HP8753A measurement help required

David

For a rudimentary measurement, first select S11 under measure.

Leave test port 1 open. Next select Cal and to perform a response calibration by selecting "open" Next under Format, select Smith Chart.

You can now place a DUT at the test port and see S11 directly or else, by selecting marker, read the R +- jX at the marker position.

G,

Thanks for that.

The Test port is an "N" connector. How can I attach a capacitor to it ?

I tried setting up as you suggest and used a length of coax from Test port to component.

When I select Smith chart I get what looks like child's scribble all over the chart.

Regards

David

g wrote:

David

I'd suggest using a solder pot type N female. Connect the solder-N to your test cable and calibrate with the DUT absent.

That probably indicates that it is measuring correctly. Your DUT is being rotated (transformed) by the length of cable. Calibrating with the cable and N fitting (sans DUT) first will eliminate that, leaving your measurement plane located *at* the DUT instead of some distance down your test cable from it.

G,

I connected a coaxial cable (about 300mm length) to the test port.

Sweep was set between 300kHz and 1.3Ghz

The measurement was set up for Smith Chart to measure S11

Auto Scale was applied

A 1-port calibration was performed. (Q: What do I use for a load at the end of the flying coaxial lead. The precision Load I have is "N" type) I used 2 x 100 Ohms resistors as "Load" for Cal.

I attached a 27pF leaded capacitor (with leads cut short as possible) to end of coaxial cable.

When I looked at the response I saw a Circle that indicated the component became inductive around 500MHz (I presume this is SRF).

The odd thing is that the capacitance reads only 3.1pF at 1Mhz and does not read 27pF until 369 MHz ?

g wrote:

David

Not much point in using a load for measuring a low loss/reactive component like a capacitor. Just use the Response:Open cal and nothing else. That should leave you seeing a high impedance after the cal and before the device is connected.

Perhaps. Or maybe you tried to do a full S11 cal rather than response only? You really don't need the load to get pretty close for measuring a reactance. Using a load will help your accuracy for well matched devices. Using open gives you a measurement for small capacitances and using a short for small inductance. You can do an open:short:load cal if you have all standards in the same connector type for which you have a defined cal kit in the instrument. Otherwise just use response cal.

What do you see before you attach the DUT? Should look pretty much like a dot on the right side of the Smith Chart and read out as a high Z with perhaps a small capacitance portion. This represents the modeled fringing capacitance of an "open".

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 :)

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.

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.

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 ?

F> David

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

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.

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.

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.

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,

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" c> David wrote:

Hi Dave,

Do you know this site?:

pf

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