Sky+HD box not working with Optima Multiswitch

Stephen H wrote:

Min dBuV Max dBuV C/N ratio db
Analogue TV PAL 60 80 43
Digital UHF DTT 45 65 26
FM (stereo) 57 74 45
FM (mono) 40 74 25
DAB (Band II) 45 70 20
Sat IF analogue 47 77 15
Sat IF digital 47 77 12

So I reckon your LNB signals are too high for the multiswitch to handle
properly without introducing distortion.



In practice most switches will manage +25dBmV (85dBuV) and in fact they
are often used like
that where cable runs are very long. But it really is the top limit, and
I would normally
set a maximum of +18dBmV (78dBuV) where cables are long, resulting in
receiver levels of perhaps 5dBmV. This is far more than the receiver
really needs, but it helps fight any crap picked up at the bottom end.

Bill

Stephen H wrote:

If you are actually using a QUATTRO LNB with the multiswitch, there will
not be any tone or voltage switching between the QUATTRO LNB and the
Multiswitch so you can use normal attenuators between the QUATTRO LNB
and multiswitch.

As long as they pass DC without starting to smoke.

Bill

On 13/03/2013 03:39, Bill Wright wrote:
Stephen H wrote:

The signal between the LNB and multiswitch is peaking at 76.8dBuV, but
is peaking at 80.6dBuV after the multiswitch so it looks as the
multiswitch is adding 4dB gain to the signals so it appears to be an
amplified multiswitch

They all amplify except for a few specialist ones that require external
pre-amplification. They have to amplify because the internal losses are
high; about 20dB is normal.

Bill

Bill's comment has reminded me to say a few words about multiswitch design.

As Bill has rightly pointed out, you can have up to 20dB of internal
loss within the multiswitch and with the gain appearing to be 4dB from
inputs to outputs, there clearly is an amplifier in the multiswitch with
a gain of 24dB.

Now In order to minimise degrading of the C/N and the S/N ratios, the
input signals are amplified first internally before
multiplexing/splitting to the various outputs.

So that means your input signals is going in at 76.8dBuV,
straight into the signal amplifier(s) (which will be a 2 stage design)
and then coming out at a level of 76.8 + 24 = 100.8dBuV

before then going through the switch multiplexers/splitters and
suffering 20dB loss to exit the multiswitch at 100.8 - 20 = 80.8dBuV.

So with an already strong signal in the first place, you're
over-amplifying and introducing intermodulation products...

I know you said you have only 5m of cable between LNB and multiswitch,
but I suspect your dish is bigger than you actually need for 28.2E. What
size is it?

Stephen

"Stephen H" wrote in message
...
On 13/03/2013 03:39, Bill Wright wrote:
Stephen H wrote:

The signal between the LNB and multiswitch is peaking at 76.8dBuV, but
is peaking at 80.6dBuV after the multiswitch so it looks as the
multiswitch is adding 4dB gain to the signals so it appears to be an
amplified multiswitch

They all amplify except for a few specialist ones that require external
pre-amplification. They have to amplify because the internal losses are
high; about 20dB is normal.

Bill

Bill's comment has reminded me to say a few words about multiswitch
design.

As Bill has rightly pointed out, you can have up to 20dB of internal loss
within the multiswitch and with the gain appearing to be 4dB from inputs
to outputs, there clearly is an amplifier in the multiswitch with a gain
of 24dB.

Now In order to minimise degrading of the C/N and the S/N ratios, the
input signals are amplified first internally before multiplexing/splitting
to the various outputs.

So that means your input signals is going in at 76.8dBuV,
straight into the signal amplifier(s) (which will be a 2 stage design) and
then coming out at a level of 76.8 + 24 = 100.8dBuV

before then going through the switch multiplexers/splitters and suffering
20dB loss to exit the multiswitch at 100.8 - 20 = 80.8dBuV.

So with an already strong signal in the first place, you're
over-amplifying and introducing intermodulation products...

I know you said you have only 5m of cable between LNB and multiswitch, but
I suspect your dish is bigger than you actually need for 28.2E. What size
is it?

Stephen



That is one of the best posts you are likely to see in this group this year.

Stephen H wrote:

Now In order to minimise degrading of the C/N and the S/N ratios, the
input signals are amplified first internally before
multiplexing/splitting to the various outputs.

