Re: [time-nuts] DIY VNA design, directional coupler
Alexander wrote: 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 Yeah, but look at its 1/f input voltage noise corner -- it's at 2 or 3 MHz!!! So the baseband input noise density is over 1000nV/sqrtHz at 1Hz, with potentially devastating effects on the phase noise performance in phase measurement applications. (See Figures 18 and 56 of the cited datasheet.) Even if the internal amplifier topology exhibits relatively low AM-PM conversion, starting that far behind is not a promising way to design a low-PN widget. NFL Best regards, Charles ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] DIY VNA design, directional coupler
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 73 Alex On 8/21/2016 1:29 PM, Attila Kinali wrote: On Sun, 21 Aug 2016 10:04:10 -0700 Alex Pummerwrote: 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/ ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] DIY VNA design, directional coupler
On Sun, 21 Aug 2016 10:04:10 -0700 Alex Pummerwrote: > 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/ -- It is upon moral qualities that a society is ultimately founded. All the prosperity and technological sophistication in the world is of no use without that foundation. -- Miss Matheson, The Diamond Age, Neil Stephenson ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] DIY VNA design, directional coupler
directional coupler/circulator could be made with high bandwidth [ up to 1GHz ] operational amplifiers, that circulator will work from DC.. driving A/D converter input asymmetrically; drive trough a Guanella-choke, but match the output of the choke 73 Alex On 8/21/2016 3:21 AM, Attila Kinali wrote: On Sun, 21 Aug 2016 04:46:10 + (UTC) Bob Albert via time-nutswrote: I was interested in this, but my needs are mostly below 100 MHz. I wonder what could be done similarly for this lower range... As Orin mentioned, there are some designs for that range out there, best known are probably the two Orin listed (N2PK and the VNAW by DG8SAQ). Although these are good designs, they are not as good as the one by Henrik Forstén. Henrik addresses many issues that the other leave out for simplicity. What I would do instead is use Henrik's design and do some adaptions. There are four parts that limit the frequency at the lower end: the signal sources, the filters for the sources, the mixer and the directional couplers. For the signal source there are two choices: DDS and down-mixing. The DDS is probably the obvious choice and delivers good results, but limits the maximum frequency if you have price limit. The down-mixing approach uses one of the PLL's with VCO as the original design uses, but only within a limited range, eg around 200MHz. This signal can then be down-mixed using a crystal oscillator (or another PLL+VCO) and a suitable mixer (eg LTC5512 or a DIY diode mixer). Advantage of this is, that the spurs of the PLL+VCO can be surpressed to a large extend, as the frequency range is quite narrow relative to the output frequency of the PLL+VCO. For the directional couplers, the approach used with Henriks design will not work for low frequencies, as this type of coupler needs a length of approximately lambda/4 to work optimally. I.e. they would become unweildingly large. The two choices I am aware of for the lower frequency ranges are transformer based directional couplers or resistive bridges. Transformer based couplers have the disadvantage of a non-flat frequency response and an upper and lower frequency limit, given by the characteristics of the transformer (number of windings/inductance and the used ferrite). Their advantage is that they have very little loss. Resistive bridges on the other hand have a loss of 3db (respectively a -6dB signal at each output), but are totally flat down to DC and up to several hundred MHz or even GHz if RF resistors are used. Most of the above mentioned methods have a lower frequency limit somewhere in the range of 20kHz and ~100kHz. If you want to go below that limit, you will need to adapt the circuit further: For the signal source the DDS approach is the only one that will result in a good SNR at a reasonable price. Easiest way to go is to use a 16bit DAC at >1MHz and an uC or FPGA to feed it (but use some low jitter oscillator as clock source for the DAC). The other components in the signal path that are limiting are the baluns and mixers. I would get rid of those two all-together and digitize the signal from the directional couplers directly using an ADC with >1Msps and 16-18bit. If you limit yourself to the range of 10Hz-20kHz, you can do all this using audio ADC/DACs and get a very high performing system. Attila Kinali ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.