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.

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?


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?

In rec.radio.amateur.antenna 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.

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?


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?

Assembly, test, calibration kit, expansion board, presentation storage
case, cables, etc.


--
Jim Pennino

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

On 12/29/2015 8:17 PM, Jeff Liebermann wrote:
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.

The issue with noise from a DDS is that all noise is not the same. A
DDS is used to produce a sine wave, preferably of a single frequency. A
typical DDS has a phase accumulator word of some number of bits which
establishes the accuracy of the frequency being produced. Then some or
all of those bits are used to produce digital samples of the sine wave.
The quantization noise of the sample produces noise which is fairly
evenly spread across the spectrum, but related more to the clock rate
than the carrier frequency. The quantization noise from the phase word
produces noise which has significant content very close to the carrier.
This noise can not be easily filtered and so is of great concern. The
fewer phase word bits used (called phase truncation) to produce the sine
values the greater the close in noise.

For the most part you seem to be describing noise created by an ADC or
DAC which is dependent on the number of bits in the sine wave sample.

--

Rick

On Tue, 29 Dec 2015 21:20:24 -0500, rickman wrote:

On 12/29/2015 8:17 PM, Jeff Liebermann wrote:
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.

The issue with noise from a DDS is that all noise is not the same. A
DDS is used to produce a sine wave, preferably of a single frequency. A
typical DDS has a phase accumulator word of some number of bits which
establishes the accuracy of the frequency being produced. Then some or
all of those bits are used to produce digital samples of the sine wave.
The quantization noise of the sample produces noise which is fairly
evenly spread across the spectrum, but related more to the clock rate
than the carrier frequency. The quantization noise from the phase word
produces noise which has significant content very close to the carrier.
This noise can not be easily filtered and so is of great concern. The
fewer phase word bits used (called phase truncation) to produce the sine
values the greater the close in noise.

Notice that I didn't use the term "noise" anywhere in my comments.
That was intentional as I was directly addressing the comments about
"multiple harmonics". I didn't want to dive deep into how a DDS
synthesizer works mostly because I don't understand it very well. I
have a few cheap eBay DDS synthesizer boards, some grand ideas, and no
time to play.

For the most part you seem to be describing noise created by an ADC or
DAC which is dependent on the number of bits in the sine wave sample.

Exactly. Ignoring noise and jitter, if the DDS DAC has enough bits
and is sufficiently linear, the harmonics will not be a problem for a
VNA which does not have enough display resolution to where the noise
is going to be a problem.

A common DDS chip is the AD9850 which uses a 10 bit DAC and 14 bits
for that phase after truncation:
http://datasheet.octopart.com/AD9850BRS-Analog-Devices-datasheet-88235.pdf
10 bits puts the harmonics theoretically about -60dB down from the
carrier. However, that won't happen as there are other sources of
noise, errors, distortion, junk, non-linearity, etc. At about 3MHz, I
recall seeing about -40dB, which became worse above 25 MHz.

Argh... back to (paper)work.



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

On 12/29/2015 10:29 PM, Jeff Liebermann wrote:
On Tue, 29 Dec 2015 21:20:24 -0500, rickman wrote:

On 12/29/2015 8:17 PM, Jeff Liebermann wrote:
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.

The issue with noise from a DDS is that all noise is not the same. A
DDS is used to produce a sine wave, preferably of a single frequency. A
typical DDS has a phase accumulator word of some number of bits which
establishes the accuracy of the frequency being produced. Then some or
all of those bits are used to produce digital samples of the sine wave.
The quantization noise of the sample produces noise which is fairly
evenly spread across the spectrum, but related more to the clock rate
than the carrier frequency. The quantization noise from the phase word
produces noise which has significant content very close to the carrier.
This noise can not be easily filtered and so is of great concern. The
fewer phase word bits used (called phase truncation) to produce the sine
values the greater the close in noise.

Notice that I didn't use the term "noise" anywhere in my comments.
That was intentional as I was directly addressing the comments about
"multiple harmonics". I didn't want to dive deep into how a DDS
synthesizer works mostly because I don't understand it very well. I
have a few cheap eBay DDS synthesizer boards, some grand ideas, and no
time to play.

You mentioned "distortion" which is the problem with a DDS. The OP's
use of the term "harmonic" shows his lack of familiarity with DDS
technology. It is very simple really. A step size is set by the value
added to an accumulator on each clock cycle. The accumulator is allowed
to wrap around. This generates a ramping value representing the phase
of a vector. The number of bits used in the accumulator and phase step
in conjunction with the clock rate set the frequency resolution of the
ramp.

The next step is to turn the phase into a sine sample. Often a lookup
table is used. Since the number of entries in the table is limited this
limits the phase resolution used to generate the sine sample. The
number of bits in the output of the table set the resolution of the
amplitude. These two resolutions generate very different noise patterns.

