Quote:
Originally Posted by Argus25
Jeremy,
Regarding the remark in post 55 about the capacitance of the resistor. I thought in this situation the reason the HF response would roll off would be due to the capacitance of the circuit the series resistor feeds (in this case the coaxial cable and input it feeds and any capacitance to ground from the proximity of the resistor body to ground) and therefore any tiny self capacitance of the series resistor itself would be helpful, and that to help maintain the HF response it would require a small capacitor in parallel with the resistor, especially for larger values of the resistor, much like what is required in x10 scope probes. In those the ratio of the capacitive divider formed for AC voltages of high frequencies roughly matches that of the resistive divider for DC voltages, to try to get the response flat.
Hugo.
|
The problem is actually the other way around... it's the tiny parallel capacitance of the 1500R resistor that causes the probe to show an increasing response with increasing frequency. The little shunt cap compensates for it to try to flatten the response to -30dB up to about 2GHz. This capacitor is a shunt cap across the coax after the series 1500R resistor.