Electronics

Guest · Posted: Fri Feb 04, 2005 2:01 am Post subject: Analog, multichannel RF recorder?

I need to record some real-world RF waveforms. All-digital approach
looks *very* disk intensive!

Wondering if there are multi-channel analog recorders adequate for the
job. Pointers to "better" digital solutions are appreciated as well.

Scenario:

I have 8 antennas receiving ~1GHz signals, ~5 MHz bandwidth each
signal.

Each antenna drives 3 RF->IF sections, for a total of 24 IF outputs.
IF is around 75MHz.

IF is undersampled (Fs=60MHz) with 12-bit A/D converters.

60M samples/second * 12 bits/8 bits/byte) = 90 Mbytes/sec

24 channels * 90 Mbytes/sec = 2.16 GB/sec
(That's 129.6 GB/minute or 7.776 Terabytes/hour, and I'd like to record
a full hour of data.)

24 channels is ideal, but 4 channels is absolute minimum.
Could record either the 75MHz IF output, or 1GHz RF straight from the
antennas.

I then need to play these back (4-24 channels in parallel) for testing
my receiver (al RF or just IF).

[Analog in and out is easiest. Digital record/playback approach is OK
(no wow & flutter problem) as long as resolution remains around 12-bits
and there are no artifacts of original sampling, because I need
"authentic" signals coming thru the IF and into my A/D converters for
accurate testing.]

[I could also test my DSP section only, by recording my own A/D
converter outputs, given an affordable disk system with sufficient I/O
bandwidth. With 8 channels at 90MB/s = 720 megabytes/second, I could
record about 6 minutes of data on one 250 GB drive, which is an
acceptable starting point for me.]

Tall order I know, but I'm open to creative solutions. Thanks a ton for
*any* help or references you can provide.

mj

John O'Flaherty · Guest

jjohnson@cs.ucf.edu wrote:

Jerry Avins · Guest

jjohnson@cs.ucf.edu wrote:

rutiger · Guest

Could you not write to a RAID rather than a single disk?

john · Guest

jjohnson@cs.ucf.edu wrote:

Guest

In sci.physics jjohnson@cs.ucf.edu wrote:

Guest

Sorry to have not been more accurate, precise, and clear. First, I
think I should have said "bandpass sampled" instead of undersampled.
IF output is centered at 75 MHz, +/- about 15MHz of signal BW (spectrum
from 60MHz to 90 MHz).

Sampling at 60MHz shifts that down to baseband (0-30MHz) where the DSP
begins. The 5 MHz BW I quoted is after some of the filtering in our
DSP.

I was hoping to capture analog on a multichannel recorder so I could do
repeated playback thru the RF->IF analog chain, and iteratively tweak
components (including my A/D converters), without having the artifacts
of other (possibly inferior) A/D converters corrupting my
"gold-standard" reference signals.

Digital capture also means I have to hook up 24 modulators to replay
the RF. I wanted to avoid degrading my reference signals as much as
possible. Replaying and modulating captured baseband means the next
time I look at the signals in my DSP, they will have been thru my IF
twice, plus somebody else's modulator, which munges things a bit. Thus,
capturing original Rx antenna output would be ideal.

Also, capturing the data will be expensive, as it requires rental of an
airplane for a couple of hours. Swapping disks every 2-5 minutes does
not give me much continuous, uninterrupted data to work from. (I want
to replay an entire flight segment thru certain maneuvers and across a
good strectch of airspace.)

Having said that, if there is an "easy" (affordable, off-the shelf)
RAID solution I can write to from my ADC converters (129.6 GB/minute
for at least 10 minutes), I could use that for replay into my DSP
datapath simulations. Not ideal, but far better than nothing.

Hope this better explains my needs and rationale. But as I mentioned
before, all feedback is appreciated.

Thanks again,

mj

Repeating Rifle · Guest

jjohnson@cs.ucf.edu wrote:

Mark · Guest

If you have 8 antenna's why do you have to record 24 channels.

Can't you record the 8 signals before they go into the 2 way converters
(whatever those are)?

Saves you a factor of 3.
Mark

Robert Baer · Guest

jjohnson@cs.ucf.edu wrote:

Mike Yarwood · Guest

"Mark" <makolber@yahoo.com> wrote in message
news:1107582077.694507.65160@l41g2000cwc.googlegroups.com...

