Jim Lux wrote:
>On 7/18/12 1:49 AM, Ian White GM3SEK wrote:
>> A few points from this and earlier postings....
>>
>> 3. True Vector Network Analyzers that do offer - and indeed, require -
>> OSL calibration at the exact location where the measurement is being
>> made. The instrument then compensates for its own inaccuracies and
>> presents the user with a much truer result. This is a defining feature
>> of the true VNA that kicks it into a totally different league of
>> accuracy.
>>
>> (For example, there is almost no difference in accuracy between the
>> home-built N2PK VNA and a high-end laboratory VNA. The only significant
>> differences are in extremely difficult measurements where it is highly
>> likely that both analyzers are wrong.)
>>
>I would suggest that there is a significant difference between two
>subflavors of the latter, depending on whether they used a tuned RF
>detector or not. The TAPR VNA, for instance, has a broadband detector
>scheme, so while it's great at measuring something like S21 of a filter,
>if there's out of band interfering signals (e.g. measuring S21 between
>two antennas), then there's a potential problem.
>
>I think, also, that the lab VNA will probably have somewhat better
>dynamic range than the less expensive kilobuck ones. For the vast
>majority of uses (as you point out), whether you can accurately measure
>the gain and phase of an S21 of -60dB is probably not a big deal.
Thanks for the thoughtful comments, Jim. That last point is entirely
fair: we *should* be able to connect an "antenna analyzer" to an
antenna, without it falling over due to strong received signals.
The TAPR VNA is no longer available and seems to have been left behind
by other, more advanced designs.
The dynamic range of the N2PK VNA is much better because it is
essentially a robust direct-conversion receiver with a very narrow
detection bandwidth. A dynamic range of 80-90dB is routinely achievable
on the bench; and when connected to an antenna, out-of-band signals need
to reach the blocking threshold in order to upset impedance measurements
significantly. The main spurious responses of the N2PK involve LO
harmonics, but they are still at a low level and they only appear in
sweeps covering more than a 2:1 frequency ratio. Next along, the DG8SAQ
VNWA is essentially an SDR using audio detection in a sound card, so
once again the dynamic range can be quite good.
The big step up in cost and complexity is from those simple
receiver/detector architectures to a full superhet design. Superhet
architectures are the main reason why laboratory VNAs have a better
dynamic range across their entire frequency coverage - but also the main
reason why they cost tens to hundreds of kilobucks.
>Most of what you get with the lab box is better user interfaces, built
>in computational features, and better remote control/export of data. I
>don't think there's any LabView or Matlab drivers for the kilobuck
>class VNAs. The latter sort of thing is what you get when you spend the
>10-20k for the latest Agilent, R+S, or Anritsu portable boxes, which
>performance wise are comparable to any of the tuned detector kilobuck boxes.
>
Well, not entirely. PC-driven test equipment has the potential to be far
more flexible than an all-hardware box, because you can choose between
several different kinds of control software to meet your needs of the
moment. For example, N2PK wrote his original control software for DOS,
and it was very much "bare-metal" to meet his needs as the original
hardware developer. Then G4PMK and I wrote something completely
different: VNA 4win was a "virtual front panel" that behaved much more
like the hardware VNAs that we already knew. But the big difference with
the "virtual front panel" was that whole areas could be completely
reconfigured to give an optimized control interface for each specific
function. That is something a hardware panel just can't do. Next up was
G8KBB with MyVNA, which gave the same N2PK box a completely different
UI, more of a "Windows" look that has been followed by the AIM software
and others.
Only PC-controlled test equipment can achieve such total changes in
personality... and not forgetting the potential for remote control using
your existing wireless network.
As for the LabView and MatLab drivers... I am not aware of any, perhaps
because the amateur approach is more commonly to hack together a
special-purpose program to meet one specific need. But there's no doubt
that more powerful all-purpose drivers could be written if needed. On
the output side, there are no problems as the VNA control software can
already dump all of its data out to a file.
>Oh yeah, you can also get an E-Cal widget for your Agilent, which is
>pretty convenient. One could build such a thing pretty easily, though.
From an amateur point of view, there isn't much need for a gadget to
speed up the calibration cycle because calibration doesn't use up a lot
of your time anyway. And before you could contemplate building an E-Cal
unit, you'd need a precision-calibrated VNA to bootstrap yourself into
the project.
--
73 from Ian GM3SEK
http://www.ifwtech.co.uk/g3sek
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