In the last few weeks, I've had my old HP 432A power meter on the bench along with its 478A thermistor sensor. I normally only use it about once a year to check the 1mW (50 MHz) reference on another power meter.
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However, I've spent a bit more time looking at the 478A sensor and wondered if it was worth sharing some of the knowledge gained and also some of the options for reducing the measurement uncertainty compared to using a standard sensor at 50 MHz.
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I've looked at the input match, the efficiency and also at how the 478A has had some changes internally over the years. One obvious option to improve the input match at 50 MHz is to buy a special version with option H75 or option H76. These are probably quite rare and expensive. Another option is to improve the input match of a standard sensor by adding a low loss (external) matching section inline with the standard sensor. I have done this and have achieved good results. I'll show the design process and test results in this thread.
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The standard sensor has a fairly grim spec for input VSWR at 50 MHz of about 1.3:1. However, it shouldn't be this poor at 50 MHz if you look at what is inside the sensor and model the VSWR. In theory, an older version of the 478A (like mine) should manage a VSWR of about 1.07:1 at 50 MHz. The later versions might be closer to 1.05:1 at 50 MHz. So in this sense, there probably isn't much point trying to improve the match if the aim is to measure a 1mW reference from a typical HP power meter. This is because the source VSWR of the 1mW reference is likely to be much lower than its spec of about 1.06:1. So the mismatch uncertainty should be negligible. Only a calibration house should really be concerned here, but I thought I would have a go at improving my sensor anyway.
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The following posts show the thought process and the modelling and the test results.
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I've attached a VSWR plot of the input VSWR of an early HP 478A sensor. This was taken with a lab VNA set to a really slow sweep time of about 20 seconds, although slowing the sweep time from less than 1 second to 20 seconds didn't seem to make much difference.
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You can see there is a kink in the VSWR at 50 MHz and this has always bugged me because it causes a bump in the efficiency across about 40-60 MHz. I've lived with this 'bump' issue ever since I first got this sensor back in the 1990s. But now I've managed to get rid of it and I'll show what causes it and how to upgrade an older sensor like mine to get rid of it.
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Regards
Jeremy
