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4G filter measurement



 
 
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  #21  
Old March 10th 13, 11:23 AM posted to uk.tech.digital-tv
[email protected]
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Default 4G filter measurement

On Sun, 10 Mar 2013 12:09:51 +0000, "J. P. Gilliver (John)"
wrote:

In message , SpamTrapSeeSig
writes:
[]
I assume if you cascade filters, the basic loss (insertion loss) is too
high, so that you don't gain anything. And what about phase shifts


Well, the dB losses do just add, so whether you gain enough _relative_
rejection before you lose too much of the wanted signal will depend on
your situation (how strong the wanted signal is, how deaf your
receiver).


In theory they add, but practical filters tend to have poor impedance
matches. They work fine berween 75/50 ohm terminations, but when you
cascade two filters the reactive mismatch at the interface degrades
the performance considerably, both in-band and out-of-band. It's not a
good thing to do.
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  #22  
Old March 10th 13, 12:13 PM posted to uk.tech.digital-tv,uk.tech.broadcast
dave
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Posts: 32
Default 4G filter measurement

On 10/03/13 10:21, Jim Lesurf wrote:

If anyone finds a five quid filter with

max loss in band 3dB ; ch60 loss 3 dB; 4G adjacent channel rejection =
20 dB, then let me know! I'll go searching for the magicians who provided it! :-)


Would a notch filter be a better bet than a straight LPF? All that's
needed presumably is to nix out a 5MHz chunk depending on location.
--
Dave

  #23  
Old March 10th 13, 01:03 PM posted to uk.tech.digital-tv
Jim Lesurf[_2_]
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Posts: 4,326
Default 4G filter measurement

In article ,
wrote:
On Sun, 10 Mar 2013 12:09:51 +0000, "J. P. Gilliver (John)"
wrote:


In message , SpamTrapSeeSig
writes: []
I assume if you cascade filters, the basic loss (insertion loss) is
too high, so that you don't gain anything. And what about phase shifts


Well, the dB losses do just add, so whether you gain enough _relative_
rejection before you lose too much of the wanted signal will depend on
your situation (how strong the wanted signal is, how deaf your
receiver).


In theory they add, but practical filters tend to have poor impedance
matches. They work fine berween 75/50 ohm terminations, but when you
cascade two filters the reactive mismatch at the interface degrades the
performance considerably, both in-band and out-of-band. It's not a good
thing to do.


Agreed. May be useful to add the following at this point:

When I did the measurements referenced at the start of this thread I used
two 6dB pads. One on the 'RF sig gen side' and another on the 'output
side'. I'd previously checked the pad were reasonably OK.

The measured values I plotted are the difference between having the filter
between the pads and having a female-female F connector 'barrel' there.

When cascading items like filters or amps having pads may be a good idea if
you want the behaviour to 'add up' in the way you'd expected. What you get
without pads may be 'better' or 'worse' depending on what you're after! :-)

Slainte,

Jim

--
Please use the address on the audiomisc page if you wish to email me.
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc http://www.audiomisc.co.uk/index.html

  #24  
Old March 10th 13, 01:24 PM posted to uk.tech.digital-tv,uk.tech.broadcast
Jim Lesurf[_2_]
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Posts: 4,326
Default 4G filter measurement

In article , dave
wrote:
On 10/03/13 10:21, Jim Lesurf wrote:


If anyone finds a five quid filter with

max loss in band 3dB ; ch60 loss 3 dB; 4G adjacent channel
rejection =
20 dB, then let me know! I'll go searching for the magicians who
provided it! :-)


Would a notch filter be a better bet than a straight LPF? All that's
needed presumably is to nix out a 5MHz chunk depending on location.


It may well be that combining a channel notch filter (or two!) with
something like the AE5100 would be better. But I doubt a simple 2nd order
(i.e. LC) would do enough.

The problem here is that each 4G channel is 5MHz wide, and you need to eat
most of that. But the DTTV in ch60 is only about 3.5 MHz away from the
center of the lowest such 4G channel, and just about 1 MHz away from its
edge. You also need to avoid fouling the edge of the ch60 mux with
something so nasty that it simply upsets the ability of the DTTV receiver
to correct and get the data it needs to work. This tiny gap of about 1 MHz
really is insanely small for made-by-the-million domestic kit! Particularly
combined with interference levels vastly bigger than the wanted signals.

It would matter much less in many commercial/pro/defence situations as
people could spend a lot more money on a filter. Don't worry about the
cost, the Government are paying! :-) But for 5 quid you won't get much I
fear.

The basic problem is the need to go from 'pass with little loss' to 'reject
by 20dB or more' in the space of about 1 MHz at 790 MHz, combined with a
flat low-loss pass response for the 790 MHz region.

A Saturn V and a Nov 5th rocket with a stick are both 'rockets'. But their
prices and performances do differ a little. :-)

Here we want Saturn V performance in the size and price ranges of something
you can launch from a milk-bottle...

The 'good' filters would be likely to use multiple resonators or something
more fancy like a crystal/ceramic/saw approach to make a high-order filter
that can have steep sides whilst cutting out a reasonably width. Some of
these designs can be 'printed'. The real costs tend to be in development
and setting up tight production quality lines. But - even when making a few
million off - making something like this in the 5 quid-a-pop range would be
a challenge I suspect. Particularly when it may lose its mass market in
just a few years... Why would a maker invest in this when they expect the
market to evaporate again after a couple of years?

