Interesting piece of test equipment

A few months ago I picked up a used Agilent E4406A VSA to play around
with
and I thought I’d share a few of my observations. In short: I think
it’s a
very good piece of equipment for the money, and I’d love to see it
develop a
broader following in the experimenter community.

The E4406A was initially intended for use as a transmitter tester,
covering
the 7 MHz to 4 GHz range (5 Hz-4 GHz with the B7C I/Q input option).
I’d
seen them all over eBay and the used test equipment houses for
relatively
cheap and, encouraged by an Agilent forum post (link below), when they
got
cheap enough I bit the bullet. I think the market is hampered by the
fact
that it’s viewed (partly, rightly so) as a very specialized piece of
gear,
and supply is definitely outstripping demand. They can be often be
found
for sub-$1000.

The number one thing that attracted me to the instrument is that, as
it’s
intended to be a VSA, the LO is not swept and has pretty good phase
noise
characteristics…certainly better than anything else in the price
range.
This obviously works to its detriment as a general-purpose spectrum
analyzer, but for narrow spans (<=10 MHz) it’s great. I’ve had good
luck
retrieving the spectrum trace in Python (using PyVisa) and piecing
together
multiple shots for a broader span…the instrument has an Ethernet
interface
and tunes very quickly, so grabbing a good swath (100s of MHz) doesn’t
take
much time at all. If it was all I had for spectrum analysis, I could
certainly make it work. John Miles, KE5FX, has done a lot of work to
support it with his tools, link below.

There’s good potential for pulling out the I/Q data for offline (or even
near-real-time) analysis. Agilent’s older 89601A VSA software supports
this
box, and I’ve successfully made some very good DQPSK measurements using
the
trial version. I haven’t gone as far as sniffing the Ethernet to see
what’s
being sent across, but I’d imagine they’re just using the standard calls
which are well documented in the Programmer’s Manual. Getting Python to
talk to the instrument was trivial, so I think there’s some really
interesting work that could be done here.

One reason I mention this on an SDR forum is that there’s potential to
use
the instrument as a tuned front-end for an IF digitizer. There’s
provision
to take the 321.4 MHz IF out, and although my unit didn’t come with the
IF
option, it took less than an hour to figure out how to wire the
connector on
the RF deck to a back panel connector. Happy to provide guidance here
if
anyone’s interested. Quick measurements show the IF bandwidth to be
fairly
flat to 100 MHz or so, though I haven’t done full testing and there may
be
some gotchas in various modes or at certain frequencies. It’s obviously
wider than 10 MHz.

One thing to note: since this is a transmitter tester, there’s no preamp
and
the noise figure is almost guaranteed to be lacking. In fact, there’s a
beefy 7 dB pad right at the front panel input connector, just in case
there
was any doubt as to its intended application. That’s certainly not an
insurmountable limitation.

All in all I think this instrument is definitely worth a look. It’s not
a
replacement for a general-purpose spectrum analyzer, but has other
attributes that might make it very valuable to the radio/SDR
experimenter.
–Ward

http://forums.tm.agilent.com/community/viewtopic.php?t=3857&f=50 (the
forum
post that sparked my interest)
http://forums.tm.agilent.com/community/viewtopic.php?f=31&t=25805&p=75890
(further
info)
http://tech.groups.yahoo.com/group/hp_agilent_equipment/message/36988
(John’s
take, and links to his software)

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