[time-nuts] DIY VNA design, directional coupler

Alexander Pummer alexpcs at ieee.org
Sun Aug 21 17:57:40 EDT 2016

there is a current mode feedback device [which does not follows the gain 
bandwidth product role ] and has 1000V/usec rise time 92dB THD at 30MHz  
3nV/rtHz noise, see here http://www.ti.com/lit/ds/symlink/ths4271.pdf, I 
used it as a medical color Doppler application

Guanella's choke and Guanellas balun are two different animals, the 
balun has a cross DC path, the choke does not have, it's other name is 
1:1 transmission line transformer,  in conjunction with an A/D converter 
the choke has the function to prevent a current path to the ground via 
one of the the differential inputs, also used in high dynamic range 
medical ultrasound application



On 8/21/2016 1:29 PM, Attila Kinali wrote:
> 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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