Category Archives: Case Study

PC Board Resonance

 How many of you have beat down a harmonic at one end of the spectrum, only to have an otherwise low harmonic rise up above the limit at the higher end of the spectrum? This is often termed the “ballon effect”, where squeezing one end of a balloon makes it expand at the other end. This is usually due to board resonances within the PC board itself.
I recently received an interesting observation from fellow EMC consultant, Mike Farnet, following an experiment he performed on reducing the emissions from a client’s embedded ARM processor board with Ethernet. There were strong 25 MHz harmonics from the PHY circuit, as is usual for these low-cost boards. The original harmonic was peaking strongly at 150 MHz. Here is his discussion:I use a 5407 EMCO GTEM and a Rigol DSA-815 TG+EMC spectrum analyzer.  I use LabView to collect the data from my spectrum analyzer.

I am working on a 25 MHz issue on an embedded ARM board with Ethernet.  The strongest offending harmonic is at 150 MHz.  See Figure 1.

Figure 1 – The harmonic profile before the capacitors were changed.

I was growing tired of waiting to collect 16000 data points for the 3 positions in the GTEM and was contemplating limiting the scan window to the 150 MHz target for faster debugging when the scan in Figure 2 told me “Bad Idea.”

Figure 2 – The harmonic profile after the capacitors were changed.

For my answer, click here…

Case Study – Radiated Emissions from LCD Displays

I recently had a chance to troubleshoot a controller display with very high emissions in the 90 to 300 MHz region. This was a product inherited from an acquisition, so they had to fix the design after the fact – not an uncommon situation.

I’ve dealt with the issue of radiating LCD displays before, so had some ideas on an approach already. LCD displays use a high-speed “dot clock” signal to drive the individual pixel elements. This is frequently in the order of a few hundred MHz and is routed via a ribbon cable from the clock driver and processor circuitry. Unfortunately, and for reasons beyond me, most LCD display modules are generally comprised of a sandwich of two floating pieces of aluminum – a front bezel and rear shield and mounting plate. These plates often have a gap between them as well as sometimes lacking a bonding connection back to the chassis or product enclosure. This results in “floating pieces of metal”, which couple energy and radiate. On top of that, the OEM display assembly was packaged in a plastic enclosure, which was inserted into a large rectangular hole in the metallic controller housing and clamped from the rear. In other words, we have this large radiating assembly with no easy way to bond it to the primary metal controller housing!

I first probed around the whole assembly to confirm the ribbon cable and front bezel were the dominant radiators. I used the Beehive Electronics h-field probes connected directly to the handheld TTi PSA2701T spectrum analyzer. This took just a few minutes and proved the cable was the “hottest”, with large emissions also radiating out the front of the display.

Closeup showing the ferrite choke on the dot-clock cable.

I treated the dot-clock cable first, because that was easiest. Unplugging the cable from the PC board, I slipped a small flat ferrite choke (Laird 28R0898-100) over the cable and reconnected it. This immediately reduced the emissions as much as 12 dB. The was no effect on the display quality.

The lower frequencies, while improved, were still fairly high, so the next step was to disassemble the display getting right down to the LCD module itself. After confirming the front bezel and rear shield were indeed floating, I used copper tape to temporarily bond the two together. I then confirmed the LCD module was supported by plastic pins, so was not bonded to the rest of the display assembly. I took additional copper tape and ran it between the rear shield of the LVD module and steel sub-chassis of the display. I also ran several pieces out from the sub-chassis to the inside of the metal enclosure. The result was that all metal pieces were now bonded together. After connecting the tape to the enclosure, the emissions had been reduced from 9 to 25 dB! We later “cost-reduced” by removing most of the unnecessary fixes. After a few experiments, we were able to remove all the copper bonding between the display assembly and controller housing, but the two nearest the source. After eight hours work…a compliant product!

Closeup of the corner of the LCD module, showing the gap around the edge of the module. Copper tape was temporarily applied as a troubleshooting fix.