VNA help!

Updated October 2005

New for November 2005! This material used to have it's very own web site, vnahelp.com, maintained by Barry, an Agilent employee. His site is no more, but he has donated all of his helpful hints to Microwaves101, in exchange for a Laurel and Hardy Handshake, and perhaps a coffee mug. We have't done any editing to these notes.

TRL Calibration reference plane - thru vs reflect

According to HP Product Note 8510-5A:
"The phase reference can be specified by the Thru or Reflect during the TRL 2-PORT calibration. SET REF: THRU corresponds to a reference plane set by Thru standard (or the ratio of the physical lengths of the Thru and Line) and SET REF: REFLECT corresponds to the Reflect standard."

This option only matters when you are doing a TRL calibration, and it determines where the network analyzer's reference planes will be assigned; that is, the physical locations which are assigned to be the "test ports" of the network analyzer system. Where these are located will obviously effect the resulting measurements of your DUT (primarily it effects the phase angles of your measurements).

If your TRL calibration kit contains a zero-length THRU, then use this to set the reference planes. If the THRU has length, and this length is accurately known, then you can still use it. If your REFLECT standard is very well defined, like a precision short circuit, then you can use it to set the planes. The idea here is to choose whichever one will give you the more precise definition of the physical locations of your reference planes. For example, if your REFLECT standard were a microstrip open circuit, that would be a poor choice - probably better to use the THRU.

Use cal kit standard #5 for sliding load

If your calibration kit contains a sliding load, you should be aware that some models of the HP 8753 and HP 8720 network analyzer families will only calibrate properly if the sliding load is standard #5. If you assign the sliding load to any other standard number, the calibration will not work - the error corrected data will be grossly wrong in the sliding load's frequency range. Furthermore, on these VNAs standard #5 can only be used as a sliding load - when you attempt to measure #5 during a calibration, the VNA will ask you to "set the slide" no matter how you defined the standard. This problem is corrected in the latest models.

Copy complete standard or class in VNA Cal Kit Manager

You can use the Cut, Copy, and Paste commands in VNA Cal Kit Manager's Edit menu to copy and paste a complete standard or class definition. This is handy if you want to re-use the standard definition from an existing cal kit in a new one you are constructing.

In the Standard Definitions page, if you click the mouse on a standard number in the far left column of the table, the entire row is selected and highlighted. You can then cut, copy, or paste the entire row to or from the Windows Clipboard. You can do the same with class assignments in the Class Assignments page. You cannot paste a standard into a class, or vice versa.

Display negative impedance Smith chart with 1/S conversion

Most HP network analyzers can display reflection coefficient on an inverted Smith chart, where the negative resistance values are inside the unit circle. This is usually the preferred presentation when designing negative-resistance oscillators or amplifiers. Just select 1/S in the conversion menu from the network analyzer's front panel.

Effect of "mixer bounce" or "sampler bounce" on high dynamic range measurements

Nearly all VNAs suffer to some degree from a measurement problem known as "mixer bounce" or "sampler bounce". This manifests itself as a false response in the measurement of a very good filter, making the filter's stopband rejection appear worse than it really is. The problem is caused by leakage of the VNA's LO signal and its harmonics out of the test ports, and IF frequency modulation sidebands on that LO leakage.

If you set a VNA to a CW frequency and an S12 measurement, you can connect a spectrum analyzer to port 1 and find the LO leakage. If you connect a coupler between ports 1 and 2 so that there is a large signal going into port 1 and you can observe the port 1 LO leakage through the coupled arm, you may be able to see that the LO leakage now has sidebands on it at the VNA's IF frequency. Better VNAs have high LO to RF isolation in their receivers to minimize this leakage.

So why is this modulated LO leakage a problem? Suppose you are measuring a highpass or bandpass filter with excellent stopband rejection (there is seldom a problem with lowpass filters). While the VNA is measuring S21 at the stopband frequencies, there is a large RF signal going into port 1, assuming the filter is reflective. Harmonics of the port 1 LO leakage land in the filter's passband, and are passed through to port 2. The IF sidebands on these LO harmonics are down converted in the port 2 receiver channel, and cause an IF response, which appears on the screen.

What can be done about this?
First, look for an operating mode which "turns off" the unused receiver channels. If the port 1 receiver is not working, it will not leak the modulated LO signal out of the test port. In some HP network analyzers, this mode is available and is called "alternate A and B" (A and B refer to the receiver channels for ports 1 and 2 respectively).
Second, if your VNA has direct receiver access jumpers, you can remove these and place pads in front of the R channel and A channel receivers, probably 12 to 20 dB is a safe bet. These will both reduce the RF signal going into these receiver channels, and reduce the LO leakage coming out of them. You will need to re-calibrate the VNA with the pads in place.
Finally, try reducing the VNA power level a little bit below maximum. Although you usually want to use maximum power for best dynamic range, you may find for some VNAs and some DUTs, that reducing the power a few dB actually gives you better dynamic range by decreasing problems like mixer bounce.

What if I need to purchase a GPIB card?

For use with VNA Cal Kit Manager, the GPIB-PCII/IIA card from National Instruments is a good deal at $395, provided your computer has an open ISA slot. It can be configured to work without IRQ or DMA if you use its "Basic Configuration 4 " in the Windows 95 Device Manager. Change the switches and jumpers as explained in Appendix A of the " Getting Started with Your GPIB-PCII/IIA and the NI-488.2 Software" manual. Be sure to purchase the model with the correct software driver for Windows 95/98.

Alternatively, purchase one of the PCI or USB GPIB products from NI or Hewlett-Packard. These are truly plug and play, and will not require any additional IRQ or DMA resources.

Why doesn't an open or short look like a dot on the Smith chart?

After performing a one-port or two-port calibration on your network analyzer, what do you expect to see when you measure the open standard from the calibration kit? Some people expect to see a perfect dot at the edge of the Smith chart. After all, a perfect open has a reflection coefficient of one at an angle of zero degrees at all frequencies. And since you are measuring the open that you calibrated with, it should look perfect, right? This sounds reasonable, but in fact the measurement of the open will look like an arc instead of a dot. This is a common source of confusion for many VNA users.

This measurement result is actually correct - there is nothing wrong with the network analyzer system or the calibration. You see, the cal kit's open standard is not an ideal open. It has nonlinear fringing capacitance, electrical length, and possibly some loss due to radiation. All of these imperfections are included in the model of the open as described in the calibration kit. Because the goal of the calibration is to make your measurement system give accurate results, and the open standard is not ideal, it would be inaccurate for the calibrated measurement system to report that it is an ideal open.

In most cal kits, this same argument applies to the short standard as well. For mechanical construction reasons, the actual short circuit is a small distance away from the calibration plane, so the short standard looks like a short circuit at the end of a very short transmission line. When you measure it, the result is also an arc on the Smith chart.