One tool every EMC test lab should own is a harmonic comb generator. A comb generator is simply a device that will produce a set of harmonically related CW signals whose spacing is based on a fundamental oscillator frequency. For example, if we were to start with a 10 MHz clock oscillator and feed the digital output into a coax connector, we’d produce a series of CW higher-order harmonics spaced every 10 MHz apart. Generally, the harmonic amplitudes produced are fairly consistent, so they may be used as a frequency and amplitude calibrator.
Comb generators are most often used for ensuring your semi-anechoic test chamber is reading correctly from day to day. Simply place the generator on the turntable, measure specific harmonics each day and record the trend data. I’ve occasionally found loose coax connectors or bad coax cables by comparing the current readings with past data. This would also fulfill the requirement for “equipment verification testing” as specified in ISO 17025. However, there are several other interesting uses for these generators, especially if you build yourself a small one.
Caution: Most comb generators radiate across a wide spectrum, significantly exceeding radiating noise limits specified in any CISPR or MIL standard. If connected to an antenna, use it only in a shielded environment to avoid interference with other electronic communications systems.
Comb Generator Theory – We all know that fast digital signals produce a range of harmonics. Assuming the rise and fall times of the square wave are straight up and down, an infinite number of harmonically-related basis functions, or sine waves are required for this theoretically “square” wave. Digital circuitry today uses rise and fall times of sub-nanoseconds, which can generate harmonics ranging from several hundred to thousands of MHz.
If we take a simple crystal oscillator and capacitively couple the output to a coax connector, we’ve just created a pulse generator. The capacitor differentiates the square wave, allowing only the edges to pass as positive and negative-going spikes. These pulses result in a “comb” of harmonics spaced at half the fundamental frequency.
The better comb generators generally use a capacitively coupled Schottky diode, a step recovery diode (SRD), or even a high frequency (2 to 3 GHz) emitter-base junction, following the digital clock signal. When these semiconductor junctions come out of reverse-bias, they “snap” on with a very fast edge, on the order of picoseconds for SRDs (Figure 1).
A Simple DIY Comb Generator
Simple harmonic comb generators are easy to make. Most simple comb generators utilize the fast edges from a crystal oscillator or oscillator module.
This is the lowest-cost design I’ve seen, courtesy of my friend and fellow EMC consultant, David Eckhardt (Reference 3). I use this for some of my EMC seminar demos (Figure 2). Simply start with a crystal oscillator module and couple the output through a small capacitor to the diode. The value is of little importance and can range from 27 pF to 100 nF. This will generally produce nice harmonics up through 300 MHz, or more. For example, a 10 MHz oscillator will produce positive and negative spikes every 5 MHz, resulting in harmonics at intervals of 5 MHz.
If the oscillator is near a 50% duty cycle (rare), the even-order harmonics will be suppressed to some degree. Adding a high-current driver will usually square up the edges better and create higher frequency harmonics. Adding a single diode or back-to-back diodes will also get you higher in frequency. The better comb generator designs will use Schottky or step recovery diodes (SRDs).
This was built on a small perf-board and includes a 5V regulator, so it can be powered from a 9V battery. Figure 3 shows the board. I used a variable capacitor for coupling to the diode, but adjusting it had little effect. I’d probably use a fixed 27 pF capacitor next time.
Using a Rohde & Schwarz MXO44 (1 GHz bandwidth) oscilloscope, I show both the time domain and frequency domain simultaneously. Figure 4 shows the waveform at the output. As you can see, the generator creates a relatively fast edge – on the order of 5 ns.
Favorite Commercial Comb Generators
I’ve evaluated quite a few harmonic comb generators through the years. Here are a few of my recent favorites.
Picotest J2150B Harmonic Comb Injector
Picotest recently introduced an upgraded (J2150B) harmonic comb injector that’s not only designed to reveal power integrity issues, but also has interesting uses in EMC measurement and troubleshooting [1]. It also has several unique features that really set it apart from all other comb generators:
- It is sized about the same as a USB thumb drive and derives 5V power from any USB port.
- The generator has three 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. This is useful for power integrity characterization.
- Mode 1 steps through three different frequency steps (1 kHz, 100 kHz and 8 MHz) in order to better reveal circuit resonances.
Figure 5 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 parasitic or structural resonances. The various modes are described in Figure 6.
Mode 1 is the default mode at power-up, and steps through modes 2 through 4 in order to excite all harmonic frequencies, thereby quickly revealing problem resonances. Once resonant frequencies are identified in Mode 1, impulse functions with frequencies of 1 kHz, 100 kHz, and 8 MHz in Modes 2 through 4 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 7 shows a typical harmonic output up to 1 GHz when using Mode 4 (8 MHz dithered fundamental). There are useful harmonics out beyond 1 GHz. The pulse rise time is about 300 ps.
The Picotest J2150B harmonic injector is quite a versatile device and 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. The price is $495.
Tekbox TBCG4 Comb Generator
Tekbox Digital Solutions recently developed a small harmonic comb generator with switchable 5, 10 and 20 MHz comb frequencies, model TBCG4 [2]. The ability to adjust the comb frequency allows a greater or lesser number of combs. More combs allow greater resolution when it comes to measuring resonances in structures or cables. When comparing EMC chambers or performing validation tests, fewer numbers of combs is usually sufficient.
The TBCG4 is powered by a standard USB-C port in the back. This comb generator is rather unique, because the harmonic comb amplitudes are relatively consistent. This flat harmonic response may be an advantage when it comes to characterizing EMC chambers. The price is $517.
Comb Generator Applications
Cable Resonance – 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 an RF current (or large H-field) probe, you can measure the harmonic content using a second current (or H-field) probe. The cable resonance is indicated by the peak in harmonics. Figure 10 shows the overall setup. In this case, I’m using a Siglent SSA 3032X spectrum analyzer and a pair of large Beehive Electronics H-field probes. The position of the probes is not critical. There is a peak resonance of this wire at 200 MHz. Note that you can also use a pair of RF current probes clamped around the wire as an alternative to the H-field probes. Please refer to my previous article on measuring cable resonances for more details [3].
Portable Operation – One really neat feature of harmonic comb generators is that they may be powered from USB battery packs (Figure 11). 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.
When you’re having a product tested for radiated emissions at a third-party test lab, it’s also valuable to have one of these comb generators on hand to validate the test lab’s measurement chamber before they start testing. You want to be sure the chamber measures the same as the last time you were there. You can also use it to compare other test labs’ chambers. There are several other applications for harmonic comb generators, including shielding effectiveness measurements of sample materials and evaluating the actual enclosure shielding effectiveness [4].
