>Hi,
>I have an LMS based adaptive equalizer which mostly works well but it
>doesn't tolerate low frequency inputs too well. The channel consists
>of two transformers (high pass) and cable (low pass) and when there is
>long periods of no-change in data (which is possible unfortunately
>even though the data is scrambled) and the SNR goes down during these
>long periods of no change. I think this is because the equalizer's
>bandwidth is not high enough ie it is not able to compensate for the
>high pass behavior of the transformers. Any idea how to approach this
>problem ?
>
Something I have run into in the past is line transformers that have
nonlinearitities at low frequencies. For example, if the transformers
high pass cutoff is 3kHz, it could have a nonlinear response below that
frequency. If you are transmitting signal energy below 3kHz, you can
generate a noise floor due to non-linearities above 3 kHz.
In a past application, we prefiltered the signal before transmission
through the transformer to avoid raising the noise floor. It may be
tempting to say "that region will be filtered out by the transformer
anyway", and this is true, but it will raise your noise floor outside that
band in the process and afterwards it too late to do anything about it.
Reply by mk●April 23, 20062006-04-23
On Sun, 23 Apr 2006 17:17:22 +0800, Steve Underwood <steveu@dis.org>
wrote:
>Many, many, systems over the years have been anywhere
>from a little quirky to totally unusable because they required DC
>response of a channel that doesn't offer it.
I seem to be dealing with one of them, one cast in stone by those
people who need to be in an instution instead of an institute of the
triple something letter ;-)
Reply by Jerry Avins●April 23, 20062006-04-23
Steve Underwood wrote:
> mk wrote:
...
>> I know how to generate DC free codes but I don't think you paid enough
>> attention to my post. The system definition is what it's and it's as
>> close to cast in stone as it can get and it can't be changed.
>
>
> I don't think you paid enough attention in physics lessons. :-) You can
> set the design in stone as much as you like. Making it work properly is
> another matter. Many, many, systems over the years have been anywhere
> from a little quirky to totally unusable because they required DC
> response of a channel that doesn't offer it.
At least one system failed because all it could pass was approximately
DC. The first transatlantic telegraph cable had a frequency response of
1/N Hz, where N was not a very small number. The first commercially
successful cable pushed the speed up to two words per minute. They could
signal no faster no matter what the spec (or board of directors) said.
Jerry
--
Engineering is the art of making what you want from things you can get.
�����������������������������������������������������������������������
Reply by Steve Underwood●April 23, 20062006-04-23
mk wrote:
> On 22 Apr 2006 19:56:18 -0700, "Mark" <makolber@yahoo.com> wrote:
>
>
>>mk wrote:
>>
>>>On 21 Apr 2006 18:56:57 -0700, "Mark" <makolber@yahoo.com> wrote:
>>>
>>>
>>>>Jerry Avins wrote:
>>>>
>>>>>mk wrote:
>>>>>
>>>>> ...
>>>>>
>>>>>
>>>>>>I understand that in general transformers are band-pass but within my
>>>>>>frequency of interest they are high-pass. The ones I am using have an
>>>>>>insertion loss of max -1dB 1-100 MHz. My baud rate is 62.5 MHz and the
>>>>>>cable has around 16 dB loss at that point.
>>>>>
>>>>>
>>>>When there is no change to the data for a while, what pattern is sent
>>>>over the line?
>>>>
>>>
>>>I've been talking about the wire data so no change happens on the
>>>wire.
>>>
>>>
>>>>Does this pattern have a DC component?
>>>>
>>>
>>>You bet :-(
>>>
>>>
>>>>As you note the xformers will not pass DC and the Adaptive EQ cannot
>>>>correct for this,, you need to use a randomizing pattern that ensures
>>>>there is no DC component.
>>>
>>>Interestingly there is a coder & scrambler in the system but alas it
>>>don't work :-( and they are what they, can't be changed. I am applying
>>>dc-offset correction at the AFE (to save ADC bits) so my input is not
>>>going to droop but equalizer output error still becomes higher during
>>>these events.
>>
>>If you know you are trying to send data that contains a DC component
>>through transformers, then I think you have identifed the problem...
>>
>>there are various coding solutions to this....try to look it up...
>
>
> I know how to generate DC free codes but I don't think you paid enough
> attention to my post. The system definition is what it's and it's as
> close to cast in stone as it can get and it can't be changed.
I don't think you paid enough attention in physics lessons. :-) You can
set the design in stone as much as you like. Making it work properly is
another matter. Many, many, systems over the years have been anywhere
from a little quirky to totally unusable because they required DC
response of a channel that doesn't offer it.
Regards,
Steve
Reply by mk●April 23, 20062006-04-23
On 22 Apr 2006 19:56:18 -0700, "Mark" <makolber@yahoo.com> wrote:
>
>mk wrote:
>> On 21 Apr 2006 18:56:57 -0700, "Mark" <makolber@yahoo.com> wrote:
>>
>> >
>> >Jerry Avins wrote:
>> >> mk wrote:
>> >>
>> >> ...
>> >>
>> >> > I understand that in general transformers are band-pass but within my
>> >> > frequency of interest they are high-pass. The ones I am using have an
>> >> > insertion loss of max -1dB 1-100 MHz. My baud rate is 62.5 MHz and the
>> >> > cable has around 16 dB loss at that point.
>> >>
>> >>
>> >When there is no change to the data for a while, what pattern is sent
>> >over the line?
>> >
>> I've been talking about the wire data so no change happens on the
>> wire.
>>
>> >Does this pattern have a DC component?
