On 08/18/2010 12:28 PM, glen herrmannsfeldt wrote:
> Tim Wescott<tim@seemywebsite.com> wrote:
> (snip)
>
>> "If I take my 8MHz wide signal and chop off everything
>> except the middle (or lower) 5 to 6MHz, why do I lose information?".
>
>> Well gee, I don't know -- why does whacking off over 1/4 of
>> your signal degrade it?
>
> That description sounds much like vestigial sideband modulation,
> carefully designed not to lose information.
Yes, but in vestigial sideband modulation the two sidebands are redundant.
--
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
See details at http://www.wescottdesign.com/actfes/actfes.html
Reply by Zeph80●August 18, 20102010-08-18
>On 08/18/2010 12:02 PM, Zeph80 wrote:
>>>> On 08/18/2010 08:56 AM, Zeph80 wrote:
>>>>> I'm simulating a DVB-T OFDM 8 Mhz BW transmitter with 2K tones,
DVB-T
>>>>> sample rate is 9MHz. I'm doing this by using a 4K IFFT and 18 MHz
>>> sample
>>>>> rate (I arrange the IFFT input with 1K tones (worth 4 MHz) having
>> QPSK
>>>>> data, then zero out all the central tones, and put remaining QPSK
data
>>> on
>>>>> the last 1 K tones ( worth the remaining 4 MHz BW).So I get a
>>> base-band
>>>>> OFDM spectrum to begin with. I'm then up-sampling the output by 20 ,
>>> and
>>>>> then filtering with a low-pass interpolation filter. What I do not
>>>>> understand is,I only get the desired output if my LPF filter has
>>> cut-off
>>>>> frequency of 8 MHz. If I set the cut-off to 5 or 6 MHz I do not get
>> the
>>>>> output desired. If my 8 MHz signal is centered at 0 ( 4 MHz on each
>>> side),
>>>>> why can't I filter at a frequency of 5 or 6 MHZ.
>>>>
>>>> You haven't answered your question already?
>>>>
>>>> "If I take my 8MHz wide signal and chop off everything except the
middle
>>
>>>> (or lower) 5 to 6MHz, why do I lose information?".
>>>>
>>>> Well gee, I don't know -- why does whacking off over 1/4 of your
signal
>>>> degrade it?
>>>>
>>>> --
>>>>
>>>> Tim Wescott
>>>> Wescott Design Services
>>>> http://www.wescottdesign.com
>>>>
>>>> Do you need to implement control loops in software?
>>>> "Applied Control Theory for Embedded Systems" was written for you.
>>>> See details at http://www.wescottdesign.com/actfes/actfes.html
>>>>
>>> I'm sorry, I don't understand what you mean. I think the way I'm
feeding
>>> the inputs to the IFFT results in the 8 MHz being centered around 0.
So
>> on
>>> each side I have only 4 MHz. So why can't I apply a LPF with cut-off of
>> should be greater than 0.5fs. I did not understand why, but I guess
this
>> related to my question.
>
>That 0.5 Fs is the Nyquist limit, and if you don't know what its about
>then you have a lot to learn.
>
>This may help. It's aimed at someone who knows that there's a Nyquist
>limit but has misconceptions. You're fortunate, in that you don't seem
>to have misconceptions to dispell!
>
>http://www.wescottdesign.com/articles/Sampling/sampling.html
>
>--
>
>Tim Wescott
>Wescott Design Services
>http://www.wescottdesign.com
>
>Do you need to implement control loops in software?
>"Applied Control Theory for Embedded Systems" was written for you.
>See details at http://www.wescottdesign.com/actfes/actfes.html
>
I did not think I was violating Nyquist by using a filter cut-off frequency
lower than 0.5 fs.I thought as long as my sample frequency is twice the
signal bandwidth,I can filter my signal with any cut-off frequency as long
as it includes the 1 sided spectrum.
