How to Translate MIL-STD-461 Step Size to EMI Analyst™ Frequency Resolution

MIL-STD-461 specifies maximum frequency step size for susceptibility tests, as do many other EMI specifications. The intent is to subject the equipment under test to enough test frequencies that any design vulnerabilities are exposed. Better to find out in the lab than in the field.

Align Analysis and Testing

It makes sense, then, that you would want to run your EMI Analyst™ analyses with the same precision as the test spec. If the analysis has too few frequencies, a susceptible condition might be overlooked. Analyze the design at too many frequencies, and you get little benefit for the additional computation time.

Direct Correlation

Fortunately, there is a direct relationship between the unintuitive way MIL-STD-461 specifies maximum frequency step size, and the intuitive way EMI Analyst™ uses frequency resolution.

Step Size Versus Resolution

The values in the center column are from MIL-STD-461F, Table III. Step size is specified in terms of a multiplier of the tuned frequency (f0) of the signal source. Different versions of MIL-STD-461 use different multiplier values, so be sure to check your version. Each test frequency is based on the prior test frequency and the multiplier. As frequency increases, step size increases. Thus, tests are performed at logarithmically spaced frequencies.

The values in the right-hand column of the table are the corresponding resolution values used in EMI Analyst™. For example, the screen shot below from EMI Analyst™’s Frequency form shows settings that are equivalent to MIL-STD-461F.

Simple Conversion

To convert from MIL-STD-461F Step Size to EMI Analyst™ Resolution, the formula is:

For example, from 30 MHz to 1 GHz, the formula returns, Resolution = 461.67. Round up to the next
 integer to assure maximum step size requirements are satisfied.

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