Re: [Discuss-gnuradio] DC component

[Gaetano Mendola]

On Fri, Dec 23, 2011 at 5:17 PM, Marcus D. Leech  wrote:
>>
>> Please do not get me wrong. I believe that the work from Ettus and the
>> contributors to GNU-Radio are revolutionary!  The equipment with the
>> interface to GNU-Radio is not only priced at a level that is affordable to
>> many amateurs and hobbyists, it opens up a brand new world for... (the list
>> is too long to list here)
>>
>> In fact, I am so impressed that I desire to contribute to the community.
>> First by contributing to these forums and second by eventually posting
>> projects to CGRAN!
>>
>> My intent was to let Gaetano know of the potential for spurious signals so
>> that he can properly select a center frequency that is free of these little
>> nuisances.
>>
>>
> Yup, understood.
>
> At lot of the folks on this list come at SDR from a background in
> software/digital, where the "vagaries" of
>  the analog world are entirely foreign to them, and they may see the
> existence of such things as "spurs" as
>  some kind of horrible defect, rather than an inevitable annoyance.  So
> I felt an explanatory note was in order.
>
> Another subset on this forum come from a background where they're used
> to dealing with lab-calibrated
>  instruments that their corporate  lords and masters (or university
> administration) have spent significant
>  money on, so their performance expectations (along certain vectors,
> anyway) will be driven by what they've
>  seen their $40-$100K lab instruments do.
>
> The ham radio community is used to dealing with this.  As radios became
> more and more broadly-tunable,
>  it was no big surprise that "birdies" (a peculiar ham-radio term for
> "spurs") became more and more frequently
>  observable, since it was no longer possible to "tune" the underlying
> "birdy-producing" mechanisms to produce
>  those "birdies" outside the band of interest, since the "band of
> interest" became larger and larger.

Thank you for your reply, I will soon look at those utilities to see
if I can minimize
the effect.

-- 
cpp-today.blogspot.com

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Re: [Discuss-gnuradio] DC component

[Marcus D. Leech]

>
> Please do not get me wrong. I believe that the work from Ettus and the
> contributors to GNU-Radio are revolutionary!  The equipment with the
> interface to GNU-Radio is not only priced at a level that is affordable to
> many amateurs and hobbyists, it opens up a brand new world for... (the list
> is too long to list here)
>
> In fact, I am so impressed that I desire to contribute to the community.
> First by contributing to these forums and second by eventually posting
> projects to CGRAN!
>
> My intent was to let Gaetano know of the potential for spurious signals so
> that he can properly select a center frequency that is free of these little
> nuisances.
>
>   
Yup, understood.

At lot of the folks on this list come at SDR from a background in
software/digital, where the "vagaries" of
  the analog world are entirely foreign to them, and they may see the
existence of such things as "spurs" as
  some kind of horrible defect, rather than an inevitable annoyance.  So
I felt an explanatory note was in order.

Another subset on this forum come from a background where they're used
to dealing with lab-calibrated
  instruments that their corporate  lords and masters (or university
administration) have spent significant
  money on, so their performance expectations (along certain vectors,
anyway) will be driven by what they've
  seen their $40-$100K lab instruments do.

The ham radio community is used to dealing with this.  As radios became
more and more broadly-tunable,
  it was no big surprise that "birdies" (a peculiar ham-radio term for
"spurs") became more and more frequently
  observable, since it was no longer possible to "tune" the underlying
"birdy-producing" mechanisms to produce
  those "birdies" outside the band of interest, since the "band of
interest" became larger and larger.

-- 
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org



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Re: [Discuss-gnuradio] DC component

[Evan Merewether]

Please do not get me wrong. I believe that the work from Ettus and the
contributors to GNU-Radio are revolutionary!  The equipment with the
interface to GNU-Radio is not only priced at a level that is affordable to
many amateurs and hobbyists, it opens up a brand new world for... (the list
is too long to list here)

In fact, I am so impressed that I desire to contribute to the community.
First by contributing to these forums and second by eventually posting
projects to CGRAN!

My intent was to let Gaetano know of the potential for spurious signals so
that he can properly select a center frequency that is free of these little
nuisances.

Again, I am impressed with the level of commitment and the work that the
team puts into to improving an already great product.

Evan Merewether

-Original Message-
From: discuss-gnuradio-bounces+evan=syndetix@gnu.org
[mailto:discuss-gnuradio-bounces+evan=syndetix@gnu.org] On Behalf Of
Marcus D. Leech
Sent: Friday, December 23, 2011 7:54 AM
To: discuss-gnuradio@gnu.org
Subject: Re: [Discuss-gnuradio] DC component

> I have noticed that there are some fixed frequency spurious signals in my
> N210.  These spurious signals are probably associated with the harmonics
of
> the clock.  If your DC component is at some nice even frequency like 2GHz,
I
> would suspect a spurious signal to be the cause.
>
> Evan
>
>
Spurious signals are a virtually-inevitable aspect of modern 
receivers/transmitters.  Many of us are used to radios that are
   "purpose built", and probably don't realize that most such radios 
have their own problems with spurious signals ("spurs"),
   but that they get "tweaked" in the design phase to move those 
(inevitable) "spurs" outside the operational envelope of
   the particular application at hand.  Your radio have a CPU?  Move the 
fundamental of its clock frequency so that the
   fundamental and harmonics fall outside of your band of interest.  But 
in a radio whose "band of interest" lies anywhere
   from DC up to a few GHz, that's a very tall order.