Eric Jacobsen · Guest

On Sat, 05 Feb 2005 11:56:58 GMT, Robert Baer
<robertbaer@earthlink.net> wrote:

Tom Potter · Guest

<jjohnson@cs.ucf.edu> wrote in message
news:1107460901.200754.128210@c13g2000cwb.googlegroups.com...

Simone Merrett · Guest

MJ,
My first reaction to your apparent (rx testing) problem is "WHy isnt
simulation sufficient?"
or inother words, "What physical phenomenom (affecting the signal ) is so
complex, that it can not be statistically re-created".??

Vibration and movement of and in an airplane (assuming this is part of your
system) can be fairly easily measured, modelled, and use to 'modulate' an
"ideal" signal into a pseudo-actual one. Recording the relationship between
the acutal phenomenon and the affect on signal ( just use CW) is fairly
easy, and can be used to derive a statisitcal model. (MATLAB is very helpful
here)

Noise, wow flutter etc due to ionospherics is also able to be modelled. Im
sure there are lots of papers on how this phenomenom affects signals.
In short, are you really sure recording the signal is
a) the easiest method to acheive what you need?
b) Is able to achieve the depth and dynamics of testing youd like,
when compared to simulation?

Before jumping into a recording-solution, I would justify mathematically
that I could not acheive the same testing requirements using an
easier-to-built simulation (which also probably has a far broader potential
for testing / use than a one-of recording).

Having said all that,....................

My first guess at actually doing what you want, with easily available COTS,
is to use a number of fast customized linux boxs, with maximum RAM, A/D
cards, and fast disk, then setup them as a set of rotating buffers, You will
need N = data-rate/disk-write-rate boxs, so the 1st just completes writing
all its data to disk before the last triggers it for reading to RAM again.
How you multiplex the various signals (each of 5MHz?) is another issue, but
it seems wise that you'd shift each of your individual signals to baseband
(just use mixer and LO) .

Even with this system, there is a limit to how accurate it will be for
suitable recreation of the actual signal. Even after locking the local
osciallators, accurately recording phase noise, especially for long
term-intergration, may be a problem.

Happy working!
David Merrett

<jjohnson@cs.ucf.edu> wrote in message
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Steve Underwood · Guest

Hi,

I am unclear what you are trying to evaluate. Are you gathering data for
research on the world outside the aircraft, or are you trying to
evaluate how well your own kit on the aircraft performs? It makes a big
difference to the best approach. Flights are expensive, and you need to
minimise the chance of a wasted flight. The less things between you and
your data source, the less things you might find out later were not
behaving properly. :-) In other words, record as close to the raw data
as data rates and volumes will permit.

I haven't been involved in this kind of work for some years. It used to
be that disk recording on aircraft was problematic for a number of
reasons - vibration, acceleration, pressure, etc. I guess these days it
would probably work out OK, as notebook disks seem to behave well on
aircraft. A quick Google seems to show Enertec and others still make
tape recorders for the kind of applications you are looking at. I used
to use Enertec and Ampex machines in the 80s. The Ampex ground playback
machines were good, but I had better results recording with Enertec
machines. That was a long time ago, though, and newer machines may bear
little resemblance to the ones I used. It seems these machines haven't
speeded up that much. We used to write at about 120M bits/second to
Enertec machines 20 years ago. They are talking about less than 1G
bits/second now :-) Those solutions certainly work solidly in aircraft
of various types, when used to suitable shock and vibration mounts -
recording defense system flight data is their core market.

Exactly what you should try to record is another matter entirely. This
comes back to my first question. A modern fighter gives surprisingly
modest levels of vibration, but massive acceleration. A C-130 gives
little acceleration, but vibrates hard at about 68Hz. The microphonic
effects of these things on RF kit can be interesting, to say the least.
If the aircraft operators will let you do it, tethering the aircraft and
running the RF systems with the engines flat out can be educational.
They tend not to like extended tethered tests, though. Whether done on
the ground or in the air, I would recommend trying to get a proper
understanding of how well the RF equipment works in its operational
conditions before putting too much credance in any information you
gather from the outside world.

Regards,
Steve

jjohnson@cs.ucf.edu wrote:

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