Simple trick to measure plane impedance with a VNA

-October 09, 2014

The question of time vs. frequency, as the most useful measurement domain, has long been a controversial topic.  In some cases, it leads to rather heated discussions.  The argument in favor of a vector network analyzer (VNA), a frequency domain instrument, is that the dynamic range and signal to noise ratio (SNR) of a VNA are much better than they are for a time domain reflectometer (TDR).  The argument in favor of TDR measurements is that they tend to be lower cost and are taken from a direct reading, so there is little to interpret.  Fortunately, most new TDRs can also transform measurements to S-parameters (much like a VNA) and most new VNAs can also transform to time (providing TDR equivalent data). 

Having said all of this, the measurement of a PCB plane using a VNA may not be as straightforward as you might expect.   One simple trick makes it easy. 

A 2-sided 63mil FR4 PCB is used as an example in this article.  The PCB is measured using both a VNA and a TDR.  The results are compared, the error source is identified, and the trick is employed to correct the error.

The PCB is shown in Figure 1 connected to a SD-24 20GHz/17.5ps sampling head installed in a Tektronix 11801B DSO mainframe.  The board is connected to the instrument via an SMA edge connector and a SMA male-SMA-male barrel adapter.   The instrument cursor table is enlarged and shows a characteristic impedance of approximately 3.36Ω or 10.4dBΩ.  I say approximately because the cursor resolution on this instrument is somewhat coarse, but it is pretty close to 3.36 Ω.

Figure 1 Tektronix 11801B DSO with a 20GHz SD-24 TDR head connected to PCB shows an approximate impedance of 3.36Ω.

The PCB is then connected to a Rohde & Schwarz ZNB 20GHz VNA using the same connectors as shown in Figure 2.  The general method of determining the characteristic impedance of the plane is to measure the open circuit capacitance and the short circuit inductance. The characteristic impedance is then computed as:

Rather than calculating the inductance and the capacitance, which takes time and effort, there is an easier way.  Since the capacitive impedance is falling at 6dB/octave and the short circuit inductance is increasing at 6dB/octave, we can average the short circuit and open circuit impedance at any frequency.  The result is also shown in Figure 2 as 12dBΩ or 3.98Ω.

Figure 2: The same PCB mounted to a Rohde & Schwarz ZNB 20 VNA shows a significantly different result.  Note the resonant peak of the shorted (orange) trace is at a lower frequency than the open (green) trace.

The TDR and VNA characteristic impedance measurements are almost 20% apart, which can result in degraded signal and/or power integrity.  Note that the peak resonance in the short circuit trace is at a lower frequency than the resonance of the open circuit trace.  The reason is simple.  There is no perfect short circuit.  The short at the far end of the PCB is inductive, increasing the impedance measurement and reducing the resonant frequency.  

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