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Derating factor.....



 
 
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  #1  
Old July 24th 19, 09:06 PM posted to uk.tech.digital-tv
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Default Derating factor.....

I have been reading today about derating factors for UHF amplifiers.

Apparently the amplifier gain or the input signal level must be turned down as a function of the number of channels processed....

I had understood that the "gain control" is simply an attenuator after an amp that has fixed gain.

So with 10 analogue channels requiring a derating of 9 dB, one turns up the attenuation or reduces the input signal levels?

Is this understanding correct?
  #3  
Old July 25th 19, 07:20 AM posted to uk.tech.digital-tv
stephent[email protected]
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Default Derating factor.....

Well, the 10 analogue signals coming into the televes Avant 5 are 65 to 70 dBuV.

The Avant defaults to the maximum output of 111 dBuV so it sounds like I need to reduce the output to 102 dBuV via the derating factor and adjust the slope.

Now that I had sorted out the 4g interference ingress, there is slight patterning on the higher channels which looks like intermodulation products to me.

If it makes any difference, the following multiswitches have a through RF gain of 4dB so it's clear to me there is a 16dB gain UHF amp within the multiswitch followed by a 16 way splitter if you see what I mean.
  #4  
Old July 25th 19, 08:26 AM posted to uk.tech.digital-tv
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Default Derating factor.....

P.S.

The televes Avant 5 is used as 10 single filters and for equalising the signal levels. I suspect feeding the modulators straight into the multiswitches without filterin and equalising woul be problematic.

In addition, this particular televes avant 5 also processes FM and DAb signals.

The design is such that an UHF input is required for automatic self adjustment and it sets the required gain for the FM and DAB signals.

So if there is no UHF input, the unit wi not self configure even if FM and DAB signals are present.
  #5  
Old July 25th 19, 09:15 AM posted to uk.tech.digital-tv
Brian Gaff
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Default Derating factor.....

Did the article explain why this should make any difference at all? Surely
the only issue will be intermodulation due to the actual amount of signal in
the passband driving it into non linearity.
Brian

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I have been reading today about derating factors for UHF amplifiers.

Apparently the amplifier gain or the input signal level must be turned
down as a function of the number of channels processed....

I had understood that the "gain control" is simply an attenuator after an
amp that has fixed gain.

So with 10 analogue channels requiring a derating of 9 dB, one turns up
the attenuation or reduces the input signal levels?

Is this understanding correct?



  #6  
Old July 26th 19, 06:30 PM posted to uk.tech.digital-tv
Terry Casey[_2_]
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Default Derating factor.....

In article , briang1
@blueyonder.co.uk says...

Did the article explain why this should make any difference at all? Surely
the only issue will be intermodulation due to the actual amount of signal in
the passband driving it into non linearity.


There are two factors to consider here, noise and distortionn.

For a specified signal to noise ratio there will be an optimum
input level.

The distortion level increases as the number of carriers
increases and the output level should, therefore, be derated
as the number of carriers increases.

A decent amplifier, as Bill has poinred out, will have mid-
stage adjustments for both gain and slope. This means thsat
the input level, which should be level across the band, can be
set at the optimum level and the mid stage adjustments used to
set the output levels and slope.

Setting the slope to partially compensate for cable losses
reduces the distortion levels and thus means that higher
output levels at higher frequencies can be used without
causing excessive distortion - within limits.

If you have an amplifier with no mid stage adjustments, the
only way to reduce distortion is to reduce the input level but
this, of course, also degrades signal to noise performance.



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  #7  
Old July 26th 19, 10:06 PM posted to uk.tech.digital-tv
Bill Wright[_3_]
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Default Derating factor.....

On 26/07/2019 18:30, Terry Casey wrote:

If you have an amplifier with no mid stage adjustments, the
only way to reduce distortion is to reduce the input level but
this, of course, also degrades signal to noise performance.


This being a major problem when a very large number of channels or muxes
are carried.

Bill
  #8  
Old July 27th 19, 09:49 AM posted to uk.tech.digital-tv
Jim Lesurf[_2_]
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Default Derating factor.....

In article , Bill Wright
wrote:

This being a major problem when a very large number of channels or muxes
are carried.


Yes, this is an interesting point. And one that becomes more relevant as
OfCom squeeze the muxes into an ever-narrower bandspace.

In effect, if they are all packed together you end up with a SNR determined
by all the 'other' channels having the same power level as the one carrying
the Mux you want to pick up!

Since there are a number of Muxs, this implies a effective SNR of less than
unity for an amp that has to pass them all when only one is (currently)
wanted by the user! 8-]

Fun with maths... and games for front-end designers.

Jim

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  #10  
Old July 28th 19, 10:19 PM posted to uk.tech.digital-tv
Bill Wright[_3_]
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Default Derating factor.....

On 28/07/2019 16:06, Terry Casey wrote:

Interesting point but, with some obvious exceptions, DTV muxes
were grouped into a similar bandwidth to the 11 channel
analogue groups, so there wouldn't be much difference in
levels across each group.

For example, although the slope introduced by 100m of CT100 is
5.2dB across the original Band IV - V span of 470 - 862MHz,
when the top is reduced to 700MHz it drops to 3.2dB and is
only 1.9dB from 470m - 600MHz.

Over an 11 channel group it is only slightly over a 1dB spread
in 100m of cable with a loss of ~20dB @ 862MHz.


The response variations with frequency of most aerials is a much bigger
factor.

Bill
 




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