Reply by glen herrmannsfeldt●August 18, 20102010-08-18
Tim Wescott <tim@seemywebsite.com> wrote:
(snip)
> "If I take my 8MHz wide signal and chop off everything
> except the middle (or lower) 5 to 6MHz, why do I lose information?".
> Well gee, I don't know -- why does whacking off over 1/4 of
> your signal degrade it?
That description sounds much like vestigial sideband modulation,
carefully designed not to lose information.
-- glen
Reply by Tim Wescott●August 18, 20102010-08-18
On 08/18/2010 11:36 AM, Zeph80 wrote:
>> On 08/18/2010 08:56 AM, Zeph80 wrote:
>>> I'm simulating a DVB-T OFDM 8 Mhz BW transmitter with 2K tones, DVB-T
>>> sample rate is 9MHz. I'm doing this by using a 4K IFFT and 18 MHz
> sample
>>> rate (I arrange the IFFT input with 1K tones (worth 4 MHz) having QPSK
>>> data, then zero out all the central tones, and put remaining QPSK data
> on
>>> the last 1 K tones ( worth the remaining 4 MHz BW).So I get a
> base-band
>>> OFDM spectrum to begin with. I'm then up-sampling the output by 20 ,
> and
>>> then filtering with a low-pass interpolation filter. What I do not
>>> understand is,I only get the desired output if my LPF filter has
> cut-off
>>> frequency of 8 MHz. If I set the cut-off to 5 or 6 MHz I do not get the
>>> output desired. If my 8 MHz signal is centered at 0 ( 4 MHz on each
> side),
>>> why can't I filter at a frequency of 5 or 6 MHZ.
>>
>> You haven't answered your question already?
>>
>> "If I take my 8MHz wide signal and chop off everything except the middle
>> (or lower) 5 to 6MHz, why do I lose information?".
>>
>> Well gee, I don't know -- why does whacking off over 1/4 of your signal
>> degrade it?
>>
>> --
>>
>> Tim Wescott
>> Wescott Design Services
>> http://www.wescottdesign.com
>>
>> Do you need to implement control loops in software?
>> "Applied Control Theory for Embedded Systems" was written for you.
>> See details at http://www.wescottdesign.com/actfes/actfes.html
>>
> I'm sorry, I don't understand what you mean. I think the way I'm feeding
> the inputs to the IFFT results in the 8 MHz being centered around 0.
You _think_? Have you _checked_?
> So on
> each side I have only 4 MHz. So why can't I apply a LPF with cut-off of 5
> MHz.
Are you getting a complex signal out of the IFFT? If you're only using
the real part then it is impossible for you to have unique spectra on
each side of 0Hz.
--
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
See details at http://www.wescottdesign.com/actfes/actfes.html
Reply by Tim Wescott●August 18, 20102010-08-18
On 08/18/2010 12:02 PM, Zeph80 wrote:
>>> On 08/18/2010 08:56 AM, Zeph80 wrote:
>>>> I'm simulating a DVB-T OFDM 8 Mhz BW transmitter with 2K tones, DVB-T
>>>> sample rate is 9MHz. I'm doing this by using a 4K IFFT and 18 MHz
>> sample
>>>> rate (I arrange the IFFT input with 1K tones (worth 4 MHz) having
> QPSK
>>>> data, then zero out all the central tones, and put remaining QPSK data
>> on
>>>> the last 1 K tones ( worth the remaining 4 MHz BW).So I get a
>> base-band
>>>> OFDM spectrum to begin with. I'm then up-sampling the output by 20 ,
>> and
>>>> then filtering with a low-pass interpolation filter. What I do not
>>>> understand is,I only get the desired output if my LPF filter has
>> cut-off
>>>> frequency of 8 MHz. If I set the cut-off to 5 or 6 MHz I do not get
> the
>>>> output desired. If my 8 MHz signal is centered at 0 ( 4 MHz on each
>> side),
>>>> why can't I filter at a frequency of 5 or 6 MHZ.
>>>
>>> You haven't answered your question already?
