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From: jacobfenton on 10 May 2010 12:24
I have I and Q data that I want to low pass filter. From what I have read I
would impliment the same LPF twice, one on I and one on Q. Is that the only
way to do this, or is there some trick out there I have not found out
about? Also I would assume my LPF must have linear phase?

Thanks.

-JF
From: Rune Allnor on 10 May 2010 12:29
On 10 Mai, 18:24, "jacobfenton" <jacob.fenton(a)n_o_s_p_a_m.gmail.com>
wrote:
> I have I and Q data that I want to low pass filter. From what I have read I
> would impliment the same LPF twice, one on I and one on Q.

Yes.

> Is that the only
> way to do this,

Probably not.

> or is there some trick out there I have not found out
> about?

There might be, but using the same filter on both the I and Q
channels is both simple and convenient.

> Also I would assume my LPF must have linear phase?

Why?

Rune
From: Tim Wescott on 10 May 2010 12:49
Rune Allnor wrote:
> On 10 Mai, 18:24, "jacobfenton" <jacob.fenton(a)n_o_s_p_a_m.gmail.com>
> wrote:
>> I have I and Q data that I want to low pass filter. From what I have read I
>> would impliment the same LPF twice, one on I and one on Q.
>
> Yes.
>
>> Is that the only
>> way to do this,
>
> Probably not.

Probably not, but I can't imagine an easier or lower cost one.

>> or is there some trick out there I have not found out
>> about?
>
> There might be, but using the same filter on both the I and Q
> channels is both simple and convenient.
>
>> Also I would assume my LPF must have linear phase?
>
> Why?
>
> Rune

Tell us about your application and we'll help you with answering Rune's
last question. Sometimes it is vital that one's filters have linear
phase, sometimes it matters not at all, and sometimes it's good but not
necessary.

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com
From: maury on 10 May 2010 16:11
On May 10, 11:24 am, "jacobfenton"
<jacob.fenton(a)n_o_s_p_a_m.gmail.com> wrote:

>............ Is that the only way to do this,

What you are getting into is multidimensional (n-dimensional filter)
MIMO (I and Q channels) signal processing. I would suggest you look at
Rajagopal, Ravikiran's dissertation from Ohio State, "An Algebraic
View of Multidimensional Multiple-Input, Multiple-Output Finite
Impulse Response Equalizers". He gives a good background overview, and
has a good set of references.

You may want to just implement two separate filters.


Maurice Givens
From: Rune Allnor on 11 May 2010 05:09
On 10 Mai, 22:11, maury <maury...(a)core.com> wrote:
> On May 10, 11:24 am, "jacobfenton"
>
> <jacob.fenton(a)n_o_s_p_a_m.gmail.com> wrote:
> >............    Is that the only way to do this,
>
> What you are getting into is multidimensional (n-dimensional filter)
> MIMO (I and Q channels) signal processing.

I would agree with your suggestion that one gets into MIMO stuff
if one accepts the quadrature filter as *both* an implementation
of the complex-valued arithmetics of a complex-valued filter, *and*
that the I and Q components are independent. In that case one needs
to handle the cross terms between the 'real' and 'imaginary'
components
to emulate the complex-valued filter, which means one need to get
into some sort of MIMO structure. Or use complex-valued arithmetics.

But the quadrature filter contains two real-valued sequences that
usually (always?) are not independent, but have been derived as
(emulations for) the real and imaginary components of a Hibert
transform of one real-valued signal. Each of these real-valued
signals must then be processed under the conditions of a real-
valued signals.

So far so good.

As I understand it, the fact that the I and Q component are
related through the HT, implicitly takes care of the cross-terms
in the complex-valued arithmetics. Which means that one, simple,
SISO real-valued filter is sufficient to handle the 'complex'-
valued quadrature signal.

The fact that the I and Q channels are processed individually as
real-valued data, also have some implictaions. Quadrature filters
are actually restricted by Nyquist's limit, Fs > 2B, whereas
complex-valued filters are restricted by the far more forgiving
Fs > B.

Or have I misunderstood or missed out on something?

Rune
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