[ARC5] Resistor Excursions

[Richard Knoppow]

    I suppose I should stay out of this. I don't remember what I got on 
the RX meter but I think inductance. In comparing CC to film resistors 
with leads the same length at up to 250Mhz (the limit of the instrument) 
the film resistors had less parasitic reactance. The sample chart in the 
Boonton Model 250 RX-Meter shows this. Since I was also brought up to 
believe composition resistors were better at RF it was something of a 
surprise but is evidently the case. My RX-meter fell off a bench 
sometime ago and bit the dust. Eventually, I will find another but there 
are other, newer, instruments to measure the same thing.

On 4/20/2024 1:51 PM, Tom Lee wrote:
> Let’s do some math.
> 
> As I mentioned, a 0.5W classic through-hole resistor will have a shunt capacitance of around 0.5 pF with pigtails clipped short. The uncertainty of that value is perhaps 30%.
> 
> At 10MHz the reactance will be about 30 kilohms or so. That means you would not expect a significant effect from the capacitance unless your resistor was at least some kilohms. And even a 10 k resistor will appear predominantly resistive at that frequency.
> 
> A blanket assertion that a resistor will stay resistive up to some frequency is neither trustworthy nor useful without reference to resistance value or frequency. These obviously matter. I guarantee that a megohm resistor will not be primarily resistive at VHF, yet that’s what your claim implies.
> 
> I’d like to see the papers you refer to, as I’ve had this exact type of exchange many times over the years. Somehow the actual papers never get cited, so I have no idea over what range of R and f the datasets cover. That’s precisely why I made measurements of my own. The data shows that a shunt capacitance model is the simplest one that covers behaviors reasonably well up to around a GHz. Like all models, this one is wrong. But it is also useful.
> 
> As an old boss of mine liked to say, an ounce of data trumps a pound of opinion.
> 
> Tom


-- 
Richard Knoppow
Los Angeles
WB6KBL
SKCC 19998