So that means your input signals is going in at 76.8dBuV,
straight into the signal amplifier(s) (which will be a 2 stage design)
and then coming out at a level of 76.8 + 24 = 100.8dBuV

Yes but of course the switch is designed to handle that sort of level
internally.


before then going through the switch multiplexers/splitters and
suffering 20dB loss to exit the multiswitch at 100.8 - 20 = 80.8dBuV.

So with an already strong signal in the first place, you're
over-amplifying and introducing intermodulation products...
I won't comment on Optimax switch capabilities, except to say that their
purchase price has always been very low.


I know you said you have only 5m of cable between LNB and multiswitch,
but I suspect your dish is bigger than you actually need for 28.2E. What
size is it?
Of course dish size has only a marginal effect on signal level. In fact
since the signal is going to be further above the LNB noise (which being
across the whole band has quite an effect on amplifier loading) it is
likely that a bigger dish will lead to a higher possible maximum switch
output (because it will be possible to set the levels higher).

Bill

Stephen

On 13/03/2013 20:10, R. Mark Clayton wrote:
"Stephen H" wrote in message
...
On 13/03/2013 03:39, Bill Wright wrote:
Stephen H wrote:

The signal between the LNB and multiswitch is peaking at 76.8dBuV, but
is peaking at 80.6dBuV after the multiswitch so it looks as the
multiswitch is adding 4dB gain to the signals so it appears to be an
amplified multiswitch

They all amplify except for a few specialist ones that require external
pre-amplification. They have to amplify because the internal losses are
high; about 20dB is normal.

Bill

Bill's comment has reminded me to say a few words about multiswitch
design.

As Bill has rightly pointed out, you can have up to 20dB of internal loss
within the multiswitch and with the gain appearing to be 4dB from inputs
to outputs, there clearly is an amplifier in the multiswitch with a gain
of 24dB.

Now In order to minimise degrading of the C/N and the S/N ratios, the
input signals are amplified first internally before multiplexing/splitting
to the various outputs.

So that means your input signals is going in at 76.8dBuV,
straight into the signal amplifier(s) (which will be a 2 stage design) and
then coming out at a level of 76.8 + 24 = 100.8dBuV

before then going through the switch multiplexers/splitters and suffering
20dB loss to exit the multiswitch at 100.8 - 20 = 80.8dBuV.

So with an already strong signal in the first place, you're
over-amplifying and introducing intermodulation products...

I know you said you have only 5m of cable between LNB and multiswitch, but
I suspect your dish is bigger than you actually need for 28.2E. What size
is it?

Stephen



That is one of the best posts you are likely to see in this group this year.



As well as high signal levels within the multiswitch, there is likely to
be cross-talk from one sub band to another.

Reducing the input signal levels will reduce any cross talk if it is
occurring.

I find that the poorer the screening is within a multiswitch, the more
prone it is to cross talk.

Also as a multiswitch has to have four sub bands, that's actually four
amplifiers in the multiswitch in close proximity to each other.

Stephen H wrote:

As well as high signal levels within the multiswitch, there is likely to
be cross-talk from one sub band to another.
I've tested quite a few different switches because in the early days
crosstalk was a real problem. Even low levels of crosstalk can cause
significant worsening of BER.
Nowadays the amount of crosstalk is negligible on most good quality
switches. I've never tested an Optima, as far as I can remember. They
are very cheap compared to most other makes.


Reducing the input signal levels will reduce any cross talk if it is
occurring.
If the levels of all four modes are reduced equally the amount of
crosstalk will stay the same. This might not appear to be the case with
casual tests because at low levels the crosstalk falls below the noise.
However it is still there and will still contribute to poor BER.
The killer is when the modes are unequal for some reason; for instance
when the LNB or a cable is faulty. This can cause symptoms that can
confuse. Because signal levels are generally well above receiver noise
thresholds it can appear that when strong mode 'a' is disconnected weak
mode 'b' magically starts to work. But look at the actual levels and you
will see that mode 'b' -- although it works fine with mode 'a'
disconnected at the switch -- is actually 20dB below the other modes.
The crosstalk has a 20dB 'advantage' so it affects the BER of mode 'b'.


I find that the poorer the screening is within a multiswitch, the more
prone it is to cross talk.
Well yes.

Bill

On Tue, 12 Mar 2013 18:16:28 -0500, "Ashley Booth"
wrote:

Stephen H wrote:

On 12/03/2013 22:20, Stephen H wrote:
On 12/03/2013 20:31, Ashley Booth wrote:
Bill Wright wrote:

Ashley Booth wrote:
I have just fitted a Optima MS508 multiswitch. My FoxSat HD
works fine with it but my Sky+HD reports no signal.