>>>
>>> "If I take my 8MHz wide signal and chop off everything except the middle
>
>>> (or lower) 5 to 6MHz, why do I lose information?".
>>>
>>> Well gee, I don't know -- why does whacking off over 1/4 of your signal
>>> degrade it?
>>>
>>> --
>>>
>>> Tim Wescott
>>> Wescott Design Services
>>> http://www.wescottdesign.com
>>>
>>> Do you need to implement control loops in software?
>>> "Applied Control Theory for Embedded Systems" was written for you.
>>> See details at http://www.wescottdesign.com/actfes/actfes.html
>>>
>> I'm sorry, I don't understand what you mean. I think the way I'm feeding
>> the inputs to the IFFT results in the 8 MHz being centered around 0. So
> on
>> each side I have only 4 MHz. So why can't I apply a LPF with cut-off of 5
>> MHz.
>>
>
>
>
> Page 10 of this tutorial
> http://folk.ntnu.no/htorp/Undervisning/TTK10/IQdemodulation.pdf talks
> about how the interpolation filter's cut-off frequency for a complex signal
> should be greater than 0.5fs. I did not understand why, but I guess this
> related to my question.
That 0.5 Fs is the Nyquist limit, and if you don't know what its about
then you have a lot to learn.
This may help. It's aimed at someone who knows that there's a Nyquist
limit but has misconceptions. You're fortunate, in that you don't seem
to have misconceptions to dispell!
http://www.wescottdesign.com/articles/Sampling/sampling.html
--
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
See details at http://www.wescottdesign.com/actfes/actfes.html
Reply by Zeph80●August 18, 20102010-08-18
>>On 08/18/2010 08:56 AM, Zeph80 wrote:
>>> I'm simulating a DVB-T OFDM 8 Mhz BW transmitter with 2K tones, DVB-T
>>> sample rate is 9MHz. I'm doing this by using a 4K IFFT and 18 MHz
>sample
>>> rate (I arrange the IFFT input with 1K tones (worth 4 MHz) having
QPSK
>>> data, then zero out all the central tones, and put remaining QPSK data
>on
>>> the last 1 K tones ( worth the remaining 4 MHz BW).So I get a
>base-band
>>> OFDM spectrum to begin with. I'm then up-sampling the output by 20 ,
>and
>>> then filtering with a low-pass interpolation filter. What I do not
>>> understand is,I only get the desired output if my LPF filter has
>cut-off
>>> frequency of 8 MHz. If I set the cut-off to 5 or 6 MHz I do not get
the
>>> output desired. If my 8 MHz signal is centered at 0 ( 4 MHz on each
>side),
>>> why can't I filter at a frequency of 5 or 6 MHZ.
>>
>>You haven't answered your question already?
>>
>>"If I take my 8MHz wide signal and chop off everything except the middle
>>(or lower) 5 to 6MHz, why do I lose information?".
>>
>>Well gee, I don't know -- why does whacking off over 1/4 of your signal
>>degrade it?
>>
>>--
>>
>>Tim Wescott
>>Wescott Design Services
>>http://www.wescottdesign.com
>>
>>Do you need to implement control loops in software?
>>"Applied Control Theory for Embedded Systems" was written for you.
>>See details at http://www.wescottdesign.com/actfes/actfes.html
>>
>I'm sorry, I don't understand what you mean. I think the way I'm feeding
>the inputs to the IFFT results in the 8 MHz being centered around 0. So
on
>each side I have only 4 MHz. So why can't I apply a LPF with cut-off of 5
>MHz.
>
Page 10 of this tutorial
http://folk.ntnu.no/htorp/Undervisning/TTK10/IQdemodulation.pdf talks
about how the interpolation filter's cut-off frequency for a complex signal
should be greater than 0.5fs. I did not understand why, but I guess this
related to my question.
