VNA number of points

[Jeroen]

What could be reason that Agilent's network analyzers will
only accept such weird record sizes like 201, 401, 801, 1601,
etc?

Jeroen Belleman

 

[Gerhard Hoffmann]

Am 15.04.2012 17:37, schrieb Jeroen:

What could be reason that Agilent's network analyzers will
only accept such weird record sizes like 201, 401, 801, 1601,
etc?

When you sweep from 10 to 20 MHz, the center frequency
will be 15 MHz and not
"cannotmakeupmymindwhat2writeinthemiddleofthexaxis,somethingbetween14.996and15.004"

and when you switch to center/span you won't be off-grid.


Gerhard

 

[Jeroen]

Am 15.04.2012 17:37, schrieb Jeroen:

When you sweep from 10 to 20 MHz, the center frequency
will be 15 MHz and not
"cannotmakeupmymindwhat2writeinthemiddleofthexaxis,somethingbetween14.996and15.004"


and when you switch to center/span you won't be off-grid.


Gerhard

Mmmmh. Maybe, for a linear sweep. For a log sweep, only the first
and last point are exactly on gridlines, anyway. Yet the same
restriction on the number of points applies. Maybe it's just
what people have gotten used to.

Now that we're at it, why do so many VNAs have a lower frequency
limit of 9kHz? Another weird number that pops up in Agilent,
Rhode & Schwartz and several other VNAs.

Jeroen Belleman

 

[[email protected]]

Mmmmh. Maybe, for a linear sweep. For a log sweep, only the first
and last point are exactly on gridlines, anyway. Yet the same
restriction on the number of points applies.

Even with a log scale, you have the "fence post" problem.

Maybe it's just what people have gotten used to.

Now that we're at it, why do so many VNAs have a lower frequency
limit of 9kHz? Another weird number that pops up in Agilent,
Rhode & Schwartz and several other VNAs.

So that 10kHz is in the range of the instrument?

 

[Gerhard Hoffmann]

Am 15.04.2012 19:41, schrieb Jeroen:

Now that we're at it, why do so many VNAs have a lower frequency
limit of 9kHz? Another weird number that pops up in Agilent,
Rhode& Schwartz and several other VNAs.

Some important compliance measurements start at 9 KHz.
The 9KHz is more a spectrum analyzer spec.

regards, Gerhard

 

[Jeroen]

Even with a log scale, you have the "fence post" problem.

OK, so if I should choose an even number of points, there will
be no point spot-on in the centre of the plot. So what? Is that
a reason to restrict the possible number of points to a small set
of values of the form 100*2^N + 1? It still makes no sense to me.

Am 15.04.2012 19:41, schrieb Jeroen:


Some important compliance measurements start at 9 KHz.
The 9KHz is more a spectrum analyzer spec.

regards, Gerhard

Yes, that makes sense.

Thanks,
Jeroen Belleman

 

[[email protected]]

OK, so if I should choose an even number of points, there will
be no point spot-on in the centre of the plot. So what? Is that
a reason to restrict the possible number of points to a small set
of values of the form 100*2^N + 1? It still makes no sense to me.

It's not the center fence post that's ever the issue, rather the two ends. You
must be a programmer. ;-)

 

[Jeroen Belleman]

It's not the center fence post that's ever the issue, rather the two ends. You
must be a programmer. ;-)

That's still no reason to restrict the possible choices of the
number of points, nor does it explain why it's just *that* weird
set of values!

Jeroen Belleman

 

[[email protected]]

That's still no reason to restrict the possible choices of the
number of points, nor does it explain why it's just *that* weird
set of values!

You want real hardware with an infinite number of choices of measurement
points? I don't see that those values are in any way "weird". You worry to
much.

 

[Jeroen]

You want real hardware with an infinite number of choices of measurement
points? I don't see that those values are in any way "weird". You worry to
much.

What, me? Worry? I'm just curious. That, and annoyed when it
coerces the number of points I ask for to one of these weird
values.

Jeroen Belleman

 

[Jeroen]

What could be reason that Agilent's network analyzers will
only accept such weird record sizes like 201, 401, 801, 1601,
etc?

Jeroen Belleman

The "weird" number of samples is chosen to greatly simplify the
instrument's FFT calculations.
[...]

I believe that FFTs work best with record lengths some power of
two. The limitation I'm asking about predates the availability
of FFTs on VNAs by quite a few years.

Jeroen Belleman

 

[[email protected]]

What could be reason that Agilent's network analyzers will
only accept such weird record sizes like 201, 401, 801, 1601,
etc?

Jeroen Belleman

The "weird" number of samples is chosen to greatly simplify the
instrument's FFT calculations.
[...]

I believe that FFTs work best with record lengths some power of
two. The limitation I'm asking about predates the availability
of FFTs on VNAs by quite a few years.

Power of two, plus one (DC), no?