Hello
I'm currently looking at what appears to be the canonical setup for home-testing of filters. This setup i outlined here in the "Measuring Double-Tuned Circuit Performance"-box:
http://www.robkalmeijer.nl/techniek/electronica/radiotechniek/hambladen/qst/1991/12/page29/index.html
and here in the "A Method to Measure Insertion Loss or Gain" section:
http://www.qrp.pops.net/RF-workbench-2.asp
and seems to be very common. Basically a 50 Ohms signal generator is connected to the filter which is then connected to a 50 Ohms terminated scope. Now the first link shows 6 dB attenuators both between the signal generator and the filter and between the filter and the scope. The second link only shows the first of the two and says:
I cannot really make sense out of this. I could imagine that an impedance mismatch could lead to reflections at the filter input that could mess up things. But I have no conception of what "messing up things" constitutes of.
With my limited knowledge, this is my process of thoughts which must be wrong: An attenuator would dissipate the reflection, but how can the presence of the attenuator influence the final result? In the end, there is stuff going into the filter and stuff coming out of the filter on the other end, and if you measure whatever comes out of the filter and compare it to what you put in, how is that not quantifying the loss of the filter properly? Also, when the filter is put to use, there is no such attenuators, so whatever is measured in the test setup is not how the filter eventually behaves when used...
On a related note, why is the second attenuator, between the filter and the scope, missing from the second web page?
Best regards and thank you!
I'm currently looking at what appears to be the canonical setup for home-testing of filters. This setup i outlined here in the "Measuring Double-Tuned Circuit Performance"-box:
http://www.robkalmeijer.nl/techniek/electronica/radiotechniek/hambladen/qst/1991/12/page29/index.html
and here in the "A Method to Measure Insertion Loss or Gain" section:
http://www.qrp.pops.net/RF-workbench-2.asp
and seems to be very common. Basically a 50 Ohms signal generator is connected to the filter which is then connected to a 50 Ohms terminated scope. Now the first link shows 6 dB attenuators both between the signal generator and the filter and between the filter and the scope. The second link only shows the first of the two and says:
Edit: The first link says that the attenuators present "clean" 50 Ohm impedances to the filter. I do not understand the difference between a "clean" and a "dirty" 50 Ohm termination.I show an attenuator pad in this diagram to stress that the signal generator output port must have a return loss >= 20 dB.
I cannot really make sense out of this. I could imagine that an impedance mismatch could lead to reflections at the filter input that could mess up things. But I have no conception of what "messing up things" constitutes of.
With my limited knowledge, this is my process of thoughts which must be wrong: An attenuator would dissipate the reflection, but how can the presence of the attenuator influence the final result? In the end, there is stuff going into the filter and stuff coming out of the filter on the other end, and if you measure whatever comes out of the filter and compare it to what you put in, how is that not quantifying the loss of the filter properly? Also, when the filter is put to use, there is no such attenuators, so whatever is measured in the test setup is not how the filter eventually behaves when used...
On a related note, why is the second attenuator, between the filter and the scope, missing from the second web page?
Best regards and thank you!