To investigate this further, I measured four different 346A noise sources on an E5080A VNA. These are all made in 2004 and the serial numbers are quite close together. They are all within 150 of each other.
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The reactive part of the impedance looks quite similar but the resistive part is different and you can see some spread in VSWR. I suppose it's possible they may have been stressed by reverse power at some point but this is unlikely.
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This might represent the typical spread in VSWR seen with these noise sources. It may be that one or more of the resistors in the matching section is a select on test device with the aim of achieving low VSWR and best noise response out to 18GHz.
I'm not sure how much spread to expect in the attenuation value of the 16dB attenuator at the output port of the noise source.
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