How to measure the VSWR of a video detector?

[Ohioan]

We received a video detector from someone, and she asked us if we knew how to measure the VSWR of it.

It's a little thing, with an SMA connector on each end.

I am not an RF engineer, but I do have a little bit of experience in the area. (I did microwave engineering as a co-op student many centuries ago.) So I asked a few RF engineers how to do it, and, well... they didn't seem to know, either.

Unfortunately I don't think it's as simple as connecting it to a VNA and measuring the VSWR. The spec says there must be a 300 Ω "video load" connected to the output, and the video detector must be powered using a constant current source. Which I think means I need to measure the VSWR using a modulated signal.

The best way to describe the problem is with some images. I put together some PP slides. Attached is a pdf of the slides. I have also linked to png images of the slides.

Any help/guidance/assistance you can provide would be much appreciated.

Thank you

 

[BigBoss]

The signal may be modulated or not, no matter. The input impedance is constant regardless of the signal modulation. The appropriate method is to measure the Input Impedance with CW. Being of output signal of the detector is DC or demodulated won't change anything.
Some expensive VNAs have Large Signal s-parameter measurement features but they are really expensive. If your intention is to measure Input Impedance only, a simple VNA method is sufficient.

 

[Ohioan]

The signal may be modulated or not, no matter. The input impedance is constant regardless of the signal modulation. The appropriate method is to measure the Input Impedance with CW. Being of output signal of the detector is DC or demodulated won't change anything.
Some expensive VNAs have Large Signal s-parameter measurement features but they are really expensive. If your intention is to measure Input Impedance only, a simple VNA method is sufficient.

Thanks! But as mentioned, I need to source 270 uA into the detector's output terminal. Would you agree that, in order to do this, I will need a blocking capacitor in the 300 Ohm load?

 

[G4BCH]

Set the VNA source power such that the output voltage across the 300ohm video load is 81mV, that will give you the 270uA current.

 

[BigBoss]

Thanks! But as mentioned, I need to source 270 uA into the detector's output terminal. Would you agree that, in order to do this, I will need a blocking capacitor in the 300 Ohm load?

Increase VNA Output level until you obtain desired level.
I don't know how this detector should be deployed but if you need 270uA to make it appropriate working, a DC blocking capacitor is used to prevent DC current over the load.

 

[vfone]

If you can read the name of the detector on the device, you may find its datasheet, where most probably you will find the return loss (VSWR) plot vs frequency, as is shown in the link attached at page 3.

https://www.pasternack.com/images/ProductPDF/PE8013.pdf