The good news is that most of the time, these "spurs" are quite weak, 
and are generally overwhelmed by any actual signal coming
   in from the antenna at the the same frequencies.  For modern wideband 
modulation schemes, an in-band spur that's 30-40dB below
   your nominal incoming signal make essentially no difference to the 
receive SNR.  For narrowband weak signals that are coming in
   just above the noise floor, it might be a different story.

I've attached a plot of 50MHz of spectrum (thanks to the latest 
50Msps/sc8 support in UHD) around 1.420GHz, with the receiver input 
terminated
   in a 50 Ohm lab-grade termination.

You can clearly see spurious signals spaced every 5MHz, and a stronger 
one right at 1.40MHz.  The 5MHz may be from the ethernet clock,
   not sure, but the stronger spur at 1.4GHz is very likely due to an 
even harmonic of the 100MHz master clock.  Even though this "spur"
   at 1.4GHz is 40dB "out of the noise", in most applications the 
signals themselves will *dwarf* that spur.  The other spurs, across 50MHz
   of bandwidth are no more than 20dB "out of the noise".  They don't 
worry me that much, even for applications like radio astronomy,
   where the signals are really weak.  Placing a low-noise gain chain 
ahead of the receiver, with enough gain to "swamp" the receiver spurs
   is all I need to make these go away.

It's true that a $40K laboratory-calibrated receiver like an (Agilent, 
R&S, etc) spectrum analyser will likely have fewer "spurs".  But if you
   open one of those up, you'll notice a lot of individually shielded 
sub-assemblies--that's not just for show.  They'll also do tricks like
   spreading the clocks for the control/monitoring CPU, shifting clocks 
around, to make the "spurs" move away from the current band of
   interest.  And for the most part, things like laboratory spectrum 
analysers are "deaf as a post"--they aren't designed, generally, to be
   "off-air" receivers, so they tend to be less sensitive to their own 
"spurs".




-- 
Marcus Leech
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org


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Re: [Discuss-gnuradio] DC component

[Evan Merewether]

I have noticed that there are some fixed frequency spurious signals in my
N210.  These spurious signals are probably associated with the harmonics of
the clock.  If your DC component is at some nice even frequency like 2GHz, I
would suspect a spurious signal to be the cause.

Evan

-Original Message-
From: discuss-gnuradio-bounces+evan=syndetix@gnu.org
[mailto:discuss-gnuradio-bounces+evan=syndetix@gnu.org] On Behalf Of
Marcus D. Leech
Sent: Thursday, December 22, 2011 11:42 AM
To: discuss-gnuradio@gnu.org
Subject: Re: [Discuss-gnuradio] DC component

> Hi all,I'm observing a DC component inside my spectrum as you can see
> in the two pictures in attachment (differentfrequency ranges), this DC
> is only shown when there is no active transmission (only noise).
> Consider I'm already using the UHD's advanced tuning specifying the LO
> at 3Mhz. I'm receiving a signal witha central frequency of 430MHz with
> a bandwidth of around 5Mhz, the DC due the LO should be quite away.
> Is this normal ?
>
> Regards
> Gaetano
>
You can use the "calibration" utilities that come with a modern UHD 
(latest from GIT), assuming either a SBX or WBX board, which can
   reduce the magnitude of the "DC anomaly", by calibrating the phase 
and gain errors in the mixers at various frequencies, and compensating
   in the FPGA.

http://files.ettus.com/uhd_docs/manual/html/calibration.html




-- 
Marcus Leech
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org



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Re: [Discuss-gnuradio] DC component

[Marcus D. Leech]

Hi all,I'm observing a DC component inside my spectrum as you can see
in the two pictures in attachment (differentfrequency ranges), this DC
is only shown when there is no active transmission (only noise).
Consider I'm already using the UHD's advanced tuning specifying the LO
at 3Mhz. I'm receiving a signal witha central frequency of 430MHz with
a bandwidth of around 5Mhz, the DC due the LO should be quite away.
Is this normal ?

Regards
Gaetano

You can use the "calibration" utilities that come with a modern UHD 
(latest from GIT), assuming either a SBX or WBX board, which can
  reduce the magnitude of the "DC anomaly", by calibrating the phase 
and gain errors in the mixers at various frequencies, and compensating

  in the FPGA.

http://files.ettus.com/uhd_docs/manual/html/calibration.html




--
Marcus Leech
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org



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Re: [Discuss-gnuradio] DC component and daughter-boards

[Robitaille, Michael]

Matt,

 

I have not and I hope it was a Basic RX board not a TX.

