You can use a SNA for quick and dirty tests, comparing a known, high
quality, component to the DUT, especially if you have a Storage
Normalizer, but in no way is an SNA a substitute for a VNA.
The only regime where they are used today, AFAIK, ia at frequencies above
those practical for VNAs, over 40 GHz or more.
FWIW,
-John
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On 24 December 2012 21:11, laurens_db <laurens101@...> wrote:
A well calibrated VNA is the best way of measuring S11/ S22.
That was my belief too - at least in the context of typical test
equipment. I would not be surprised if there other techniques
applicable to standards labs, which are not too practical for
commerical or amateur use.
I just opened this book:
and looked up scalar network analyzers. By the very title of the book,
you can see it is mainly devoted to VNAs, though there is a bit of
discussion about scalar network analyzers. To quote:
"Scalar network analyzers has the attribute of being very simple to
use, with almost no calibration or setup required. The scalar network
analyzers were designed to be quite flat in frequency response, and a
typical system consisted of one and the input and one at the output of
the DUT. However, for measurements of input or output match, or
impedance, the scalar network analyzer relied on a very high quality
coupler or directional bridge. If there was any cabling, switching or
other test system between the bridge and the DUT, the composite
matches of ALL were measured. There was no additinonal calibration
possible, to remove the effects of the mismatch. As test systems
became more complex and integrated, scalar network analyzers started
to fall from favor and there are virtually none sold today by
commercial instrument manufacturers"
As far as I can tell, the error correction of a VNA offers the ability
to compensate for errors than the scalar network analyzer simply does
not.
A broadband cal kit load is as close to 50Ohm as HP can make it.
Yes, although because of the limitations of broadband loads, sliding
loads are often used at higher frequencies. To once again quote from
Joel Dunsmore's book:
"The load standard is usually the most difficult to produce. <snip
lots> The sliding load, which should more properly be called a sliding
mismatch, is constructed from lengths of precision airline, The centre
conductor of the airline is typically created in such a way that it
can slide into place while the outer conductor is not yet mated, to
allow a beadless connection. The load element is typically not
resistive element, but is more commonly a tapered bead of lossy
material, that essentially makes the airline look like a lossy
element. It is designed to have an impedance which is not quite 50
Ohms, normally in the range of 26-40 dB return loss.
<snip>
"As the sliding load is moved, so its apparent impedance rotates
around the Smith Chart"
There's a diagram showing a full circle on the Smith chart, almost,
but not quite in the centre.It slightly spirals in, as the frequency
(and so loss) is incresed.
"The difference between the computed centre and the actual centre of
the Smith Chart determines the directivity error term"
I think is should be obvious what I stated earlier, that sliding loads
are impractical at lower frequencies. Looking at the manual for the
85054A 'N' cal kit, the minimum frequency of the sliding load is 1.999
GHz.
At 100 MHz (lambda = 3m), the load would need to be 1.5 m long to get
a complete half-wave on a Smith Chart. I suspect you can get away with
less than a full circle, as it only needs 3 points to make a circle,
but I think accuracy would suffer a lot if you could not get an
appreciable part of a circle.
Dave
