Werbel Microwave LLC: May 2015

Our article as published "In my Opinion" in the March 2015 issue of Microwave Product Digest.

Attenuators on VNA Cables when testing Passive Devices

Some difficulty in acquiring reliable measurements triggered a point of interest from previous experiences. Why did some technicians place precision attenuators on one or both VNA cables as part of a setup for measuring passive devices? “It’s always been there.” But like all things, there must be a reason.

Protection of equipment from power overload sensibly applies to active devices. Since I had been employed at companies that manufactured both active and passive devices, I wondered if it was merely convenient to leave it on all the time. But what effect would this have on production testing of passive devices? And what advantages or problems, if any, come from this?

A question on Linked In quickly erupted into an elaborate, technical discussion about the benefits and drawbacks of calibrating through precision attenuators on VNA cables. I thank all those who contributed their insight. Following this, I experimented on the subject, to arrive at some conclusion on the effect of calibration and production testing. More importantly, how it might help improve reliability of measurements with our VNA.

The characteristic return loss of any cable contains ripple; and this becomes more pronounced with length. Of course, the lower to the noise floor this ripple is, the better. But even with lower grade cables, the VNA calibration sets the reference planes beyond the cable, thereby eliminating this from the measurement. Or does it?

The importance of phase and amplitude stability is emphasized in the quality of the test cables. As vectors are comprised of magnitude and phase, an unexpected variation would invalidate measurements. Bending and flexing a cable, even slightly will introduce a variation against the calibrated values. Not to mention temperature and humidity drifting. And when an imperfect DUT is introduced, the problem compounds.

An indoor office environment can be made relatively stable, at least for a timeframe long enough to acquire measurements. So I am not too worried about temperature and humidity fluctuations immediately following calibration. However, slight bends in cables are unavoidable, particularly when DUT ports are located on several different mounting faces. Fixtures can aid. We found clamping the cable gently in a vise during and after calibration to be particularly helpful in securing cables while minimizing bends.

That being established, we conducted some experiment to observe the ripple effect, and answer the question as to the benefit, if any, provided by calibrating through attenuators on VNA cables.

In the experiment, we considered both 2-way and 4-way Wilkinson power dividers and a 10-dB directional coupler of our own design covering 2-18GHz, a pair of 3-foot test cables, and a pair of 10-dB attenuators specified to 18GHz.

In the first pass, an SOLT calibration was performed using both cables only. With the 2-way splitter connected directly, VSWR measured 1.4:1 or better at all ports, which is reasonable and expected for the DUT. However, there was noticeable ripple of many peaks in the through measurement, superimposed upon the expected characteristic ripple caused by multiple Wilkinson sections.

We then measured our 4-way model. There was still some cable-induced ripple present, but less pronounced, down at -6.5dB. When we measured a precision -10dB attenuator, or our 10-dB directional coupler model for that matter, the ripple was barely noticeable. So for us, this was a problem when measuring relatively low-value attenuation.

The VNA was then recalibrated SOLT with 10-dB precision attenuators on each cable end that would interface to the DUT. It is important to note the attenuators remained as part of the test set for the entire calibration. The 2-way DUT was reconnected with the padded cables. VSWR was identical to the non-cable measurements, thereby verifying the calibration. The cable-induced ripple was almost non-existent, except at the highest frequencies (above 13-GHz, probably beginning to cross over the attenuator element despite being rated to 18-GHz).

We performed one more experiment using a connector saver adapter so the DUT could be connected directly onto the VNA port 1, using only one cable without attenuator on port 2. As with the first experiment, there was ripple at -3.5 and -6.5 dB, but not as pronounced as with two, unpadded cables.

It would appear to me that even though the SOLT calibration cancels out the effect of cable ripple initially, when reflections exist on the line caused by an imperfect DUT or cable flexure, these reflections get summed with the ripple of the cable, and are reintroduced into the measurement. The attenuators proved to be a useful ripple buffer in taking S21 measurements at the expense of a reduction in dynamic range. In fact, there was noticeable noise appearing in S11 and S22 windows as high as -40dB, making this approach impractical for high-attenuation DUT’s such as our upcoming 50-dB directional coupler.

Of course, using a large smoothing aperture when recording passband ripple is cheating, and nobody ever does that. An improvement to the setup perhaps, will be to use lower value attenuators, such as 6-dB or even 3-dB, to assist in clean, passive measurements without the use of smoothing. Please write to me with your comments and suggestions.

Ernest Werbel

Werbel Microwave LLC

Werbel Microwave LLC

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Attenuators on VNA Cables when testing Passive Devices