloading and all levels were below
70DbuV. We have connected up the spectrum analyzer to the aerial and there
is no other channel coming in on the channel the Sky box is being
transmitted out on( namely channel 60). While trying a variable attenuator
on the input does solve the snowy picture on sky, it also leaves the
terretrial snowy also. While a quick answer would be to use a scart lead,
the customer has the Sky box distributed to 6 other rooms. We have tried
various sky boxes, all display the same problem.
We were considering placing a notched filter tuned to channel 60 on the
input of the Sky box. Do you guys think this will remedy the problem.

Thanks for your help.

J

"noname" wrote in message
...
Hi All,
We have a peculiar problem with one of our customers Sky installs. The
output ( via RF) of the Sky box becomes extremely snowy when the aerial
cable is attached. On checking the usual suspects, i.e. the RF output
channel interfering with another and so on, everything is perfect.
Irrespective on the Sky RF channel used the snowy picture on the Sky
channel remains the same, while the terrestrial channels are perfect. The
analog aerial levels were also checked for overloading and all levels were
below 70DbuV. We have connected up the spectrum analyzer to the aerial and
there is no other channel coming in on the channel the Sky box is being
transmitted out on( namely channel 60). While trying a variable attenuator
on the input does solve the snowy picture on sky, it also leaves the
terretrial snowy also. While a quick answer would be to use a scart lead,
the customer has the Sky box distributed to 6 other rooms. We have tried
various sky boxes, all display the same problem.
We were considering placing a notched filter tuned to channel 60 on the
input of the Sky box. Do you guys think this will remedy the problem.

If you're considering a notch filter you obviously think the noise is coming
from the aerial. If so you should be able to see it on the analyser.
Remember, though, that noise 40dB down will make the picture snowy. That
means the noise could be as low as -30dBmV. Will your analyser detect noise
at such a low level? If not you could possibly amplify the aerial feed and
see if that reveals anything on the analyser screen.

If the analyser is an old one with no DTT facilities you are unlikely to be
able to measure, or even detect, such noise levels.

If there is a masthead amplifer on the aerial this problem is common, and it
is very likely if the amplifier is two stage, with a gain figure in excess
of 22dB. Very old amplifiers are really bad for this.

Bill

In article , noname
wrote:
Hi All, We have a peculiar problem with one of our customers Sky
installs. The output ( via RF) of the Sky box becomes extremely snowy
when the aerial cable is attached. On checking the usual suspects, i.e.
the RF output channel interfering with another and so on, everything is
perfect. Irrespective on the Sky RF channel used the snowy picture on
the Sky channel remains the same, while the terrestrial channels are
perfect. The analog aerial levels were also checked for overloading and
all levels were below 70DbuV. We have connected up the spectrum analyzer
to the aerial and there is no other channel coming in on the channel the
Sky box is being transmitted out on( namely channel 60). While trying a
variable attenuator on the input does solve the snowy picture on sky, it
also leaves the terretrial snowy also. While a quick answer would be to
use a scart lead, the customer has the Sky box distributed to 6 other
rooms. We have tried various sky boxes, all display the same problem. We
were considering placing a notched filter tuned to channel 60 on the
input of the Sky box. Do you guys think this will remedy the problem.

What are the input analogue channels? There might be some mixing or other
strange effect. Seems likely if you attenuate the RF input and the problem
goes away.

Have you looked (on your spectrum analyser) for any signals, not in the uhf
bands) which might give rise to harmonics?

--
From KT24 - in "Leafy Surrey"

Using a RISC OS computer running v5.11

Hi All,
Thanks for the replies. My analyser is an old one, which might be the reason
I can't see any noise. There is a Triax 25A amp ( new model two stage amp,
we are in a fringe area in Ireland) on the aerial . As soon as I disconnect
the aerial input the signal is perfect on the Sky channel. Was thinking that
if I placed the notch filter on the input to the amp, it might get rid of
the snow on channel 60. What do you think.

Thanks for your help.


"charles" wrote in message
...
In article , noname
wrote:
Hi All, We have a peculiar problem with one of our customers Sky
installs. The output ( via RF) of the Sky box becomes extremely snowy
when the aerial cable is attached. On checking the usual suspects, i.e.
the RF output channel interfering with another and so on, everything is
perfect. Irrespective on the Sky RF channel used the snowy picture on
the Sky channel remains the same, while the terrestrial channels are
perfect. The analog aerial levels were also checked for overloading and
all levels were below 70DbuV. We have connected up the spectrum analyzer
to the aerial and there is no other channel coming in on the channel the
Sky box is being transmitted out on( namely channel 60). While trying a
variable attenuator on the input does solve the snowy picture on sky, it
also leaves the terretrial snowy also. While a quick answer would be to
use a scart lead, the customer has the Sky box distributed to 6 other
rooms. We have tried various sky boxes, all display the same problem. We
were considering placing a notched filter tuned to channel 60 on the
input of the Sky box. Do you guys think this will remedy the problem.

What are the input analogue channels? There might be some mixing or other
strange effect. Seems likely if you attenuate the RF input and the
problem
goes away.

Have you looked (on your spectrum analyser) for any signals, not in the
uhf
bands) which might give rise to harmonics?

--
From KT24 - in "Leafy Surrey"

Using a RISC OS computer running v5.11

"noname" wrote in message
...
Hi All,
Thanks for the replies. My analyser is an old one, which might be the
reason I can't see any noise. There is a Triax 25A amp ( new model two
stage amp, we are in a fringe area in Ireland) on the aerial . As soon as
I disconnect the aerial input the signal is perfect on the Sky channel.
Was thinking that if I placed the notch filter on the input to the amp, it
might get rid of the snow on channel 60. What do you think.

Well there you are then! Isn't that what I told you? Notch filters are OK
but do drift. I suggest either a double notch, stagger tuned slightly, or
better still--

1. Can you pass the aerial signals through a bandpass filter and put the Sky
box output on a channel well away from the broadcast signals (and hence the
bandpass filter's passband)? You can use a notch as well of course.

2. Do it the professional way. See
http://www.wrightsaerials.tv/Resourc...annel%20TV.pdf

This is very old but gives some idea of how you can use channel
filter/levellers to exclude interference.



Bill

Thanks for your help
"Bill Wright" wrote in message
...

"noname" wrote in message
...
Hi All,
Thanks for the replies. My analyser is an old one, which might be the
reason I can't see any noise. There is a Triax 25A amp ( new model two
stage amp, we are in a fringe area in Ireland) on the aerial . As soon as
I disconnect the aerial input the signal is perfect on the Sky channel.
Was thinking that if I placed the notch filter on the input to the amp,
it might get rid of the snow on channel 60. What do you think.

Well there you are then! Isn't that what I told you? Notch filters are OK
but do drift. I suggest either a double notch, stagger tuned slightly, or
better still--

1. Can you pass the aerial signals through a bandpass filter and put the
Sky box output on a channel well away from the broadcast signals (and
hence the bandpass filter's passband)? You can use a notch as well of
course.

2. Do it the professional way. See
http://www.wrightsaerials.tv/Resourc...annel%20TV.pdf

This is very old but gives some idea of how you can use channel
filter/levellers to exclude interference.



Bill