Products > Test Equipment
FeelTech FY6600 60MHz 2-Ch VCO Function Arbitrary Waveform Signal Generator
maxwell3e10:
--- Quote from: radiolistener on August 23, 2019, 10:07:13 pm ---
For clarification here is impedance measurement of my oscilloscope 1M input:
- at 10 MHz = 57 - j884 Ohms
- at 100 MHz = 43.9 - j96.5 Ohms
As you can see, my oscilloscope input impedance at 10 MHz is not 1 MOhm, but 57-j884 Ohm. So, you're needs to take this into account.
--- End quote ---
Hmm, interesting, how did you measure it? It seems a bit surprising. If it has ~50 Ohm real impedance wouldn't it attenuate signals by about a factor of 2? What happens if you go to even lower frequencies.
radiolistener:
--- Quote from: maxwell3e10 on August 23, 2019, 10:15:53 pm ---Hmm, interesting, how did you measure it? It seems a bit surprising. If it has ~50 Ohm real impedance wouldn't it attenuate signals by about a factor of 2? What happens if you go to even lower frequencies.
--- End quote ---
I measured it with vector analyzer. As you can see it has 1 M at DC but with AC things going to be much more complicated, and this is not ~50 Ohm, note it has reactive component (about 18 pF). So, the things are much more complicated, because input impedance depends on frequency and mismatch effect also depends on cable length and cable impedance (because it is also not exact 50 Ohm)...
BU508A:
--- Quote from: radiolistener on August 23, 2019, 10:20:05 pm ---
--- Quote from: maxwell3e10 on August 23, 2019, 10:15:53 pm ---Hmm, interesting, how did you measure it? It seems a bit surprising. If it has ~50 Ohm real impedance wouldn't it attenuate signals by about a factor of 2? What happens if you go to even lower frequencies.
--- End quote ---
I measured it with vector analyzer. As you can see, this is not ~50 Ohm, it has reactive component (about 18 pF). So, the things are much more complicated, because input impedance depends on frequency and mismatch effect also depends on cable length and cable impedance (because it is also not exact 50 Ohm)...
--- End quote ---
Maybe a Smith-chart would help? ;D
maxwell3e10:
--- Quote from: BU508A on August 23, 2019, 10:15:30 pm ---
But as I said, in this frequency range the "wrong" impedance of the cable doesn't really matter.
--- End quote ---
Yes, because the wavelength of the signals is so much longer than typical cable length.
--- Quote from: BU508A on August 23, 2019, 10:15:30 pm ---If the signal leaves the circuit one have to make sure, that the impedances are adapted, for example by using a Pi-network.
--- End quote ---
Yes, "signal leaves the circuit" means it has to propagate some distance. But within a circuit where the distances are very small, one does not need to match impedance. That is why as one goes to higher (GHz) frequencies, the circuit building blocks become smaller and there are more strip lines connecting then. Whereas for 1-10 MHz frequencies, one typically only does impedance matching at the input and output.
radiolistener:
May be... ;D But when you use proper impedance match, things will be much more easier :)
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