But again, if someone makes and markets one, I'd be happy to check it out,
and I'm sure Bill and others would also be happy to do so. I'm not holding
my breath whilst waiting, though...

Slainte,

Jim

--
Please use the address on the audiomisc page if you wish to email me.
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc http://www.audiomisc.co.uk/index.html

  #25  
Old March 10th 13, 01:39 PM posted to uk.tech.digital-tv,uk.tech.broadcast
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Default 4G filter measurement

On Sun, 10 Mar 2013 12:23:24 +0000 (GMT), Jim Lesurf
wrote:

In article ,
wrote:
On Sun, 10 Mar 2013 10:24:04 +0000 (GMT), Jim Lesurf
wrote:



Not a very effective filter but nonetheless there will be many
marginal locations where it will be adequate.

I'd suspect about as many people will have their problem fixed by a 6dB
pad!

Possibly, but not necessarily the exact same locations. An intelligent
installer would have both options (and others) available.


Yes. This is part of the problem.

The situation isn't likely to be solved by a single kind of 'free sweetie'.
(Unless a very expensive sweetie!) So viewers and riggers and shops will
have to have a bag full of assorted sweeties and try various ones to see
which might do the trick.

Given the range of prices, think of the scope for mis-selling of needlessly
expensive items...

...and for something to be fitted, only later on to not be good enough as
the situation evolves, etc.

Nightmare for all concerned. And expensive for many.

If they'd ensured a wider guard band (e.g. 10 MHz) the whole situation
would be better defined and we'd have a much higher chance that a single
reference filter design would handle almost all cases. The 'polluter pays'
principle would mean that the 10 MHz gap would be on the telco side of the
fence, and they'd be paying for the reference-design filters. But guess
what, this isn't happening... :-/

Given that Ofcom has already scheduled further mux changes, I think we
will see an adequate guardband being introduced. But it won't be
within the 4G spectrum.
  #26  
Old March 10th 13, 03:11 PM posted to uk.tech.digital-tv,uk.tech.broadcast
Jim Lesurf[_2_]
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Posts: 4,326
Default 4G filter measurement

In article ,
wrote:
On Sun, 10 Mar 2013 12:23:24 +0000 (GMT), Jim Lesurf
wrote:



Given that Ofcom has already scheduled further mux changes, I think we
will see an adequate guardband being introduced.


Can you point me at the changes you mean? The ones I've seen under
clearance plans all leave DTTV on ch59/60- in various areas, including
here. If that has changed I'd like to know, and to update the maps I've
done.

It would be welcome despite my having to faff about correcting the maps
from what the previous list of changes said.

Slainte,

Jim

--
Please use the address on the audiomisc page if you wish to email me.
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc http://www.audiomisc.co.uk/index.html

  #27  
Old March 10th 13, 03:42 PM posted to uk.tech.digital-tv,uk.tech.broadcast
Bill Wright[_2_]
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Posts: 9,381
Default 4G filter measurement

David Woolley wrote:
Brian Gaff wrote:

So if you have a 4G smartphone and you live in the shadow a tv
transmitter tha uses ch59 at reasonable power, does your phone get
deaf and not work one wonders.


The phone will have been designed with the presence of TV transmissions
on adjacent channels in mind.

I believe there is an element of frequency division multiplexing in LTE,
so the phone will probably choose a frequency that is less badly affected.

Yes but commerce rules. They'll calculate the percentage of
people/locations where the TV field strength will be at a certain level
and draw the design line there. The small minority with TV field
strength above that level will be ignored. They don't care you know.
It's all about money.

Bill
  #28  
Old March 10th 13, 03:44 PM posted to uk.tech.digital-tv,uk.tech.broadcast
Bill Wright[_2_]
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Posts: 9,381
Default 4G filter measurement

David Woolley wrote:
Bill Wright wrote:
Jim Lesurf wrote:
I've just done a quick measurement on the SAC AE5100 filter that I
bought
from CPC as a "4G filter". Results at...

http://jcgl.orpheusweb.co.uk/temp/AE5100.png

Useless then, on either graph. But why are they different?

I wonder how representitive this will be of all the other filters
that will
now doubtless flood onto the market... Not an encouraging thought...


There's going to be a run on snake oil. Stock up now.

Here's a bit of background:


...

In most cases 12dB is not going to be enough. A little thought about
these figures will show you that filters are not going to solve the
problem for those who watch TV on channels 58, 59, and 60.

Bill


No message from you here, David.

Bill
  #29  
Old March 10th 13, 04:15 PM posted to uk.tech.digital-tv,uk.tech.broadcast
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Default 4G filter measurement

On Sun, 10 Mar 2013 16:11:17 +0000 (GMT), Jim Lesurf
wrote:

In article ,
wrote:
On Sun, 10 Mar 2013 12:23:24 +0000 (GMT), Jim Lesurf
wrote:



Given that Ofcom has already scheduled further mux changes, I think we
will see an adequate guardband being introduced.


Can you point me at the changes you mean? The ones I've seen under
clearance plans all leave DTTV on ch59/60- in various areas, including
here. If that has changed I'd like to know, and to update the maps I've
done.

It would be welcome despite my having to faff about correcting the maps
from what the previous list of changes said.

I don't know of anything different to those.
What I was saying is that if ch60 is found to unworkable, Ofcom has an
opportunity to revise those plans and include a phasing out of ch.60
at the same time without looking quite so red-faced.
 




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