 

Mike

 






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Re: [Discuss-gnuradio] DC component and daughter-boards

[Robitaille, Michael]

In the post:

http://lists.gnu.org/archive/html/discuss-gnuradio/2005-04/msg00303.html
 

 

Matt indicates that "Feed your signal to the + input, and send the reference
voltage
(Vdd/2, 1.65V, available on the board) to the - input.  The problem here
is that the ADC expects the signals to be +/- 1V from the reference,

1.65V.  Your signal will need to be properly biased."

 

Is this a +1.65V you feed to the -input (negative)?

 

Why does the negative input require a positive voltage bias?

 

Mike

 

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Re: [Discuss-gnuradio] DC component and daughter-boards

[Damien B.]

Hi,
thanks for the solution, maybe i'll try with a different transformer
to shift the cutoff.

> > When i'm inspecting the my FIR output, it's really difficult to
> > measure a 30ns (one sample) delay if the signal is not clean.
> 
> This is unrelated to the DC component issue.

Blame it on my bad English... In fact it's partially true: to test a
FIR the easiest solution (that i know) is to send a square-ware as
input and observe the output but if the signal is deformed (at the
limit it's looking like an integration), measures are difficult.

Cheers,
Damien


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Re: [Discuss-gnuradio] DC component and daughter-boards

[Matt Ettus]

> Count me as interested, and actually wondering why your diff-amp solution 
> wasn't the default?  I wonder what the cost differential between the AD8132 
> and the transformer used?

Actually, the differential amps are _cheaper_ than the transformers.  I
went with the transformers because they allow for very high IF
frequencies -- the ADCs are spec'ed to 400 MHz, and I have done direct
sampling as high as 1.2 GHz (with about 15-20 dB of loss).  The
differential amps don't perform well past 70 MHz or so.  Even under 70
MHz, you'll get cleaner signals with the transformers, since they have
no distortion or noise.

> Speaking of, what is the usable low-frequency cutoff for those transformers 
> according to spec?

I think they have a 3dB cutoff of around 100 kHz or so.  If you need a
lower cutoff you can switch them out for other Minicircuits transformers
which have cutoffs as low as 10 kHz (like the T1-1T), at the expense of
a lower frequency response on the high end.  Most should be pin compatible.

Matt


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Re: [Discuss-gnuradio] DC component and daughter-boards

[Lamar Owen]

On Thursday 28 April 2005 13:48, Matt Ettus wrote:
> I have seen very little interest in that.

Count me as interested, and actually wondering why your diff-amp solution 
wasn't the default?  I wonder what the cost differential between the AD8132 
and the transformer used?

Speaking of, what is the usable low-frequency cutoff for those transformers 
according to spec?
-- 
Lamar Owen
Director of Information Technology
Pisgah Astronomical Research Institute
1 PARI Drive
Rosman, NC  28772
(828)862-5554
www.pari.edu


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Re: [Discuss-gnuradio] DC component and daughter-boards

[Matt Ettus]

Damien B. wrote:

> Matt in a previous thread you were talking about making d/b that let
> DC component pass:
> 
> 
>>If there is enough interest, I might make such a set of boards.  If someone 
>>else
>>is interested in the design, the BasicRX/TX are a great starting point, and I
>>can offer guidance.

I have seen very little interest in that.


> When i'm inspecting the my FIR output, it's really difficult to
> measure a 30ns (one sample) delay if the signal is not clean.

This is unrelated to the DC component issue.


> Would it
> be possible to make a quick but not perfect hack of the basicTX and
> RX, something like:
>- removing transformer
>- Short-cut pin 3 to 6 and 1 to 4
> the resulting impedance would be 40 Ohms (50 // 50+200+50), but is it
> really bad for low freq square wave < 2MHz ?

Sort of.  You would remove the transformer first.  Then you have 2
options --

1>  Feed your signal to the + input, and send the reference voltage
(Vdd/2, 1.65V, available on the board) to the - input.  The problem here
is that the ADC expects the signals to be +/- 1V from the reference,
1.65V.  Your signal will need to be properly biased.

The better option is:

2>  Put an AD8132 or similar differential amplifier in the circuit.  You
can find the appropriate connections in the AD8132 datasheet.  This is
what I would do if I did make a DC version of the BasicRX

Matt


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[Discuss-gnuradio] DC component and daughter-boards

[Damien B.]

Hi,

Matt in a previous thread you were talking about making d/b that let
DC component pass:

> If there is enough interest, I might make such a set of boards.  If someone 
> else
> is interested in the design, the BasicRX/TX are a great starting point, and I
> can offer guidance.

When i'm inspecting the my FIR output, it's really difficult to
measure a 30ns (one sample) delay if the signal is not clean. Would it
be possible to make a quick but not perfect hack of the basicTX and
RX, something like:
   - removing transformer
   - Short-cut pin 3 to 6 and 1 to 4
the resulting impedance would be 40 Ohms (50 // 50+200+50), but is it
really bad for low freq square wave < 2MHz ?

And be sure to send a positive signal...

I dont know how the converter will deal with the DC component and the
2's complement internal encoding though.

Thanks, cheers
Damien


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