Review: Tekbox TBCG4 harmonic comb generator

The Tekbox TBCG4 harmonic comb generator produces a wide range of known frequencies for EMC immunity testing and routine checking of EMC chambers.

I’ve used harmonic comb generators for decades in various applications, including antenna measurements, cable resonance measurements, and shielding-effectiveness testing. I also find them useful for checking the daily consistency of test chambers used in radiated-emissions tests by placing a short monopole or dipole antenna on the RF output inside the chamber.

Until now, I’d been using Applied Electromagnetic Technology’s small USB-powered comb generators for these applications. I also use them in live demonstrations of basic EMC design issues to show issues with gaps in the PCB return plane and problems stemming from long pig-tailed cable-shield connections. Unfortunately, the comb generator I used was discontinued several years ago, and I’ve been hoping another company will develop a replacement. That day arrived when Tekbox Digital Solutions developed the model TBCG4 generator with switchable 5 MHz, 10 MHz, and 20 MHz combs (Figure 1).

Why three comb frequencies? 5 MHz has just enough resolution to observe peak resonance in structures such as cables. The 10 and 20 MHz selections work well for general-purpose use for measuring EMC chambers, antennas, shielding effectiveness, or live demos during my presentations. The 20 MHz comb selection also produces the largest amplitude harmonics. When using the 5 MHz comb selection, energy spreads across the useful band, so the overall amplitude of the combs decreases.

TBCG4 basic specifications

- Base frequencies: 5 MHz, 10 MHz, 20 MHz, selected through a slide-switch on the front panel
- Characterized comb spectrum: 5 MHz to 1 GHz
- Output amplitude of 20 MHz comb spectrum: –19 dBm ±3 dB
- Output amplitude of 10 MHz comb spectrum: –28 dBm ±3 dB
- Output amplitude of 5 MHz comb spectrum: –38 dBm ±3 dB
- Spectrum flatness from 5 MHz to 1 GHz: < 10 dB
- Pulse rise time: < 300 ps
- Output: 50 Ω, SMA socket
- Power supply: USB-C
- Current consumption: 120 mA
- Dimensions, L × W × H: 56 mm × 81 mm × 33 mm

Performance measurements

I connected the Tekbox TBCG4 harmonic comb generator to channel 1 of the Rohde & Schwarz MXO44 1 GHz bandwidth oscilloscope, which was adjusted for a 50 Ω input impedance. Harmonic combs were measured at 5 MHz, 10 MHz, and 20 MHz. As the comb frequency increased, I noticed that the amplitude level also increased. See Table 1.

Table 1. Measured characteristics versus comb frequency.

Comb Freq (MHz)	Amplitude (dBµV)	Harmonic Peak (V)
5	65	0.40
10	78	1.1
20	88	1.6

Comb generator pulse with ringing — Figure 2. Overall view of the test setup using the Rohde & Schwarz 1 GHz oscilloscope. The expanded 20 MHz pulse shows the 500 ps rise and fall time. The horizontal time base is 500 ps/div. There’s a 20 MHz ring wave following the main pulse.

Figure 2 shows an expanded 20 MHz pulse with rise and fall times of 500 ps. Figure 3 compares the time and frequency domains. I was impressed with the flatness of the combs compared to frequency. Most other comb generators I’ve characterized have greater decreases in amplitude and one or more notches in the emission spectrum.

To test the maximum usable comb frequency, I used a Siglent SSA 3032X spectrum analyzer with a Tekbox DC block to protect the input from any DC biases from this comb generator. I set the start and stop frequencies to 10 MHz and 3.2 GHz, respectively.

flat response to 20 MHz — Figure 3. This screen capture shows the time domain trace of 20 MHz pulses and the frequency domain trace showing the harmonic combs from 20 MHz to 1000 MHz. I’ve never seen such a flat amplitude response in any other comb generator I’ve used or reviewed.

Figure 4 shows the relative amplitude versus comb frequency. It’s easy to observe the amplitude decrease as the comb frequency is lowered from 20 MHz to 5 MHz. Interestingly, there seems to be a sinusoidal response on the harmonic tops. The response is relatively flat out to 1 GHz.

Comb generator response to 2 GHz — Figure 4. Plotting the TBCG4 from 10 MHz to 3.2 GHz shows usable combs out to at least 2 GHz. The yellow trace is 5 MHz combs, the violet trace is 10 MHz, and the aqua trace is 20 MHz combs. There’s about a 10 dB decrease in average amplitude as comb frequency is reduced.

Cable resonance measurement

Injecting signals into a cable — Figure 5. This test setup uses the Tekbox TBCG4 harmonic comb generator to inject RF harmonics into the cable (left). The H-field probe on the right connects to a spectrum analyzer to show the cable resonance.

To demonstrate using a comb generator to measure cable resonance, I took two Beehive 100C H-field probes and coupled them to a 1.2 m (48-in.) long BNC cable laid on the test bench. The TBCG4, set to produce a 5-MHz comb signal, drove one probe to excite the cable. I connected the other probe to the spectrum analyzer’s input in Figure 5 to measure the resonance.

The analyzer was set to a 9 kHz RBW to increase resolution and start/stop frequencies of 10 and 500 MHz. Markers were placed on the cable resonances of 99 (1/2-wave) and 199 MHz (1/4-wave). Other minor resonances were noted and could be cable-related or nearby parasitic elements (Figure 6).

Because the 1.22 m (48-inch) long cable is being measured in a somewhat free space, the 1/2-wave resonance should be close to 117 MHz or so. Why is it lower at nearly 100 MHz? The answer is the velocity factor, which, for most BNC cables with Teflon center insulation and outer plastic cover, is around 0.8. The speed of light is slower, making the resonant frequency lower.

resonance plot for a BNC cable — Figure 6. A 1.22 m (48-inch) long BNC cable resonance plot shows ½-wave and ¼-wave resonances.

EMC chamber measurements

I mentioned that an important use of a comb generator was the measurement or characterization of EMC semi-anechoic chambers. Connecting an electrically short monopole or dipole antenna to the generator output and placing it on a turntable as a regular product can reveal chamber resonances or defects in system cabling. The smart test lab will measure these daily to ensure proper system measurement consistency.

The Tekbox TBCG4 is now my new “go-to” comb generator. Its more universal USB-C port powers it, and it can be easily battery-powered using one of the many “portable USB batteries” and used as a source for antenna and chamber measurements. The range of comb frequencies is near ideal for most measurements I commonly perform.

Highly recommended. Price: $599.