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How does VNA get the phase angle of S11

 

I understand how a (Wheatstone) bridge gets a voltage or magnitude relative to reactance difference. But how does it get the phase of the that vector.
I think there are some hard ways of getting that phase but surely there are some practical ways of doing that.
 

I haven't looked in detail at the nanoVNA method, but normally you would mix down to a low frequency and do IQ sampling. This is a complex number that can easily be converted into a magnitude and phase.
 

Thanks for asking the question. I am familiar with them methods of old. Like sala nimi outlined. The R ch and the A and B ch's are down converted to a low IF and these are applied to the inputs of phase detectors. With the new synthesizer IC's available today the phase of signals can be shifted and the phase can be indirectly determined. I am hoping someone will give us the details!
 
Edited

On Wed, Oct 2, 2019 at 11:32 PM, sala nimi wrote:


how does it get the phase of the that vector
magnitude = sqrt(re*re + im*im)

phase = atan2(im, re)

where:
re - real part of complex S11
im - imag part of complex S11
 

You have the test signal from the SI5351 send to the bridge but also to the "reference" mixer, in the reference mixer it is mixed with the output of the Local Oscillator (LO) which also comes from the SI5351.
The unbalance of the bridge goes to the "reflection" mixer where it is mixed with the same LO.
The LO is at 5kHz offset from the test signal so the output of both mixers is a IF signal at 5kHz.
Both IF's are then read into the controller using the ADC and in a DSP algorithm in the controller the IF's are going to DSP mixers where they are mixed with two 5kHz signals 90 degrees apart (I/Q mixer) to create a DC signal.
The DC signals (I and Q) are the real and imaginary part of the complex downconverted reference and reflection input to the HW mixers. As you do not know the actual angle of amplitude of the test signal the complex test signal is divided by the complex reference signal to eliminate the phase and amplitude uncertainty.
The outcome of this division is the unbalance of the bridge scaled relative to the reference signal, both in magnitude and angle.
Attached a small pdf that contains the block diagram of a VNA using the same mechanism

SimpleVNA.pdf
SimpleVNA.pdf

Bob Albert
 

All this technology for fifty bucks?

On Wednesday, October 2, 2019, 11:36:26 PM PDT, <erik@...> wrote:

You have the test signal from the SI5351 send to the bridge but also to the "reference" mixer, in the reference mixer it is mixed with the output of the Local Oscillator (LO) which also comes from the SI5351.
The unbalance of the bridge goes to the "reflection" mixer where it is mixed with the same LO.
The LO is at 5kHz offset from the test signal so the output of both mixers is a IF signal at 5kHz.
Both IF's are then read into the controller using the ADC and in a DSP algorithm in the controller the IF's? are going to DSP mixers where they are mixed with two 5kHz signals 90 degrees apart (I/Q mixer) to create a DC signal.
The DC signals (I and Q) are the real and imaginary part of the complex downconverted reference and reflection input to the HW mixers. As you do not know the actual angle of amplitude of the test signal the complex test signal is divided by the complex reference signal to eliminate the phase and amplitude uncertainty.
The outcome of this division is the unbalance of the bridge scaled relative to the reference signal, both in magnitude and angle.
Attached a small pdf that contains the block diagram of a VNA using the same mechanism
 

To check if I understand this correctly I ask, There is also a phase difference between reference and difference voltage from a bridge. I should have remembered that. And now they are mixed to 5kHz to make it easier for DSP and so on.
 

Indeed. Downmixing retains phase difference
 

they are mixed to 5kHz to make it easier for DSP and so on
They are mixed down for the analog-to-digital converter (ADC),
which arguably makes DSP harder, since it has to sort opposite side band suppression.

Here is a collection of articles that I find helpful:


.. specifically:

* Kurt Poulsen's 30 minute video of Michael Knitter's presentation for FA-VA 5,
which has architecture similar to nanoVNA


* DF8OE's opposite side band suppression discussion
 

Thank you all for helpfull posts and links.

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