Review: Tekbox LISNs

-November 28, 2015

EMI emissions testing includes both radiated and conducted. While radiated emissions is the usual nemesis for product designers and EMI engineers, it’s also useful to have a way to measure conducted emissions. This is normally done using a spectrum analyzer (or EMI receiver) and a line impedance stabilization network, or LISN, which helps match the power line to 50 Ohms. Tekbox Digital Solutions recently introduced LISN models for both line-operated and DC-operated equipment. I’ll be reviewing their model TBLC08 (line-powered 50uH LISN) and model TBOH01 (DC 5uH LISN).

Model TBLC08 50uH Line-Operated LISN

The model TBLC08 LISN is about the same size as a very small printer (Figure 1). It comes with a line cord, coaxial cable with N-to-BNC connectors, and a nice instruction manual (available as a download here). The product seems well built and has all the usual controls; Line-Off-Neutral control, Earth inductor In/Out, power switch, RF output port, and an artificial hand connection. It also includes a 150 kHz high pass filter to reduce lower-order power line harmonics, as well as a 10 dB attenuator and transient limiter, which is a great addition and missing on most other LISNs. Turning power on/off on the unit under test or switching the LISN from line to neutral can create large transients that can destroy the sensitive front end circuitry of spectrum analyzers. The transient limiter can help protect your analyzer from harm.

The test power connector may be customized for different worldwide locations. The review unit had the normal NEMA 5-15 connector for U.S. markets.

The rear panel has a large grounding braid included and a standard IEC power line receptacle. There are fuses for both the line and neutral circuits.


Figure 1 - The Tekbox model TBLC08 is enclosed in a well built enclosure.

The frequency range is 9 kHz to 30 MHz, can handle up to 240V / 50-60Hz, and up to 8A of current. The unit meets IEC 61010-1 for product safety. The artificial hand is designed for 220pF and 511 Ohms and the impedance is 50 Ohms with the standard 50uH and 5 Ohm design as specified by CISPR 16.


Figure 2 - The Tekbox TBLC08 LISN was built using high quality components.

The interior is laid out neatly and all components appear to be the highest quality (Figure 2). The left-hand circuit board attached to the front panel includes a 10dB attenuator, along with the 150kHz high pass filter and transient limiter.

Performance

I set up the system to measure a typical switching power supply in an Keysight MSO-X 3102A oscilloscope (Figure 3). Note that for the purposes of this review, I did not test over a ground plane or in a shielded chamber. Thus, the results may differ somewhat from a normal compliance test. However, testing the general performance and signal balance was sufficient. I also compared the results with an EMCO 3810/2 LISN of similar specifications. Both produced very similar results.


Figure 3 - The system under test included a Keysight MSO-X 3102A oscilloscope and Rigol DSA815-TG spectrum analyzer.

Both line and neutral measurements appeared well balanced (Figure 4). The green display line is approximately at the FCC Class A limit. We’re looking from 9kHz to 30MHz, so emissions below 450kHz should be ignored for commercial equipment.


Figure 4 - Measurement of the line (yellow) and neutral (violet) emissions. The balance between the two is very good.

I also tested the 10dB attenuator (Figure 5) and found it to be more like 14 to 26 dB of loss below about 200 kHz. To Tekbox’s credit, they spec the attenuator performance from 150kHz and higher. At the higher frequencies, it indeed appears to be a 10 dB difference (Figure 5).


Figure 5 - A comparison of the neutral circuit with (violet) and without (yellow) the 10 dB attenuator. The attenuator is much more lossy below about 200kHz.

I also measured a low cost “wall wart” switching power supply (Figure 6). This unbranded unit does not include a line filter and the results are obvious! This is a good reason to stick with well-known manufacturers.


Figure 6 - A measurement of an inexpensive unbranded “wall wart” power supply. Note the emissions exceeds even the FCC Class A test limit. The aqua trace is the ambient noise level.

Model TBLC01 5uH DC-Operated LISN

The model TBLC08 LISN is a palm-sized rectangular box (Figure 7). It comes with binding posts for the DC in/out and comes with a nice instruction manual (available as a download here). A DC LISN is mainly used for testing 24 to 28 volt military and aerospace equipment to MIL-STD-461 or DO-160 standards. Normally, two of these are required for compliance testing.


Figure 7 - The Tekbox model TBLC01 5uH LISN fits in the hand.

The frequency range is 100kHz to 110MHz, but useful to 1 GHz (see manual). The DC resistance is just 35mOhms, with maximum voltage rating of 200V and current of 10A.


Figure 8 - The Tekbox model TBLC01 5uH LISN with cover removed. The quality of the components is exceptional.

Summary

Both units were impressive. The only improvement I’d like to see is a more robust front folding feet on the TBLC08, as they seemed a bit floppy in the folded position. I was very pleased with the apparent build and component quality. These LISNs are first-rate in my mind and with their modest pricing, should fit most company’s budgets. While marketed mainly for pre-compliance usage, I don’t see why they would not be suitable for compliance testing, as well. The 5uH model TBLC01 is $249 and the line-operated model TBLC08 is $820. The U.S. distributor is Saelig Electronics (link below). Recommended.

References

Tekbox Digital Solutions

Model TBLC01 (5uH)

Model TBLC08 (50uH)

Saelig Electronics

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