>> >
>> You bet :-(
>>
>> >As you note the xformers will not pass DC and the Adaptive EQ cannot
>> >correct for this,, you need to use a randomizing pattern that ensures
>> >there is no DC component.
>>
>> Interestingly there is a coder & scrambler in the system but alas it
>> don't work :-( and they are what they, can't be changed. I am applying
>> dc-offset correction at the AFE (to save ADC bits) so my input is not
>> going to droop but equalizer output error still becomes higher during
>> these events.
>
>If you know you are trying to send data that contains a DC component
>through transformers, then I think you have identifed the problem...
>
>there are various coding solutions to this....try to look it up...
I know how to generate DC free codes but I don't think you paid enough
attention to my post. The system definition is what it's and it's as
close to cast in stone as it can get and it can't be changed.
Reply by Mark●April 22, 20062006-04-22
mk wrote:
> On 21 Apr 2006 18:56:57 -0700, "Mark" <makolber@yahoo.com> wrote:
>
> >
> >Jerry Avins wrote:
> >> mk wrote:
> >>
> >> ...
> >>
> >> > I understand that in general transformers are band-pass but within my
> >> > frequency of interest they are high-pass. The ones I am using have an
> >> > insertion loss of max -1dB 1-100 MHz. My baud rate is 62.5 MHz and the
> >> > cable has around 16 dB loss at that point.
> >>
> >>
> >When there is no change to the data for a while, what pattern is sent
> >over the line?
> >
> I've been talking about the wire data so no change happens on the
> wire.
>
> >Does this pattern have a DC component?
> >
> You bet :-(
>
> >As you note the xformers will not pass DC and the Adaptive EQ cannot
> >correct for this,, you need to use a randomizing pattern that ensures
> >there is no DC component.
>
> Interestingly there is a coder & scrambler in the system but alas it
> don't work :-( and they are what they, can't be changed. I am applying
> dc-offset correction at the AFE (to save ADC bits) so my input is not
> going to droop but equalizer output error still becomes higher during
> these events.
If you know you are trying to send data that contains a DC component
through transformers, then I think you have identifed the problem...
there are various coding solutions to this....try to look it up...
Mark
Reply by Verizon●April 22, 20062006-04-22
"mk" <kal*@dspia.*comdelete> wrote in message
news:m15j42lb2gid729j2ukjoqsghde8oa0t97@4ax.com...
> On 21 Apr 2006 18:56:57 -0700, "Mark" <makolber@yahoo.com> wrote:
>
> You bet :-(
>
>>As you note the xformers will not pass DC and the Adaptive EQ cannot
>>correct for this,, you need to use a randomizing pattern that ensures
>>there is no DC component.
>
> Interestingly there is a coder & scrambler in the system but alas it
> don't work :-( and they are what they, can't be changed. I am applying
> dc-offset correction at the AFE (to save ADC bits) so my input is not
> going to droop but equalizer output error still becomes higher during
> these events.
One technique that may work for this problem is to treat the equalizer
output with
partial response processing. This will put a zero at DC and eliminate that
part of
the spectrum from the equalizer's response. This process helps with energy
around
DC that is hard to equalize, but I confess I don't know what long periods of
energy
only at DC will do.
Reply by mk●April 21, 20062006-04-21
On 21 Apr 2006 18:56:57 -0700, "Mark" <makolber@yahoo.com> wrote:
>
>Jerry Avins wrote:
>> mk wrote:
>>
>> ...
>>
>> > I understand that in general transformers are band-pass but within my
>> > frequency of interest they are high-pass. The ones I am using have an
>> > insertion loss of max -1dB 1-100 MHz. My baud rate is 62.5 MHz and the
>> > cable has around 16 dB loss at that point.
>>
>>
>When there is no change to the data for a while, what pattern is sent
>over the line?
>
I've been talking about the wire data so no change happens on the
wire.
>Does this pattern have a DC component?
>
You bet :-(
>As you note the xformers will not pass DC and the Adaptive EQ cannot
>correct for this,, you need to use a randomizing pattern that ensures
>there is no DC component.
Interestingly there is a coder & scrambler in the system but alas it
don't work :-( and they are what they, can't be changed. I am applying
dc-offset correction at the AFE (to save ADC bits) so my input is not
going to droop but equalizer output error still becomes higher during
these events.
Reply by Mark●April 21, 20062006-04-21
Jerry Avins wrote:
> mk wrote:
>
> ...
>
> > I understand that in general transformers are band-pass but within my
> > frequency of interest they are high-pass. The ones I am using have an
> > insertion loss of max -1dB 1-100 MHz. My baud rate is 62.5 MHz and the
> > cable has around 16 dB loss at that point.
>
>
When there is no change to the data for a while, what pattern is sent
over the line?
Does this pattern have a DC component?
As you note the xformers will not pass DC and the Adaptive EQ cannot
correct for this,, you need to use a randomizing pattern that ensures
there is no DC component.
Mark
Reply by Jerry Avins●April 21, 20062006-04-21
mk wrote:
...
> I understand that in general transformers are band-pass but within my
> frequency of interest they are high-pass. The ones I am using have an
> insertion loss of max -1dB 1-100 MHz. My baud rate is 62.5 MHz and the
> cable has around 16 dB loss at that point.
Thanks for the cable information. How does the longest pulse compare to
the transformers' time constant? You can increase the time constant by
resistively loading the transformer or, better, by choosing a turns
ratio that will make the cable load it more heavily.
Jerry
--
Engineering is the art of making what you want from things you can get.
�����������������������������������������������������������������������