There are times when an increase in harmonic content can’t completely be explained by circuit or PC board design. If you’ve already done a good EMC design and are still getting radiated emission problems, then perhaps resonances in the product enclosure are, in effect, amplifying the internal harmonics. This internal amplification can cause a myriad of mysterious couplings internally to your product with resulting radiated emissions.
Any metal structure can become resonant if driven by a noise source. For example, I’ve seen the tines on a microprocessor heat sink resonate in the 2+ GHz region. More commonly, you’ll discover resonant modes created by the product enclosure. For example, for a rectangular enclosure, we have:
Where: epsilon = material permittivity, mu = material permeability and m, n, p are integers. Cavity resonance can only exist if the largest cavity dimension is greater, or equal, to one-half wavelength. Below this cutoff frequency, cavity resonance cannot exist. In this configuration (where a < b < c), the TE011 mode is dominant, because it occurs at the lowest frequency at which cavity resonance can exist.
The resonant frequency of the circular cavity is 1.225 GHz, very close to the calculated 1.274 GHz.
To read more about constructing a simple demonstration of resonance, click here…
If there’s been one spectrum analyzer that’s created buzz lately, it’s the Rigol DSA815 budget ($1,295) spectrum analyzer, which tunes from 9 kHz to 1.5 GHz. A tracking generator option will run an extra $200 and the EMI option, which provides quasi-peak detection and the three EMI bandwidths (and especially excites us EMC engineers), is an extra $600. There are a number of other options available.
I was able to get my hands on a review unit and am putting it through it’s paces in the next couple weeks. However, with all the discussion of this product lately, I thought I’d give a few first impressions. The first thing that struck me was the compact size. It’s no larger than some of the smaller-size budget oscilloscopes with dimensions of 14″ wide by 7″ tall by just 5″ deep. The second thing that hit me was the weight of the unit for it’s size at 9.4 pounds. Obviously, there’s some shielding inside. The user guide is supplied on CD or available as a download from their web site. I added the guide to my iPad for easy reference.
I loved the large color screen (800×480 WVGA). The controls are arranged in logical groupings, duplicating to a large degree that found on Agilent spectrum analyzers. Besides the usual Frequency, Span and Amplitude buttons, major groupings include Control, Marker, Measure, Utility and Edit (numeric and text input). A vertical column of soft keys on the right side select secondary functions. Down along the left side you’ll find a column of analyzer status icons. There’s also a “Help” key that will describe each control for you.
Major “banner” specs include:
- All digital IF technology
- 9 kHz to 1.5 GHz frequency range
- Up to -135 dBm displayed average noise level (DANL)
- Resolution bandwidth (100 Hz to 1 MHz)
- 1.5 GHz tracking generator option (-20 to 0 dBm)
- EMI filter and quasi-peak detector option (200 Hz, 9 kHz and 120 kHz BWs) [updated 7/7/12]
- Connectivity: LAN (LXI standard), USB host, USB device, GPIB (option)
- Ultra Spectrum (PC) software option (provides additional analysis, including a waterfall display)
After getting some hands-on time in a local EMC test house where we performed some real conducted and radiated emissions measurements, I published a more detailed “first look” in Test & Measurement World’s EMC Blog. A full review will be upcoming on their web site soon. All in all, this appears to be very useful for EMC troubleshooting and pre-compliance testing – and at an affordable price, to boot!