Hello,
All this talk of termination resistors.
Is this with a resistor intentionally placed across the secondary winding plus the resistance to ground internal to the VNA?
That means that we have the external resistor across the secondary coil in parallel with the instrument internal resistor to ground.
Different related topic.
It is not so much about adding additional resistance to extend or reduce bandwidth it is about resistance being used to tune or critically dampen the native inductance of the coil. Another way to think of it goes like this. Inductance is often added to RF circuit to increase Q and extend bandwidth with inductive peaking.
Thanks DT
All this talk of termination resistors.
Is this with a resistor intentionally placed across the secondary winding plus the resistance to ground internal to the VNA?
That means that we have the external resistor across the secondary coil in parallel with the instrument internal resistor to ground.
It is not so much about adding additional resistance to extend or reduce bandwidth it is about resistance being used to tune or critically dampen the native inductance of the coil.
What about also *driving* the transformer from a very low impedance !
I did it the primitive way and I added a voltage divider with 50 Ohms impedance for the generator and only a few Ohms to drive the transformer.
Loss is welcome because levels should be very low and it also dampens out resonances.
See here:
https://electronicprojectsforfun.wordpress.com/injection-transformers/
What about also *driving* the transformer from a very low impedance !
Yes, it extends the bandwidth some, but my VNAs all have 50 Ohm outputs, so an external amplifier with direct-coupled output is required to do it.
my measurement go up to a few 100kHz only. They are mostly for linear power supply for measurement circuits where low noise is an issue (like preamps for noise measurements, ...).
For the "braid error" issue I am working on another approach injecting current and measuring voltage response of a PSU. In these cases I do not need a transformer,
but an active injector, the same ideas like the PicoTest ones. Stability can then be inferred by extracting data from the Nyquist plot of the output impedance.
This is work in progress, but in September I can get my hands on a Keysight E5061B-3L5 (VNA from 5Hz to 3GHz), and then I can properly measure all my homebrew stuff without improvisation.
There are nice appnotes from Keysight how a VNA can be used to measure milliohm impedances in PDN networks. I learned a lot from those.
Now that the conversation regarding the termination resistor is near complete take a look at page 8 figure 4.1 of the Omicron Lab injection transformer manual. The recommended injection resistor value is between 1R and 10R.
If you look at the Signature Bode 100 gain vs Phase Margin chart, the point where gain is equal to 0dB the PM gives a very good indication of power supply stability. Doesn’t the injection transformer phase effect the PM of the power supply? The short answer is no.
Now looking at injection transformer phase measurements:
The transformer phase plot shows the phase difference between the transformer primary and secondary. Some folks here say that the injection transformer is no longer useful when phase exceeds some value plucked from the chart. This needs a closer look.
If you look at the Signature Bode 100 gain vs Phase Margin chart, the point where gain is equal to 0dB the PM gives a very good indication of power supply stability. Doesn’t the injection transformer phase effect the PM of the power supply? The short answer is no.
Thanks DT
The frequency response (-3dB) for the video channels is 50Hz ... 78MHz, but the 45° phase shift response is from 50Hz to 14MHz - some tricks happening here - Note the bumps in the frequency response above 1MHz. This response looks the same for all three video channels. Termination was 50 Ohm for all measurements.
Pretty good stuff this is!
I'm guessing you're using the 50 Ohms term in the 3577A. If you terminate the xfmr right at its output and set the analyzer to 1MegOhms, the mag/phase correlation at the high end should make more sense.
Now looking at injection transformer phase measurements:
The transformer phase plot shows the phase difference between the transformer primary and secondary. Some folks here say that the injection transformer is no longer useful when phase exceeds some value plucked from the chart. This needs a closer look.
If you look at the Signature Bode 100 gain vs Phase Margin chart, the point where gain is equal to 0dB the PM gives a very good indication of power supply stability. Doesn’t the injection transformer phase effect the PM of the power supply? The short answer is no.
Thanks DT
What you write reminds me of a hefty discussion following a Texas Instrument Application note where they used a garden variety line transformer as an injection transformer and claimed its characteristics are irrelevant because it was "outside the loop".
It is OK to claim that (provided the secondary resistance is small enough) it does not alter the characteristics of the loop, but what it does alter is the amplitude and phase you measure at the loop output. You could calibrate this out using a VNA, but it is definitely a precision issue. When you look at the characteristics of the magnetic materials used for such transformers, the range of mu for a given frequency is 2 to 1. You can expect an coresponding low precision range for band corners. If your measurements are to be trusted, the phase shift induced by your transformer must be a) small and b) well known.
In general, I'd prefer to see the transformers response when fed and terminated by its nominal impedance (75 Ohm for video stuff). I don't have the minimum loss pads to match to 3577A's 50 Ohm ports, so for now I'll stay with the 50 Ohm measurements.
And for the records, I'm making the measurement from Output to R input, in contrary to using a power divider and A/R as I prefer to see the system response over the transformers response fed from a virtual zero ohm source.
Using the power divider and A/R measurement gives different results with notably extended low frequency response.
To me, characterizing a xfmr is set up like an FRA measurement; A/R, source Z same as it will be used, no power divider, high-Z inputs, and xfmr terminated in whatever the desired load is.
I'm not using a power divider, either. What is your "virtual zero ohm" source ?