Product enclosures have frequency resonances that can produce unwanted EMI. Absorption of materials in the cavity can reduce EMI. Use a cookie tin to compare materials before inserting them into your product.
As operating frequencies approach microwaves, enclosures can appear as resonant cavities and amplify EMI emissions. When I was working on space shuttle communications systems, inserting microwave absorber material into the cavities was a common practice. Doing so reduced the EMI created by chains of separately isolated compartments.
In normal product design, we also observe cavity or structural resonances created by smaller shielded products or products with attached cables. In this article, we’ll be experimenting with “cookie tin” resonance and the best materials to dampen this resonance. Cookie tins (Figure 1) are readily available and make good demonstrations of cavity resonance. I borrowed this idea from my colleague, Lee Hill.
To construct this, I drilled two holes for chassis-mount BNC connectors, one in the center and one off to the side. The actual positions are not critical. You just need a little separation between the two. Solder a short (1 cm long) stub to each connector, as shown in the picture. The tracking generator transmits on one, and the analyzer receives on the other.
Equipment required for this demo includes a spectrum analyzer with a tracking generator, or you can use a vector network analyzer. In this case, we’ll use a Siglent SSA 3032X analyzer set to frequency limits of 1 GHz to 1.6 GHz and a resolution bandwidth of 120 kHz. Using two BNC cables with N-BNC adapters connected, as shown in Figure 2.
Calibration — set the frequency limits. Connect the two cables together with a BNC barrel as shown in Figure 3 to calibrate the system and with the preamp turned off, press the “TG” button (upper right) and ensure the level defaults to -20 dBm. It should light up and display a ragged line, representing the loss through the two cables. In the softkey menu, press Normalize. You should end up with a straight line along the top of the display representing a normalized “zero” loss across frequency. This procedure removes cable losses from the measurement.
Calculation — it’s always best when you can predict the outcome of a measurement. For a circular cylinder with dimensions in Figure 4, the resonant frequency is:
For a ≥ h/2.03 (and 9 cm ≥ 6/2.03), then we may use the following to calculate the resonant frequency [1, 2].
Resonant frequency,
Where f = frequency (GHz), h = height (cm), a = radius (cm), µ0 = 8.854×10-12, and ∈0 = 4π×10-7
For this example, a = 9 cm and h = 6 cm (typical cookie tin) would equal,
So,
And f = 1.275 GHz.
Measurement — remove the BNC barrel and connect the two coax cables as shown in Figure 2. You should see a sharp resonance at 1.275 GHz. In our case, we measured 1.236 GHz, which is very close to the calculated frequency. This resonant frequency may differ depending on the size of your tin.
Damping Experiments
My usual demonstration during live seminars is to throw a large ferrite choke into the middle of the tin to help absorb and dampen out the resonance.
For this example, we’ll use one of the large Fair-Rite 0431176451 round cable-core assemblies with #31 material (Figure 5). This exhibits relatively high impedance from 100 MHz to 500 MHz. Whether closed or open (as tested) doesn’t seem to matter. It’s just the bulk ferrite itself that absorbs or dampens the cavity resonance. Figures 6 and 7 show the before (no damping) and after (with the large ferrite choke).
You can see the resonance was almost completely damped, and the resonant frequency shifted lower to 1.136 GHz. The ferrite dropped the resonant peak by 22 dB (27.09 minus 4.99).
I performed experiments with other ferrite absorber samples. Here are a few results. I kept Marker 1 on the original resonant peak for reference.
In Figure 8, I added a 4 in. x 6 in. (10 cm x 15 cm) piece of MAJR Products RF absorber into the bottom of the tin. This is a thick, flexible hybrid of stainless steel mesh and ferrite absorber. It reduced the resonance by 18 dB and shifted the resonant frequency to 1.136 GHz.
In Figure 9, I added an 8×8 cm piece of NEC/Tokin FK2(03) RF absorber into the bottom of the tin. This is a thin, semi-flexible ferrite absorber. It reduced the resonance by only 8 dB and hardly changed the resonant frequency.
In Figure 10, I added a 12 x 12 cm piece of Würth Elektronik 354003 RF absorber to the bottom of the tin. This is a semi-flexible ferrite absorber. It reduced the resonance by 11 dB and hardly changed the resonant frequency. I later dropped a sealed packet of five of these samples into the tin, and the peak resonance doubled to 22 dB. Thickness matters!
These flat, flexible ferrite absorbers are most effective above 2-3 GHz and are more suitable for damping the higher resonant cavity frequencies used in microwave modules.
Summary
In this experiment, we calculated the cavity resonance of a circular cookie tin and confirmed the calculation by using the tracking generator feature of a spectrum analyzer. This cavity resonance can be damped by using discrete ferrites or ferrite-loaded materials. The more ferrite “bulk”, the better damping.
Next time, we’ll test a few other interesting items for resonance.
References
[1] Terman, Radio Engineers’ Handbook, McGraw-Hill, 1943.[2] Jordon, Reference data for Engineers: Radio, Electronics, Computers and Communications, Sams, 7th edition, 1985.