If I connect a Promax Prodig-5 to either lead from the switch
I get a good signal. Also if I bypass the multiswitch and
connect the LNB direct to the Sky box it works fine.

Any ideas?

Ashley

Switch could be intolerant of out-of-spec or almost out-of-spec
13/18V or 22kHz. If you can be bothered get a splitter with DC
pass on one leg and use the DC leg to feed the switch whilst
monitoring the other leg using the suspect receiver.

Bill

Had a bit more time to play with it.

It appears that the switch is faulty on the Hi Vert circuit.

Here is my analyzer looking at 12207 MHz Vert direct to my LNB:
http://flic.kr/p/e2GnPH

And here it is via the multiswitch:
http://flic.kr/p/e2GnZe

This seems to be a mixture of V & H. I'll check the voltage and
tone on the port with a scope.

Ashley



does the multiswitch have gain/attenuator controls on it? some
multiswitches do.

The signal between the LNB and multiswitch is peaking at 76.8dBuV,
but is peaking at 80.6dBuV after the multiswitch so it looks as the
multiswitch is adding 4dB gain to the signals so it appears to be an
amplified multiswitch

I may be wrong but there appears to be intermodulation artefacts
present on the output at the multswitch.

Twiddling the gain/attenuator knobs on the multiswitch should help.
Failing that put a 6dB attenuator on the inputs to the multiswitch.

As it is, you're starting with a crackingly strong signal anyway so
with 4dB gain, its possibly enough to introduce intermodulation
artefacts and/or overdrive the multiswitch

Also there is a limit to how strong a signal you can input into a
sat receiver. This limit can vary from receiver to receiver.

Regards

Stephen



I've done some digging for a table that shows what the min and max
strengths should be for various signal types:

How much signal do I need?

Min dBuV Max dBuV C/N ratio db
Analogue TV PAL 60 80 43
Digital UHF DTT 45 65 26
FM (stereo) 57 74 45
FM (mono) 40 74 25
DAB (Band II) 45 70 20
Sat IF analogue 47 77 15
Sat IF digital 47 77 12

So I reckon your LNB signals are too high for the multiswitch to
handle properly without introducing distortion.

Thanks for those figures. I had 53-80dBuV as the range.

A reason for the high input is that there is only about 5m of cable
between the LNB and the multiswitch.

There are no gain controls on the multiswitch.

Why is only 1 of the 4 circuits affected by the high signal?

I have no attenuators that pass the DC and tone. I might try the old
damp rag over the LNB trick when I've time.

Ashley

That table appears here
http://www.megalithia.com/elect/fieldstr.html
and is accredited to CAI.
It equates DAB with Band II, which makes it slightly suspect.

--
Graham.

%Profound_observation%

Graham. wrote:

That table appears here
http://www.megalithia.com/elect/fieldstr.html
and is accredited to CAI.
It equates DAB with Band II, which makes it slightly suspect.

It says that the min for digi satellite is 47dBuV. That figure is far
too high.

Bill

Bill Wright wrote:

Stephen H wrote:

Now In order to minimise degrading of the C/N and the S/N ratios,
the input signals are amplified first internally before
multiplexing/splitting to the various outputs.

So that means your input signals is going in at 76.8dBuV,
straight into the signal amplifier(s) (which will be a 2 stage
design) and then coming out at a level of 76.8 + 24 = 100.8dBuV

Yes but of course the switch is designed to handle that sort of level
internally.


before then going through the switch multiplexers/splitters and
suffering 20dB loss to exit the multiswitch at 100.8 - 20 =
80.8dBuV.

So with an already strong signal in the first place, you're
over-amplifying and introducing intermodulation products...
I won't comment on Optimax switch capabilities, except to say that
their purchase price has always been very low.


I know you said you have only 5m of cable between LNB and
multiswitch, but I suspect your dish is bigger than you actually
need for 28.2E. What size is it?
Of course dish size has only a marginal effect on signal level. In
fact since the signal is going to be further above the LNB noise
(which being across the whole band has quite an effect on amplifier
loading) it is likely that a bigger dish will lead to a higher
possible maximum switch output (because it will be possible to set
the levels higher).

Bill

Stephen



Update

I tried to fault find by putting a Global SPLIT8AF active spliter, with
another output going to the speccy, between the affected LNB lead and
the multiswitch so I could check wether the fault was caused by the
multiswitch or LNB. No fault was found in this signal.

I put everything back to normal and the fault had gone! I don't want to
touch it again in case it comes back.

--
Ashley