A better calibration technique may be to add a known amount of RF noise
(eg from an amplified noise diode or similar) to one of the mixer inputs.
This allows calibration without switching the sound card preamp gain to
a value different from that used to make measurements.
The added RF noise only has to be white over the spectrum analyser
bandwidth.
This allows the frequency response of the entire measurement setup to be
determined.
Bruce
Bob Camp wrote:
Hi
I did a search for *input* DC coupled sound cards and came up empty handed.
There are *output* DC coupled cards out there.
I have a pile of old Audigy 2's that are ripe for destruction. Essentially they are free
at this point in their life. I've also got a E-MU 0202, an Xonar DX, a couple of Presonus
firewire boxes running around. As the price goes up, the probability of destructive mods
goes down pretty fast. The Xonar is probably the one that will be in the "final
product".
-----------
The lock box provides an easy way to bump one source off far enough for a beat note
calibration. Having a DC coupled "scope" mode would be very nice for doing an
accurate calibration. Sine wave approximations are one step to many.
---------
You could model the input to the sound card. I'll certainly try that on some of
the more expensive boxes. Also going to 1 Hz is not an every day need. Most of
these cards should do just fine at 10 Hz with no mods.
--------
Still looking for software.
Bob
On Feb 18, 2010, at 8:31 PM, John Miles wrote:
Got it. So you're planning to have a way to supply an offset frequency to
generate a beatnote for calibration at some known reduced amplitude,
correct? That, and an FFT program that understands how to normalize to 1 Hz
BW (and correct the window's noise-equivalent bandwidth) should be all you
need. Bonus if the FFT program allows the calibration factor to be added to
whatever numbers it draws on the Y-axis.
What brand/model sound card are you looking to use? With a white-noise
source you might be able to add a calibration step that takes the effect of
the input coupling capacitor out of the measurement, so you wouldn't need to
modify the card itself. There was an article in QEX a year or so ago on the
basic idea. Again, this all assumes you have some control over the
software.
There may also be some DC-coupled sound cards at that end of the market,
too -- I haven't looked lately.
-- john, KE5FX
-----Original Message-----
From: [email protected] [mailto:[email protected]]on
Behalf Of Bob Camp
Sent: Thursday, February 18, 2010 5:10 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Sound Card Spectrum Analyzer
Hi
Ok, A bit more info:
1) Quadrature PLL using an RPD-1 DBM and a home brew lock box.
2) Willingness to accept that I'm measuring a pair of oscillators
3) Plenty of sources at the appropriate frequencies
4) First took a shot at this in 1975 (I forget the Fluke app note
number ...) been doing it ever since
5) Appropriate preamp between the RPD and sound card is a work in progress
6) Sound card is a 192 KHz / 24 bit / ~110 db snr class card
7) Sound card will get butchered for the application.
8) Should be able to hit -165 ish floor, -120 ish at 1 Hz
Except for the 16 bit limitation, Baudline looks like it's got
what I need.
Bob
On Feb 18, 2010, at 7:53 PM, John Miles wrote:
Unfortunately there's no way to build a sound-card application that can
measure phase noise in the general case without a lot of additional
hardware. Baseband PN analysis with an FFT presupposes that
you have some
external means of downconverting the DUT signal to DC with a superior
reference at the same frequency, tuned with a quadrature PLL.
There must be
a suitable high-pass filter and LNA to block any DC residuals
and preamplify
the remaining part of the noise sideband.
Further, it's often the case that noise close to the carrier is strong
enough to keep you from being able to use enough gain to see
the broadband
floor, so you actually need more than one high-pass filter ahead of the
sound card in many cases. These switchable filters were
mandatory with the
old 13-bit signal analyzers like the 3561A, and may still be
needed today if
you want to look down to 1 Hz. If you restrict your offset
range to (say)
100 Hz to 20 kHz and require a 24-bit sound card, you can
probably get away
without the switchable HPFs.
It'd be helpful to know exactly what sorts of measurements you
need to make,
and on what devices. PN measurement is a *lot* of work, on both the
software and hardware sides. Much of it goes into developing a suitable
calibration process. Take a look at the 3048A manuals
sometime, realizing
that the 3048A hardware itself is not very complicated...
-- john, KE5FX
-----Original Message-----
From: [email protected] [mailto:[email protected]]on
Behalf Of Bob Camp
Sent: Thursday, February 18, 2010 4:18 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Sound Card Spectrum Analyzer
Hi
Both are very cool programs. Both are full of all sorts of neat
features. As far as I can see, neither one has the features I'm after.
More or less - I want it to run like a clunky HP audio analyzer
rather than a very cool tool for ham radio.
Bob
On Feb 18, 2010, at 6:33 PM, Don Latham wrote:
You have looked at:
spectran and spectrum lab ?
Don
Bob Camp
Hi
Assuming I have a decent sound card, and a computer, the next
thing I need
is software. If I want:
Required:
1) non- commercial
2) 1 Hz normalization
3) good low frequency processing (decimation ahead of the fft)
4) low cost
Much preferred:
5) a non-evil OS
6) Rational performance on a non-quad core system
7) free
8) rational calibration
9) scope view.
10) reasonable graphics
11) active support by the author
The application is measuring phase noise. That what makes 2&
3 pop up on
the list.
I've looked at a lot of programs and they all seem to be
pretty slick. The
ones I've looked at so far don't quite hit the mark for phase
noise. I'm
pretty sure that there are others on the list who have dug
into this same
issue already.
Bob
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--
Dr. Don Latham AJ7LL
Six Mile Systems LLP
17850 Six Mile Road
POB 134
Huson, MT, 59846
VOX 406-626-4304
www.lightningforensics.com
www.sixmilesystems.com
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