On Tue, 29 Dec 2015 22:14:45 -0000, "gareth"
wrote:

What is interesting is the simplicity of the approach (ignoring the hidden
cost of the ubiquitous PC), but it ignores the multiple harmonics that
come out of DDS chips.

One of the nice things about DDS is the lack of harmonics and
distortion. Given sufficient bits, you won't see much in the way of
harmonics. If there are are any harmonics, it's treated as
"distortion" which in this case means unwanted junk signals. See the
section on "dynamic performance".
http://www.embedded.com/design/configurable-systems/4025078/Understanding-analog-to-digital-converter-specifications
SNR(dB) = (6.02*N) + 1.76 where N = number of bits
So, if you have a 8 bit DDS, all the junk will be down:
(6.02*8)+1.76 = 50 dB
I think that's sufficient for most VNA applications. Of course, you
can introduce other forms of distorition (jitter, non-linearity,
clipping, symmetry, etc) errors in stages after the DDS.

So, if you were to working at, say, 3MHz, how would youknow that you were
not actually analysing responses at 6, 9, 12, 15, etc MHz?

Ummm... That's what a spectrum analyzer is used for.

http://sdr-kits.net/DG8SAQ/VNWA/Baier_VNWA2_QEX.pdf

But it does raise one question, and that is, who is fooling who about
its purchase price of ooo £450, for there cannot be more than about £40
in its component costs?

Jim Pennino listed a good start on what's missing. If you're going to
make it portable, add shock mounting, drop proof case, coax adapters,
leash, etc.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558