This article is written by Kenneth Wyatt for EDN.
Picotest has a very unique harmonic comb generator (called the J2150A harmonic comb injector) that’s not only designed to reveal resonances in power supply and voltage bus designs (power integrity), but also has interesting uses in EMC measurement and troubleshooting. It also has several unique features that set it apart from all other comb generators:
- It is sized about equal to a USB thumb drive and derives 5V power from any USB port.
- The generator has five different frequency modes, from 1 kHz to 8 MHz.
- Several of the modes are “dithered”, so as to help fill in the gaps between harmonic combs.
- Mode 1 steps through three different frequency steps (1 kHz, 100 kHz and 8 MHz) in order to better reveal circuit resonances.
Figure 1 – The Picotest J2150A harmonic comb injector is a little larger than a USB thumb drive
Introduction
Figure 1 shows the size of the device. By pressing the large top button, you’ll cycle through the five modes (indicated by three LEDs). The generator output terminates in an SMA connector. Because the device is completely self-contained, it only requires a 5V supply from any USB port, including USB battery packs. This would allow portable operation, as we’ll see later. Useful harmonic content can easily cover up to 1.5 GHz, and higher. No external software is required for operation.
The device was originally designed to help reveal circuit resonances in switching and linear power supply designs, as well as power bus resonances. These resonances are normally below a few MHz, due to the large capacitances normally used in these circuits. However, there may be much higher resonances in the hundreds of MHz due to circuit board, cable, and other structural resonances. The various modes are described in Figure 2. Mode 1 is the default mode at power-up, and steps through modes 2 through 4 in order to excite all harmonic frequencies – thus quickly revealing problem resonances. Once resonant frequencies are identified in Mode 1, Modes 2 through 4 impulse functions with frequencies of 1 kHz, 100 kHz, and 8 MHz may be used to zero in on specific resonances. All modes 1 through 4 include pulse width dithering to help fill in gaps between combs. Mode 5 is the exception and is a simple 50% duty cycle square wave running at 10 kHz. Modes 2 and 3 are high enough resolution to accurately identify circuit resonances. However, the 8 MHz mode would be useful in determining higher-frequency resonances for EMI troubleshooting, as I’ll describe shortly.
Figure 2 – There are five modes, settable by pressing the top button on the device (image courtesy, Picotest.com).
Figure 3 shows a simplified block diagram of the system. A RISC processor controls the clock frequencies and dithering algorithm. Pulses from the processor are shaped and run into a driver circuit that creates a 470 ps rise and 270 ps fall time (typical). The output is DC-coupled, so the signal may be used to modulate other Picotest voltage and current injectors. In order to AC-couple the output, a Picotest P2130A DC Blocker (500 Hz to 8 GHz) may be used.
Figure 3 – A simplified block diagram of the harmonic injector (image courtesy, Picotest.com).
Figure 4 shows a typical harmonic output up to 1 GHz when using Mode 4 (8 MHz dithered fundamental). A Rohde & Schwarz RTE 1104 oscilloscope was used for this capture. This mode would be useful for verification of EMI test chambers, as I’ve written in the past (see References). Figures 5 and 6 show the spectrum when using Mode 2 (1 kHz dithered fundamental).
Figure 4- The harmonic content out to 1 GHz, using Mode 4 (8 MHz dithered fundamental).
Figure 5 – Here, we’re measuring the harmonics out to 10 MHz, using Mode 2 (1 kHz dithered fundamental).
Figure 6 – Screen capture from Figure 5.
EMC Applications
There are several EMC-related applications of harmonic comb generators. One useful application is measurement of cable, or other structural, resonances. By injecting the harmonics into the cable with a current probe, you can measure the harmonic content using a second current probe. The cable resonance is indicated by the peak in harmonics. Figure 7 shows the overall setup. In this case, I’m using a Tektronix RSA306 spectrum analyzer and a pair of Fischer Custom Communications F-33-1 current probes. The position of the probes is not critical. As you can see (Figures 8 and 9), there is a peak resonance of this 1.2m long BNC cable at 96 MHz. Please refer to my previous article on measuring cable resonances for more details (see References).
Figure 7 – The setup for measuring cable resonances. The harmonics are injected into the right-hand current probe and the cable response is detected by the left-hand probe.
Figure 8 – The advantage of the Picotest harmonic injector is that it may be adjusted between coarse and fine frequency combs. Here, we’re using the 8 MHz combs and are probably lucky to catch the peak at 96 MHz. Normally, this coarse setting may miss the exact peak resonance.
Figure 9 – Here, we’re measuring the same cable resonance, but with 100 kHz combs. As you can see, the resolution in the resonant peak is much better.
One really neat feature of the harmonic injector is that it may be powered from USB battery packs (Figure 10). This allows completely portable operation. I’ve commonly used harmonic comb generators to characterize EMI chambers. For example, it’s wise to place these on your turntable and spot-check the measurement accuracy of a chamber prior to taking compliance measurements. By performing daily measurements, loose connectors or broken cables may be identified. They may also be used to compare the measurement performance of two, or more, chambers. I’d suggest using several ferrite chokes clamped around the coax cable in order to help decouple the generator from the antenna.
Figure 10 – Because the harmonic injector may be run directly from a USB power source, it can serve as a self-contained emission source. By connecting a small antenna, it can verify proper operation of semi-anechoic chambers, for example.
Power Integrity Applications
Power integrity testing and troubleshooting is really where the harmonic injector shines. By injecting closely-spaced harmonic combs into the power bus or control loop of a linear or switching power supply, it’s possible to reveal troublesome resonances that could potentially cause instability or EMI due to ringing on switched waveforms. Picotest has one- and two-port probes that may be used, along with the harmonic injector, to hunt for problem resonances within your power bus circuitry.
In Figure 11, I’m injecting 100 kHz square wave (Mode 5) into the 3.3V power bus from a linear regulator supplying a 125 MHz crystal oscillator. With the spectrum analyzer centered on 125 MHz, you can readily observe a 7 MHz resonance as sidebands of the clock. By adding a 15 uF capacitor to the 3.3V bus, the resonance is eliminated.
For more information on power integrity testing, please refer to Steve Sandler’s new book, Power Integrity – Measuring, Optimizing, and Troubleshooting Power Related Parameters in Electronics Systems. I reviewed this book a few months ago (see References).
Figure 11 – Test setup for measuring resonances within the power bus structure.
Figure 12 – Power bus resonances at 7 MHz are shown as sidebands to the 125 MHz clock.
Figure 13 – By adding an additional 15 uF capacitor to the 3.3V bus, the resonance is eliminated.
Summary
The Picotest J2150A harmonic injector is quite a versatile device and while designed for power integrity testing and troubleshooting, may also be used in several EMC applications. The small size and battery-operation using USB power packs makes this even more versatile. The stepped and manually-controllable frequencies of 1 kHz, 10 kHz, 100 kHz, and 8 MHz help separate this comb generator from the crowd. This is the perfect troubleshooting tool to help reveal resonances in power bus and power supply circuitry, thus maximizing stability and minimizing EMI from these sources. It’s also very useful as an EMC/EMI troubleshooting tool.
