The Tekbox TBCG1 comb generator is a low-cost solution for "quick and dirty" harmonic measurements from a standard RF source.
Harmonic comb generators have been available for a long time and are useful for many purposes. They produce a fixed series of harmonic signals with stable frequency and amplitude over a range of frequencies. One major use is to check the proper day-to-day measurement accuracy of anechoic chambers, to compare the measurements from one chamber to another, or to compare different antennas.
Tekbox Digital Solutions recently developed an affordable pocket-sized generator (Model TBCG1), which produces harmonics every 100 MHz. It is powered by a common 9V battery (Figure 1).
Figure 1 The Tekbox harmonic comb generator is truly pocket-sized.
I’ve written extensively on how to use harmonic comb generators for various applications (references 1-4). Figure 2 shows the typical harmonic combs as tested about 1 m away from a spectrum analyzer with short antenna on the input. I made sure to align the generator antenna polarity with the receiving antenna.
Figure 2 This graph shows the typical harmonic comb frequencies at about a 1m test distance and looking from 50 to 2500 MHz.
I recently used the generator as a fixed source in which to compare the performance of several EMI antennas. Before I started the testing, I needed to determine the generator antenna type and polarization. Removing the cover reveals a shortened wideband dipole (half of a “bowtie” design) in the horizontal polarity (Figure 3).
Figure 3 With the cover removed, you can see the dipole antenna.
In setting up for the antenna testing, I used Velcro fastener to attach the generator to a PVC pipe stuck in the ground at a height of about 1 m (Figure 4). The antennas were placed 10m away and the harmonic combs were measured using each antenna.
Figure 4 The Tekbox comb generator was used as a standard frequency source to compare several EMI antennas for accuracy. The generator was attached to the PVC pipe with Velcro.
As I mentioned, another major application would be to assess the measurement accuracy of semi-anechoic chambers. While the Tekbox model can do this, because of the 100 MHz harmonic spacings, it would only be useful for a gross measurement. Other models from AET or Com-Power would offer more accuracy, because they produce closer-spaced harmonic combs, usually at 10 MHz spacings, or better.
For example, the “top of the line” AET Model USDS spherical comb generator with spherical dipole elements has adjustable harmonic spacings of 10, 64, 100, and 133 MHz (Figure 5). This was used to compare two 10-m chambers to discover the differences in measurement (Figure 6). Even with the most accurate generator, duplicating an ideal “point source,” the chamber-to-chamber correlation was surprisingly different at the lower frequencies!
Figure 5 The Model USDS spherical dipole from AET is an example of a “top-end” harmonic comb generator.
Figure 6 You can use a harmonic comb generator to compare two semi-anechoic chambers. This plot shows the difference in measurements between two 10-m chambers. There can be surprising differences, especially at the lower frequencies!
Of course, the big advantage of the Tekbox TBCG1 is the price, at just $99 through distributors like Saelig Electronics in the U.S. and other distributors worldwide. Other commercial generators cost upwards of $4,000. It is small enough to fit in your EMC troubleshooting kit and uses commonly available 9V batteries. It can also be used to quickly compare chamber measurements from one day to the next and I would recommend using it to check 3rd-party test facilities to ensure something obvious like a disconnected coax cable or loose connectors hasn’t occurred since the last time you brought in a product to test.
Some disadvantages to consider would be the relatively wide-spaced harmonic combs at 100 MHz spacing, which would prevent higher-resolution measurements. Since the obvious position would be to lay the device flat on a turn table, the horizontal polarity would be easiest to measure. There’s no real way to mount it in the vertical polarity, except by use of tape or other attachment. One other thing I noted during the antenna testing was that the battery was only good for about an hour, so have extras on hand.
The Tekbox TBCG1 is a low-cost solution for “quick and dirty” harmonic measurements from a standard RF source. The harmonic amplitudes stay consistent within normal daytime operating temperatures and the device could save hours by ensuring chamber measurements have not gone awry prior to making any compliance measurements. Recommended.
—Kenneth Wyatt is president and principal consultant of Wyatt Technical Services.