[time-nuts] DIY VNA design, directional coupler

Attila Kinali attila at kinali.ch
Sun Aug 21 16:29:33 EDT 2016

On Sun, 21 Aug 2016 10:04:10 -0700
Alex Pummer <alex at pcscons.com> wrote:

> directional coupler/circulator could be made with high bandwidth [ up to 
> 1GHz ] operational amplifiers, that circulator will work from DC..

While this is a valid option, it would then become the element in the
system that limits dynamic range. It's better to use a "noiseless"
passive circuit that has very little distortion.

Also keep in mind that even if the opamp has an GBW of 1GHz or more,
the slewrate kicks in quite early and in this case would limit the
maximum signal strength severely. There is a reason why GHz amplifiers
use so much power.

> driving A/D converter input asymmetrically; drive trough a 
> Guanella-choke, but match the output of the choke

The Guanella balun, like all other transformer based baluns,
has the same upper and lower frequency limits: The inductance
sets the lower limit (more inductance -> lower frequency) and
the loss in the ferrite sets the upper limit (non-linear and thus
can be quite abrupt). Another issue here is symmetry of output
over frequency (c.f. [1]). I don't know how good the Guanella
baluns are in reality, but this is definitly something that should
be looked at.

			Attila Kinali

[1] http://www.markimicrowave.com/blog/2013/07/why-buy-a-high-quality-baluntransformer-for-an-analog-to-digital-converter-adc/
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use without that foundation.
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