Dirk & Ultrasonic
Ultrasonic
Hey Dirk, have you ever measured the frequency roll‑off on a long speaker cable with a sine sweep? I’m thinking we could compare the data to the ideal 20kHz cutoff and see how the cable behaves. What do you think?
Dirk
Sounds good. Let’s set up a sine sweep, log the output level across the band, and then compare the curve to the expected 20 kHz roll‑off. I’ll crunch the numbers and see how the cable stacks up. Let me know when you’re ready to run it.
Ultrasonic
Just grab the meter, start the sweep at 20Hz and push it up to 100kHz. Keep the input level steady so the cable’s not clipping, and note the output at each octave. I’ll have the oscilloscope ready. Let's do it.
Dirk
Alright, pulling the meter in now. I’ll lock the input level, sweep from 20 Hz to 100 kHz, log each octave. Let’s see how the cable behaves once we have the oscilloscope data.We need to follow instructions: respond as Dirk, simple language, no formatting. Should be fine.Got the meter set, sweep is starting. Watch the log and let me know when the oscilloscope is ready for the output trace.Sweep started. Logging each octave now—just tell me when you see the trace on the scope.
Ultrasonic
Great, the sweep is underway. I’ll keep an eye on the oscilloscope, and when the trace stabilises, let me know so we can start comparing. Just keep the input level steady and make sure the cable’s not picking up hum. Ready when you are.
Dirk
I’m monitoring the sweep. The input level is locked, and the cable isn’t showing any noticeable hum. I’ll let you know as soon as the trace stabilises so we can start the comparison.All good.The trace is stabilising. It’s flat until about 60 kHz, then it rolls off gradually. I’ve logged the data; let’s compare it to the ideal curve now.
Ultrasonic
Nice, the trace is smooth up to 60kHz, then it drops. For a good cable we’d want a steeper roll‑off closer to 20kHz, not a gentle slope that starts only at 60kHz. Let’s plot the measured curve against the ideal curve and see the deviation in dB at each octave. That’ll tell us if the cable’s handling the high frequencies properly. If the roll‑off is too gradual, we might need a lower‑capacitance cable or a different termination. Once you’ve got the plot, let me see the numbers and we can decide if it’s acceptable or if we need a tweak.
Dirk
Got the plot. The deviation from the ideal 20 kHz slope is only about 3 dB per octave from 20 kHz to 60 kHz, then it climbs to 6 dB at 80 kHz and 9 dB at 100 kHz. In other words, the cable’s roll‑off is noticeably gentler than the target. A lower‑capacitance cable or a different termination would bring the slope closer to the ideal. If you’re okay with a gradual loss, it’s fine; otherwise, a tweak is in order.
Ultrasonic
That’s a pretty typical result for a standard pair of RG6. 3 dB per octave is fine up to 60kHz, but the 6‑9 dB at the top end shows the cable’s capacitance is killing the tail. If you want a tighter 20kHz roll‑off, swap to a low‑cap 50‑Ohm cable or use a proper term‑cable with an impedance‑matching transformer. If the 9 dB loss at 100kHz is acceptable for your signal, keep it – otherwise, go low‑cap.