Dear Sir:
I was wondering if you could tell me how Agilent makes accurate voltage measurements at high frequency.
Consider this question:
Generally, oscilloscopes make measurements with an internal impedance of 1 Megaohm or 50 ohms. I read that, in the past, that when an oscilloscope made a measurement when the internal impedance was set to 50 ohms, that 50 ohm resistance would be in placed parallel with the load. This is equivalent to using a 50 ohm feedthrough when scopes were fixed at a 1 Mega ohm input impedance. It follows logically, if one were measuring a the voltage across an external 50 ohm resistance, the equivalent resistance would be approximately 25 ohms. (I realize, the method I have just described is simplistic and cannot be practically applied after giving this some more thought). This would indicate, at this setting that one would measure the a voltage drop across a 25 ohm resistance. Can you tell me how this issue is resolved? Does Agilent use a correction factor (data processing) to obtain the correct result?
Next question:
When oscilloscopes make measurements at high frequencies, (Agilent has advertised a raw measurement bandwidth at its pre-amps at approximately 26 GHz), there are SWR issues that take place at the measurement point. Can you tell me how Agilent resolves this issue?
Edited by: SOLT_guy on Mar 11, 2013 10:43 AM
I was wondering if you could tell me how Agilent makes accurate voltage measurements at high frequency.
Consider this question:
Generally, oscilloscopes make measurements with an internal impedance of 1 Megaohm or 50 ohms. I read that, in the past, that when an oscilloscope made a measurement when the internal impedance was set to 50 ohms, that 50 ohm resistance would be in placed parallel with the load. This is equivalent to using a 50 ohm feedthrough when scopes were fixed at a 1 Mega ohm input impedance. It follows logically, if one were measuring a the voltage across an external 50 ohm resistance, the equivalent resistance would be approximately 25 ohms. (I realize, the method I have just described is simplistic and cannot be practically applied after giving this some more thought). This would indicate, at this setting that one would measure the a voltage drop across a 25 ohm resistance. Can you tell me how this issue is resolved? Does Agilent use a correction factor (data processing) to obtain the correct result?
Next question:
When oscilloscopes make measurements at high frequencies, (Agilent has advertised a raw measurement bandwidth at its pre-amps at approximately 26 GHz), there are SWR issues that take place at the measurement point. Can you tell me how Agilent resolves this issue?
Edited by: SOLT_guy on Mar 11, 2013 10:43 AM
I would like to thank you very much for replying to my question.
Can you please link me to pictorial diagram of how one would the measurement you referred to when you wrote:
" One is to connect the scope as if it were the load. In the case, the scope is providing the 50 Ohm termination and you will see what your DUT would see if it were connected, instead of the scope."
Also, when you wrote:
"The other solution would be to use a good divider. In this case, if you use good cables with matched lengths, the DUT and the scope see the same signal, with a little more than 3dB loss. If everything was connected correctly, you only have to worry about the cable loss."
There is so much incorrect information on the web and sometimes making a logical inference from incomplete information can still result in an erroneous determination. In an analogous example, you can obtain two mathematically valid roots from a polynomial equation but only one root is valid for the application that you use the equation for.
I am going to stop here because I want to obtain the rudimentary measurement information first before I focus on anything else. Please include the "divider" type (I am thinking "power splitter" but I could be wrong) if it is not set forth in the pictoral diagram.
Again, I want to thank you for your reply.
Again, thank you for your reply.
What are you trying to test?
Al
When I purchase new equipment I tend to include what all my costs are going to be (in the case of an oscilloscpe I factor in the price of a new high frequency probe - I know that Agilent will give the customer a new probe when you buy the oscilloscope. This is just an example). When other devices are required to make measurements, I factor those prices in. I also look at accuracy and the cost to obtain that accuracy and the potential versatility of the system I am going to purchase.
When you mentioned the measurement methods you described, I believe you had an example in mind. I would like to know what you had in mind because somewhere along the line I will probably have to repeat your methods.
Can you please send me pictoral diagram of what you had in mind?
Edited by: SOLT_guy on Mar 12, 2013 7:58 AM
If you are checking a source, then you connect the source to the test equipment. If you want to see the signal going to another device, then you need to split the signal, using appropriate HW, and expect that you will see lower amplitude at both the scope and on the other device.
Al