A couple of people have now mentioned that it's possible to swap the internal caps for 0.1uF parts. I'm still not sure what package will be suitable, especially for the series input cap.
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A 1206 SMD package seems huge to me and it isn't the ideal form factor to achieve low series inductance. However, I put a 100nF 1206 class 2 ceramic cap into an SMD test fixture and measured it from 1 MHz to 2 GHz to look at the package inductance and the ESR.
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See the result below. This is pretty much what I've seen in the past from this capacitor type. The series inductance is about 1nH (seen in the right hand graph) and the ESR slowly climbs from a few tens of milliohms up to about 0.5 an ohm by 1 GHz. Above this, the ESR climbs very rapidly and this is normal.
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I also put this into a simulation of a H75 thermistor head and this predicts the VSWR vs frequency. The series resonance is at about 15 MHz and I would have expected to see this up at maybe 30-40 MHz with a different capacitor package. However, these H75 sensors date back a few decades so I'm not sure what technology HP used for these caps. Has anyone taken one (eg a damaged one) apart?
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I haven't included the tiny package inductance of the thermistors so I think the overall inductance will be slightly higher than my crude model below.
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The model assumes that both of the 100R thermistors track accurately together and this allows a very low VSWR between 10 and 20 MHz. However, if they don't track perfectly when the bridge is balanced, then the VSWR will be a bit higher here in reality. One thermistor might be at 90 ohms and the other one might be at 110 ohms for example where 90 + 110 = 200R. This will degrade the VSWR of the sensor.
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Regards
Jeremy
