itsmoked
Electrical
- Feb 18, 2005
- 19,114
I have a problem and am looking for some insight.
I need to monitor plasma in a chamber. My customer has a probe that has a high source impedance, (it's plasma. DUH!) I'm required to provide a simple divider network and was directed to use 100k between the probe and the AMP input which is a J-FET with insane input resistance and with attenuation turned on another 100k to ground. With no attenuation, e.g. the bottom resistor not in-circuit everything is just peachy-keen.
The problem is when 50% attenuation is desired. Switch in that bottom 100k divider resistor and now you have a 200k load piled onto the probe. The result is a fraction of what's expected. A little pencil work shows the probe has about 1 mega-ohm of source impedance!
They don't understand this concept. I made them a little test jig to demonstrate this. Here's the circuit; Coax in to a 100k resistor to a coax out. Coax out to a little N.O. push-button to another 100k to ground. This mimics my device with the AMP but without the AMP. They can hook a scope to the output the probe to the input and see an unloaded probe then press the button and see a 200k loaded probe.
I do this board on my own time donating the parts. Today they come back with, "before we bother to do this test we want two more boards; one with 100k RF resistors and one with 1M RF resistors. They want RF resistors so there is "less noise".
Please correct my thinking here;
'RF resistors' are not about "less noise" they provide no less noise, they provide less self inductance. Correct?
'RF resistors' are typically 1k or less and not 100k or 1M. Correct?
Does anyone know where one could procure 100k and 1M RF resistors? Is it right next to the shelf covered with golden geese or is it the shelf with five leaf clovers? Maybe I'm just looking in the wrong place.
Keith Cress
kcress -
I need to monitor plasma in a chamber. My customer has a probe that has a high source impedance, (it's plasma. DUH!) I'm required to provide a simple divider network and was directed to use 100k between the probe and the AMP input which is a J-FET with insane input resistance and with attenuation turned on another 100k to ground. With no attenuation, e.g. the bottom resistor not in-circuit everything is just peachy-keen.
The problem is when 50% attenuation is desired. Switch in that bottom 100k divider resistor and now you have a 200k load piled onto the probe. The result is a fraction of what's expected. A little pencil work shows the probe has about 1 mega-ohm of source impedance!
They don't understand this concept. I made them a little test jig to demonstrate this. Here's the circuit; Coax in to a 100k resistor to a coax out. Coax out to a little N.O. push-button to another 100k to ground. This mimics my device with the AMP but without the AMP. They can hook a scope to the output the probe to the input and see an unloaded probe then press the button and see a 200k loaded probe.
I do this board on my own time donating the parts. Today they come back with, "before we bother to do this test we want two more boards; one with 100k RF resistors and one with 1M RF resistors. They want RF resistors so there is "less noise".
Please correct my thinking here;
'RF resistors' are not about "less noise" they provide no less noise, they provide less self inductance. Correct?
'RF resistors' are typically 1k or less and not 100k or 1M. Correct?
Does anyone know where one could procure 100k and 1M RF resistors? Is it right next to the shelf covered with golden geese or is it the shelf with five leaf clovers? Maybe I'm just looking in the wrong place.
Keith Cress
kcress -