Harold E. Johnson wrote:

Okay, I've read the dox but am still in the dark as to what *you* use
yours for. What makes it so indispensible?

When was the last time you needed to know a capacitor value AT the
frequency you're going to use it at? (the "C" minus the "L" of the leads)
Or the passband of a filter and it's IL and it's skirts and it's blowby and
it's group delay and it's terminating impedances?
Or needed dual high level isolated oscillators to make Ip3 measurements on a
receiver or transmitter?
Or have a precision sweep generator and trigger for a Spectrum Analyzer?
Or needed the inductance of an inductor AT the operating frequency? (The "L"
less the "C")
Or needed the series and parallel resonant frquencies of a crystal and the
Rs of same and it's "Q"?
Or needed the "S" parameters of a small signal transistor?
Or wanted to know the impedance of your antenna at the shack end of the
coax. Or, if you were smart enough to log the length of coax when you
installed it, the impedance AT the antenna?
Or needed the Zo of a length of coax?
Or the gain of an amplifier over it's frequency range and it's 3 dB corners
to the 0.01 dB?
Or needed a quadrature signal source to be the LO of that new I-Q receiver?

Or-or-or

It's a lab in a box.

W4ZCB


Thanks to the US-Europe time difference, Harold has left me very little
to add this morning!

We could also think of it in terms of the other instruments that the VNA
can replace - or beat, because it gives full vector (R-X etc)
information. Using only its reflection functions, the VNA can replace
all of the following: an MFJ, Autek or AEA computing antenna analyser;
an LF LCR bridge; a Q meter; an HF R-X impedance bridge; and an HF
return loss bridge.

As Harold pointed out, the twin DDS signal sources can replace an HF
sweeper or a medium-quality HF signal generator - or two of each, if
necessary.

The transmission measurement functions - magnitude and phase, with a
dynamic range of at least 90dB - are something I just didn't have.

Several months ago, Harold described an excellent practical example of
designing and building a crystal filter. This goes in five steps:

1. Rough out the design using available software.

2. Take a pile of cheap crystals and measure their individual
characteristics, on-frequency, using the VNA. Select the best ones for
the filter you want to build.

3. Refine the filter design according to the actual component
measurements, and use simulation software to predict how it's going to
work. Adjust other component values as required.

4. Build the filter. There is no alignment - steps 2 and 3 replaced all
that.

5. Use the VNA to confirm that it works exactly as predicted. Have
lunch.

You can see those results in the VNA documentation.

This is the professional approach to RF design: compute, measure
components, compute again, and build... and it *will* work, because
you've left almost nothing to chance. Until now, this approach had not
been open to amateurs because we didn't have the necessary test
equipment. To me, that is the real power of this instrument.

For some years, I've been trying to develop this approach, but have had
to rely on the goodwill of a friend who could make measurements using
his lab VNA at work. But obviously there have been limitations on the
kinds of things that could be measured - they had to be small enough to
fit in a briefcase, and they had to be completed to the extent that they
were plug-and-play - so there have been no antenna measurements, and no
measurements on work in progress. I'm free from those limitations now,
up to at least 50MHz... and the way is being opened to make equally good
measurements on the higher amateur bands.

Meanwhile my friend has noticed that he won't have access to the lab
forever, so guess what - he has built himself an N2PK VNA too! Like
N2PK, we're struggling to identify any significant differences in
accuracy between the two instruments.

A compact, lightweight and inexpensive VNA also opens up completely new
possibilities for field measurements that even a commercial VNA cannot
make, because it's too big, heavy, delicate and valuable to be moved out
of the lab.

When the weather improves, I plan to take the N2PK VNA outside with a
laptop and a small battery pack, and to do some really detailed antenna
work. Of the three objects to be carried around, by far the smallest and
lightest will be the VNA.

I hope that gives you a real picture of the new possibilities that are
opening up.



--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)

http://www.ifwtech.co.uk/g3sek

On Fri, 13 Feb 2004 09:01:38 +0000, "Ian White, G3SEK"
wrote:

Harold E. Johnson wrote:

[snip]

I hope that gives you a real picture of the new possibilities that are
opening up.

[snip]

Thanks, both! Sounds like a must-have now you've explained all these
remarkable capabilities. I shall await developments on parts
availability with great interest....

--

The BBC: Licensed at public expense to spread lies.

On Fri, 13 Feb 2004 09:01:38 +0000, "Ian White, G3SEK"
wrote:

Harold E. Johnson wrote:

[snip]

I hope that gives you a real picture of the new possibilities that are
opening up.

[snip]

Thanks, both! Sounds like a must-have now you've explained all these
remarkable capabilities. I shall await developments on parts
availability with great interest....

--

The BBC: Licensed at public expense to spread lies.