Re: HP8753A measurement help required

G,

Thank you for all the helpful information you have given me.

I also looked at a document from Agilent that explains how to make Test Fixtures to use as standards using microstrip.

Another site shows actual "N" connectors and SMA that are shorted, open and a Load (2 x 100R) and then another where the DUT is soldered across the back of the connector.

g wrote:
David wrote:
G,

Thanks heaps, I am starting to actually measure something now. It was frustrating as I was looking forward to receiving the VNA but found once it arrived that I didn't actually know how to use it :)

And you're only using about 1% of it at this point, there's lots more, particularly if you have time domain! (:>)

When I do the Open only Response cal I get a dot on the Resistance line on right hand side. R is jumping around but reads several "K" Ohms.

That's right. If you look carefully at 3 GHz you may notice that it is starting to rotate clockwise around the edge. That's the modelled fringing capacitance of the "open" standard you have selected as part of the Cal kit you are using.

When I connect now a 22pF leaded cap it shows 22p at low freq and starts increasing above 30MHz. The response curve of the component crosses the Resistance line at around 350MHz and the cap looks inductive above this.

By 150MHz this leaded cap is 150pF. No wonder I had problems building prototypes using leaded components at VHF in the past. I did not realise the capacitance changes so much with frequency.

The capacitance almost certainly doesn't change this much. You're seeing the actual component characteristics which are closer to series connected RLC. The L is "tuning out" the C on the way to series resonance. When you cross the horizontal axis, you are at resonance and all that is left is the ESR of the DUT.


Do I use this same methodology if I want to measure SMD caps and inductors ? Just attach the cable to PCB without component and cal for open than solder in the cap or for inductor, form a short to gnd plane , cal and then replace short with inductor ?
Yes, basically. For a really refined measurment you would use cal standards designed for the environment you are measuring in (PCB). However for many purposes, a simple resopnse cal in situ but sans DUT is enough. You use the cal device(s) most appropriate for the DUT since the measurement effectively is comparing the known cal with the unknown DUT. If you have a situation such as you mention where the DUT looks both capacitive (low freq) and inductive (higher) you may want to use more than a single device for the response cal.


Finally, What would I do to measure an RF IC in place on a PCB ? I have a SA605 Receiver IC that should have input impedance that looks like around 1K Ohm in par with 2.5 pF. As this is capacitive, would I do something like solder coax to pcb pad and have coupling cap to IC removed for OPEN cal and then insert the coupling cap to device to measure impedance.
Same thing, but use a thru cal device. You'll have to figure out how to connect the test ports together in the measurement environment. Maybe just use a couple of semirigid cables soldered to the PCB and connected by a piece of microstrip.


The goal is to make the test fixture as much like the application/measurment environment as possible and find cal standards that give you as much information about the desired paramater as possible.

I think Agilent might have some VNA basics app notes on their web site. Have you looked?

g

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Relevant Pages

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    ... That's the modelled fringing capacitance of the "open" standard you have selected as part of the Cal kit you are using. ... The response curve of the component crosses the Resistance line at around 350MHz and the cap looks inductive above this. ... When you cross the horizontal axis, you are at resonance and all that is left is the ESR of the DUT. ... Just attach the cable to PCB without component and cal for open than solder in the cap or for inductor, form a short to gnd plane, cal and then replace short with inductor? ...
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