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| FeelTech FY6600 60MHz 2-Ch VCO Function Arbitrary Waveform Signal Generator |
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| rhb:
A 50 ohm source connected to a 50 ohm cable connected to a 1 meg ohm input is going to have a reflection coefficient of 0.9999. With most of the output reflected back into the source, it will be severely distorted. A 50 ohm terminator on a tee will work OK up into the middle HF region, but as you go higher the reflection from the stub going into the scope will become an issue. To avoid that, buy some 50 ohm thru terminations. The Chinese BNC ones I bought are actually quite decent and good to over 1 GHz. If my 20 GHz SD-24 sampling head arrives today and works properly I'll add some scope traces to this post later. I'll be setting up to test SMA-F to N-F adapters, so little effort to substitute a BNC tee & terminator and a thru. |
| maxwell3e10:
All of this transmission-line considerations of cable impedance and load impedance matching apply only if the cable is long enough to be a transmission line, which means its length is comparable to the wavelength of the wave. With light speed on the order of 1 ft/nsec one doesn't need to worry about impedance matching for a 3-ft cable and 100 nsec pulses. |
| radiolistener:
Here is difference with different cable length, source is 10 dBm: Blue - through 0.15 m cable Yellow - through 1 m cable The same cables, the same signal, but with 50 Ohm input: Here is direct connection through BNC-BNC adapter (cable length 0.01 m): |
| maxwell3e10:
Thanks. Try the same for a sine with f=10 MHz, the difference will be smaller. Also would be interesting to look at square pulses. There will be some ringing on the transitions, but the flat regions will not be affected. |
| radiolistener:
--- Quote from: maxwell3e10 on August 22, 2019, 05:02:51 pm ---Thanks. Try the same for a sine with f=10 MHz, the difference will be smaller. Also would be interesting to look at square pulses. There will be some ringing on the transitions, but the flat regions will not be affected. --- End quote --- Here we are, the same cables, square wave, rise time 1 ns. Blue - 0.15 meters Yellow - 1 meter the same with 50 Ohm input: And through BNC-BNC adapter (0.01 m): As you can see, the difference between 0.15 meters and 1 meter cable is 7.96 ns delay. It is pretty clear visible. Also, as you can see, even with 50 Ohm input there is some small impedance mismatch. This is because this square wave has too wide bandwidth (more than 1 GHz or something like that) and my oscilloscope input has some impedance mismatch at about 1 GHz and above... (I'm using oscilloscope with 100 MHz bandwidth). Square wave is too hard for my scope :horse: |
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