Reply by Zeph80●August 18, 20102010-08-18
>On 08/18/2010 08:56 AM, Zeph80 wrote:
>> I'm simulating a DVB-T OFDM 8 Mhz BW transmitter with 2K tones, DVB-T
>> sample rate is 9MHz. I'm doing this by using a 4K IFFT and 18 MHz
sample
>> rate (I arrange the IFFT input with 1K tones (worth 4 MHz) having QPSK
>> data, then zero out all the central tones, and put remaining QPSK data
on
>> the last 1 K tones ( worth the remaining 4 MHz BW).So I get a
base-band
>> OFDM spectrum to begin with. I'm then up-sampling the output by 20 ,
and
>> then filtering with a low-pass interpolation filter. What I do not
>> understand is,I only get the desired output if my LPF filter has
cut-off
>> frequency of 8 MHz. If I set the cut-off to 5 or 6 MHz I do not get the
>> output desired. If my 8 MHz signal is centered at 0 ( 4 MHz on each
side),
>> why can't I filter at a frequency of 5 or 6 MHZ.
>
>You haven't answered your question already?
>
>"If I take my 8MHz wide signal and chop off everything except the middle
>(or lower) 5 to 6MHz, why do I lose information?".
>
>Well gee, I don't know -- why does whacking off over 1/4 of your signal
>degrade it?
>
>--
>
>Tim Wescott
>Wescott Design Services
>http://www.wescottdesign.com
>
>Do you need to implement control loops in software?
>"Applied Control Theory for Embedded Systems" was written for you.
>See details at http://www.wescottdesign.com/actfes/actfes.html
>
I'm sorry, I don't understand what you mean. I think the way I'm feeding
the inputs to the IFFT results in the 8 MHz being centered around 0. So on
each side I have only 4 MHz. So why can't I apply a LPF with cut-off of 5
MHz.
Reply by Tim Wescott●August 18, 20102010-08-18
On 08/18/2010 08:56 AM, Zeph80 wrote:
> I'm simulating a DVB-T OFDM 8 Mhz BW transmitter with 2K tones, DVB-T
> sample rate is 9MHz. I'm doing this by using a 4K IFFT and 18 MHz sample
> rate (I arrange the IFFT input with 1K tones (worth 4 MHz) having QPSK
> data, then zero out all the central tones, and put remaining QPSK data on
> the last 1 K tones ( worth the remaining 4 MHz BW).So I get a base-band
> OFDM spectrum to begin with. I'm then up-sampling the output by 20 , and
> then filtering with a low-pass interpolation filter. What I do not
> understand is,I only get the desired output if my LPF filter has cut-off
> frequency of 8 MHz. If I set the cut-off to 5 or 6 MHz I do not get the
> output desired. If my 8 MHz signal is centered at 0 ( 4 MHz on each side),
> why can't I filter at a frequency of 5 or 6 MHZ.
You haven't answered your question already?
"If I take my 8MHz wide signal and chop off everything except the middle
(or lower) 5 to 6MHz, why do I lose information?".
Well gee, I don't know -- why does whacking off over 1/4 of your signal
degrade it?
--
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
See details at http://www.wescottdesign.com/actfes/actfes.html
Reply by Zeph80●August 18, 20102010-08-18
I'm simulating a DVB-T OFDM 8 Mhz BW transmitter with 2K tones, DVB-T
sample rate is 9MHz. I'm doing this by using a 4K IFFT and 18 MHz sample
rate (I arrange the IFFT input with 1K tones (worth 4 MHz) having QPSK
data, then zero out all the central tones, and put remaining QPSK data on
the last 1 K tones ( worth the remaining 4 MHz BW).So I get a base-band
OFDM spectrum to begin with. I'm then up-sampling the output by 20 , and
then filtering with a low-pass interpolation filter. What I do not
understand is,I only get the desired output if my LPF filter has cut-off
frequency of 8 MHz. If I set the cut-off to 5 or 6 MHz I do not get the
output desired. If my 8 MHz signal is centered at 0 ( 4 MHz on each side),
why can't I filter at a frequency of 5 or 6 MHZ.