Jack, DC line conducted requirements exist for aircraft, MIL-STD, Bellcore, and some ETSI specifications, and each standard specifies the test set-up and equipment used.
>From you message it seems you are not so much testing to one of these specs, you are more interested in determining what levels of RF conducted emissions appear at your radio DC terminals - that way, if your emissions are below FCC 15 and EN55022 AC line conducted standards, you can make the argument that your product will not adversely impact line conducted emissions when integrated into a larger system, and that any over-limit AC conducted emissions would be the "fault" of the AC-DC power supply manufacturer or system integrator. EMC labs use the LISN because it is common to many line conducted specs in the aircraft/MIL/Bellcore/ETSI worlds, its use is common in commercial AC line conducted testing, it offers a convenient RF measurement point, and it presents a 50 ohm measurement impedance for measurement repeatability. Laboratory power supplies are often very noisy at RF frequencies, especially in the 10 kHz - 30 MHz region, just where you want to make your measurements. Testing with batteries eliminates this noise problem, but since many labs don't have them, the power supplies are used. Inserting the LISN helps filter out the lab DC supply noise, but placing an additional line filter in series with the LISN just after the lab supply helps a lot better. I don't understand what is meant by by subtracting out noise, unless it means the noise from the golden power supply is very low, and therefore not a significant contributor when you do your measurement of radio supply EMI plus lab supply noise. Testing DC-DC supplies in this way should be OK - caveats are you will be making measurements in a 50 ohm system, and results may vary when the DC-DC supply is operated with the actual AC-DC supply with its characteristic impedance (probably not 50 ohms). Also, some LISNs use ferrite cores to get required 50 uH inductance, instead of stictly air core inductance - LISN specs will give you saturation current, probably well above the current your product will draw. Best regards, Tom Cokenias Consultant, EMC & Radio Type Approvals tel 650-726-1263 fax 650-726-1252
