Differences

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+====== HP Power Sensor Information ======
+The 8481/82/85/87 basically all use the same thermocouple. HP used to sell those as spare parts for about $50 ea, now the price is up to $300 ea with a 100pcs min order (1 waffle pack.) So what is the reason for the sensors to have different frequency specifications?
+The main reason can be found in the structure the thermocouple is mounted in. It is basically a small "board" carrying a coplanar structure. While the structure on the 8481 and 82 is quite large, it becomes alot smaller for the 8485 and 87. The reason for this is that the whole structure may become resonant for frequencies above 18GHz and start to mode.
+So, if they use the same sensor, why is the 8482 specified at a lower frequency (300KHz) than the 8481 and only goes to 4GHz?
+The answer to this is fairly simple. The sensors are not DC coupled. They utilize a series capacitor to block DC. The value for this cap is fairly small on the 8481A so it will pass high frequencies but does not pass low frequencies below 10MHz well. The 8482A utilizes a cap with high capacitance. While this cap offers a low impedance for frequencies around 100KHz and higher it begins to be resonant and lossy for frequencies above 5GHz. This is why the 8482A only goes to 4GHz. The caps by the way are thinfilm and cannot be changed!
+Lothar