There are other ways of generating the sine samples. One is to
approximate by calculation. Calculations can use a series expansion or
various trig identities. These can achieve lower values of distortion
with less hardware than large table lookups.

The final step, if an analog signal is needed, is the DAC conversion. A
DAC introduces its own types of distortion, harmonics and noise.

Nothing complex really. But there are some subtleties.


For the most part you seem to be describing noise created by an ADC or
DAC which is dependent on the number of bits in the sine wave sample.

Exactly. Ignoring noise and jitter, if the DDS DAC has enough bits
and is sufficiently linear, the harmonics will not be a problem for a
VNA which does not have enough display resolution to where the noise
is going to be a problem.

That's like saying "if you ignore the tailpipe emissions, a diesel
engine is very clean". Noise and jitter *is* the limitation of a DDS.
The frequency resolution is easy to make as fine as you wish. The
limitation comes in reducing the close in spurs.


A common DDS chip is the AD9850 which uses a 10 bit DAC and 14 bits
for that phase after truncation:
http://datasheet.octopart.com/AD9850BRS-Analog-Devices-datasheet-88235.pdf
10 bits puts the harmonics theoretically about -60dB down from the
carrier. However, that won't happen as there are other sources of
noise, errors, distortion, junk, non-linearity, etc. At about 3MHz, I
recall seeing about -40dB, which became worse above 25 MHz.

For many apps, such as a lot of radio work, -60 dB is not a very good
number. I seem to recall seeing radio apps where -80 would be a more
respectable value. But then that was military radios so maybe -60 is
just fine for amateur work.

--

Rick

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.

Up to about 600MHz I think the DDSen are pure enough, but above that the
VNWA deliberately uses mixer products of some kind. As to how, without
losing all accuracy, I afraid is beyond my understanding. And down to
the genius of the inventor.



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?

The multiple spurious products around are many more than simply
harmonics. But in some way the phase of the desired product is
tracked(?).




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?

That is simply not true. In small quantities they would cost a great
deal more than 450GBP, and the VNWA is not produced in very large
quantities. Consider the costs of quite a complicated PCB in relatively
small quantities, the USB chip is quite expensive to pay for so-called
intellectual property, and requires individual licensing. The DDSs are
not cheap, neither is the hardware.


--

Roger Hayter

"gareth" wrote in message
...
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.

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?

MEA CULPA !!!!!

Sorry, but I was thinking out loud about the si570 series of chips and not
about DDS chips.


Ooops!

"gareth" wrote in message
...
"gareth" wrote in message
...
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.
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?
MEA CULPA !!!!!
Sorry, but I was thinking out loud about the si570 series of chips and not
about DDS chips.
Ooops!

The other area of befuddled thinking on my part (so many dimensions to such
a project!)
is that when working with a tracking generator, the signal from that
generator is the only
one to have to consider because it will be working in a closed, and largely
shielded circuit,
and that therefore the selectivity of instrumentation will not be
particularly relevant.

Such is the case with the VNWA, where an IF of only 1kHz at frequencies off
several MHz
might at first sight to invite a problem with severe image responses, but,
of course,
with only the one signal in consideration, there cannot be any images!

Anybody out there have experience of input from the sound card via the .NET
framework?

Also, apart from setting up ladder crystal filters, is there any real need
for us to have very
narrow IF filters in our speccy-plus-trackie set-ups?

On 12/30/2015 6:48 AM, Roger Hayter wrote:
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.

Up to about 600MHz I think the DDSen are pure enough, but above that the
VNWA deliberately uses mixer products of some kind. As to how, without
losing all accuracy, I afraid is beyond my understanding. And down to
the genius of the inventor.



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?

The multiple spurious products around are many more than simply
harmonics. But in some way the phase of the desired product is
tracked(?).




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?

That is simply not true. In small quantities they would cost a great
deal more than 450GBP, and the VNWA is not produced in very large
quantities. Consider the costs of quite a complicated PCB in relatively
small quantities, the USB chip is quite expensive to pay for so-called
intellectual property, and requires individual licensing. The DDSs are
not cheap, neither is the hardware.

I think the price is a bit high for a PCB, but 450GBP is for a tested
unit in the "presention case". The driver is not just the parts cost at
low volume, as that only goes up some. I estimate total parts cost of ~
$100. The problem is just as much the labor involved in turning the
parts into a final product. This type of assembly is done on machines
and at low volume the set up costs become a significant portion of the
total costs. Then adding profit easily brings this up to 450GBP.

I don't know how many of these units have been sold, but the Yahoo group
has 2830 members. If only half the owners are members that is still
less than 6000 units. Spread over a few years that is only 150 a month
or so. Not really high volume, but a nice workable volume.

--

Rick