Measure PDN on a budget

-March 18, 2014

As the use of microcontrollers, CPUs and FPGAs continues to grow, more engineers are facing challenges with their power distribution network (PDN) design. Many engineers are just starting to perform PDN measurement.  Typically, these measurements are performed using a vector network analyzer (VNA).  A 1-port measurement can be used for impedance levels above a few hundred mΩ.  A 2-port shunt-thru measurement is used for applications as low as 1mΩ and in some cases even lower.  Both of these measurements are based on S-parameters, allowing the cable to match the instrument impedance all the way to the point of measurement. (For background on s-parameters, head to the S-parameter collection on EDN).

There are many good analyzers available for measuring PDN
High performance VNAs are available from all of the major test equipment manufacturers, including Agilent, Tektronix, Teledyne Lecroy, and Rohde & Schwarz.  Most product lines include offerings of S-parameter measurements up to 40GHz or higher.

Many of these are above our price range
While these are all excellent instruments, and I use them myself, we don’t all have a budget that can support such an expense.  There are other options available. If you only need an instrument for a short time, you might consider renting one from one of the many test equipment rental companies.  Depending on your measurement requirements, you might be able to make use of some lower cost instruments, two of which we will discuss in this article.

What frequency range do you need to measure?
A recent poll on our Power Integrity for Distributed Systems LinkedIn group  indicated that 22% of the voters only measure PDN to 40MHz, while 33% measure to 200MHz and 44% measure to 4GHz.

Figure 1: 22% of respondents only need to measure PDN impedance up to 40MHz or less while all respondents measurements can be fulfilled with a 4GHz capability

None of the respondents currently measure PDN above 4GHz.  As this unscientific poll shows, we aren’t all designing the highest speed circuits.  That does not mean that we won’t have PDN problems, but it could limit the maximum measurement frequency, and measurement bandwidth is expensive.  A general rule of thumb for the maximum measurement frequency is based on the highest edge speed of the loading circuits.  The 3dB corner frequency is approximately

Where T is the rise or fall time of the load current.   It is good practice to measure up to at least twice this frequency.  At the lower limit, we have to be sure to include the lowest possible regulator bandwidth and measure at least an octave below that.  Many switching regulators and LDOs have measured bandwidths of a few kHz minimum.  It’s safe to say that measurement should start from about 1kHz. 

What instrument characteristics are important?
Assuming the VNA has typical SOLT (short, open, load, and thru) calibration capability, measuring PDN requires sufficient dynamic range for the measurement.  The 2-port impedance measurement has a resulting S21 magnitude of

Measuring a 1 mΩ PDN impedance results in a -88dB S21 measurement, while measuring a 10 mΩ PDN results in an S21 measurement of -68dB.  Allowing a 10dB margin for a reasonable measurement, the instrument should have a roughly 100dB dynamic range to measure 1 mΩ and roughly an 80dB dynamic range for 10 mΩ. The 2-port PDN measurement also requires a 50Ω common-mode coaxial transformer for low frequency measurements in order to break a DC ground loop that is common to all fixed-ground VNAs.

Low Frequency 1Hz-40MHz
If you are in the 22% that only need to measure PDN to 40MHz, the OMICRON Lab Bode 100 can fulfill all of you needs.  The Bode 100 offers exceptional performance in a very low cost instrument.  It includes the functions of both a frequency response analyzer (FRA) for measuring Bode plots and a VNA for measuring 1-port and 2-port impedance, including PDNs.  

Figure 2 OMICRON Lab Bode 100 has a frequency range of 1Hz to 40MHz and offers all of the features of both a VNA and an FRA. In this image, it is combined with a Picotest J2101A common mode transformer to measure impedance as low as 1 milliOhm.

The Bode 100 has better than 100dB dynamic range, allowing the measurement of a 1 mΩ PDN as demonstrated in Figure 3.  Using an external preamplifier, such as the J2180A, 250uOhm has been demonstrated using the Bode 100.

Figure 3 The Bode 100 can measure 1mΩ as shown here with when used in conjunction with the common mode coaxial transformer. The measurement is shown both with (RED) and without the coaxial transformer (BLUE).  The measured impedance is approximately 1.3mΩ and 400pH.

Mid-Frequency 20kHz-4GHz
If you need to measure PDN at frequencies up to 4.8GHz, the Copper Mountain Technologies S5048 also has a dynamic range of better than 100dB allowing accurate measurement of 1 mΩ as demonstrated in Figure 5.

Figure 4  The Copper Mountain Technologies S5048 picks up where the Bode 100 leaves off.  The S5048 is an S-parameter analyzer, with a frequency range of 20kHz-4.8GHz.

Figure 5 The S5048 can also measure 1mΩ as shown here with the common mode coaxial transformer (BLUE) and without the coaxial transformer (RED).  The measured resistance and inductance are approximately 1.3mΩ and 400pH

The S5048 includes some nice features for a very low cost instrument including log frequency sweep and 1-port and 2-port time domain functions (TDR/TDT).  The S5048 does not offer FRA capabilities and cannot measure below 20kHz.  The combination of the OMICRON Lab Bode 100 and the Copper Mountain Technologies S5048 provide a cost-effective solution for measurements of PDN up to 4.8GHz.  For lower speed devices, the 40MHz capability of the Bode 100 is sufficient, while its FRA capabilities support bode plots, power supply rejection ratio (PSRR) and other common power distribution measurements.

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