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a couple of puzzlements re calibration and connectors


barnc4br 2021/01/16 09:19

1. I have a terminating plug for a TV splitter. It is 75Ω, obviously. When I put it on my Nano (50Ω calibrated) it shows 73.2Ω 50pF. Close enough for gvmt work. But when I try to measure it on my DVM, I get nothing. The DVM reads 49.3Ω for the 50Ω calibrator load but nothing for the terminator.

2. When I put the SMA 2 F adapter on the Nano it bounces around between 1.6Ω and 0.5Ω and 2pF. Without anything or with the open cal it bounces all over the place. With a 75Ω terminator attached it show as above, 73.2Ω 50pF.

3. When I place the short cal on tight the phase oscillates between ±180°. If it is slightly loose it is stable at +180° and about 70mΩ 50pH.

Can anyone explain these?

Jim Lux 2021/01/16 09:37

On 1/16/21 9:19 AM, barnc4br@gmail.com wrote:
> 1. I have a terminating plug for a TV splitter. It is 75Ω, obviously.
> When I put it on my Nano (50Ω calibrated) it shows 73.2Ω 50pF. Close
> enough for gvmt work. But when I try to measure it on my DVM, I get
> nothing. The DVM reads 49.3Ω for the 50Ω calibrator load but nothing
> for the terminator.

You didn't say what frequency you were measuring this at.

Your TV termination probably has a DC block (50pF). This is pretty
common in systems where they supply power through the coax, since you
don't want to supply DC power to heat the termination up.



>
> 2. When I put the SMA 2 F adapter on the Nano it bounces around
> between 1.6Ω and 0.5Ω and 2pF. Without anything or with the open cal
> it bounces all over the place. With a 75Ω terminator attached it show
> as above, 73.2Ω 50pF.

This is with the connector open? At what frequency?

barnc4br 2021/01/16 09:52

1. That make sense.
2. Yes, just the adapter. #1 is with that same adapter and the terminator. It was at the default 100M to 900M

David Eckhardt 2021/01/16 21:52

A DC measurement (your DVM) of impedance does not represent RF measurement
of the same impedance. Likely there is a 50 pF blocking capacitor in
series with you 75-ohm splitter.

Example: Take a 1/4-wavelength line at a specific frequency which is open
terminated at the far end.
DVM Measurement: Open, infinite resistance (after the
line charges)
VNA Measurement at the frequency of the 1/4-wavelength:
Short, zero ohms

Or: A parallel resonant circuit:
DVM Measurement: Short Circuit
VNA Measurement at the resonant frequency of the circuit:
Close to an open circuit, nearly infinite ohms

Dave - WØLEV

On Sat, Jan 16, 2021 at 5:37 PM Jim Lux <jim@luxfamily.com> wrote:

> On 1/16/21 9:19 AM, barnc4br@gmail.com wrote:
> > 1. I have a terminating plug for a TV splitter. It is 75Ω, obviously.
> > When I put it on my Nano (50Ω calibrated) it shows 73.2Ω 50pF. Close
> > enough for gvmt work. But when I try to measure it on my DVM, I get
> > nothing. The DVM reads 49.3Ω for the 50Ω calibrator load but nothing
> > for the terminator.
>
> You didn't say what frequency you were measuring this at.
>
> Your TV termination probably has a DC block (50pF). This is pretty
> common in systems where they supply power through the coax, since you
> don't want to supply DC power to heat the termination up.
>
>
>
> >
> > 2. When I put the SMA 2 F adapter on the Nano it bounces around
> > between 1.6Ω and 0.5Ω and 2pF. Without anything or with the open cal
> > it bounces all over the place. With a 75Ω terminator attached it show
> > as above, 73.2Ω 50pF.
>
> This is with the connector open? At what frequency?
>
>
> >
> > 3. When I place the short cal on tight the phase oscillates between
> > ±180°. If it is slightly loose it is stable at +180° and about 70mΩ 50pH.
> >
> > Can anyone explain these?
> >
>
>
>
>
>
>
>
>

--
*Dave - WØLEV*
*Just Let Darwin Work*

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