Re: HP8640B -140/+10dBm instead of -130/+20dBm?
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The ALC pulling or modulation of the loop created IM in itself, hence it is worth doing. I know because I have measured the condition for both cases. There is little one can do because of the PIN attenuator system but that is a secondary IM contribution. On the 40's I have it is straight forward to accomplish, may be different in earlier models.
On Friday, October 23, 2015 9:20 AM, "Alan Ibbetson alan@... [hp_agilent_equipment]" wrote:
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A friend of mine used to work as a design engineer for HP at Queensferry
in the UK, though he didn't himself work on the 8640B. I asked him about
turning off the ALC versus using external attenuators and couplers. He said
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Turning off the ALC fixed ALC pumping. It doesn¡¯t fix the diode
nonlinearity from the detector influencing the output by being a
non-linear load, nor the non-linearity of the output amp.
Yes, a coupler with >=40dB isolation would be good. Snag is that
broadband HF ones use transformers, and to get higher frequency coverage
drives the designers to scoddy little cores. Ferrites do intermod too,
so finding a low intermod coupler is difficult.
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The Clifton Labs website that Thomas referenced has details of couplers
that don't have significant IMD so that is the path I am going to
choose, leaving the inconvenience of ALC disabling for the day when I
can build receivers with substantially more than 100dB IMD dynamic range.
Thanks, Alan G3XAQ
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