On 6/9/23 11:12 AM, Anne Ranch wrote:
Here is a new twist ....
I replaced current , unknown type , unmarked type of coax with
known, good RG6 length plainly marked in cubits and converted them to meters - approximately 35 meters.
Now when I set sweep to 10 to 60 MHz TDR "estimated length" is around 28 meters - with open ended coax.
( Not even close to 35 meters ...)
Is your coax foam or solid dielectric - RG-6 comes in both forms.
What does the trace look like? (can you save it and post it?) I'd expect around 41-42 meters if set to VF=1 (or 100%).
if it's set to, say, 0.66 VF and it's actually 0.84 (first google hit for RG-6 VF), then it will give you a shorter than real length
(35 *.66/.84 = 27.5 meters which is pretty close to what you got.
some RG-6 is foam dielectric so the epsilon of the dielectric is smaller.
The other way it can get fooled is if there's a place where the cable is damaged, causing a discontinuity that's big enough to fool the algorithm.
The algorithm does a simple argmax to find the peak
s11 = []
for d in self.app.data11:
s11.append(np.complex(d.re, d.im))
window = np.blackman(len(self.app.data11))
windowed_s11 = window * s11
self.td = np.abs(np.fft.ifft(windowed_s11, FFT_POINTS))
step = np.ones(FFT_POINTS)
self.step_response = signal.convolve(self.td, step)
self.step_response_Z = 50 * (1 + self.step_response) / (1 - self.step_response)
time_axis = np.linspace(0, 1/step_size, FFT_POINTS)
self.distance_axis = time_axis * v * c
# peak = np.max(td)
# We should check that this is an actual *peak*, and not just a vague maximum
index_peak = np.argmax(self.td)
cable_len = round(self.distance_axis[index_peak]/2, 3)
feet = math.floor(cable_len / 0.3048)
inches = round(((cable_len / 0.3048) - feet)*12, 1)
self.tdr_result_label.setText(f"{cable_len}m ({feet}ft {inches}in)")
self.app.tdr_result_label.setText(str(cable_len) + " m")
self.updated.emit()
I started looking at the software and so far it looks as there is "the original " nanoVNASaver and several "forks", some for windoze only.
It would be nice if somebody who KNOWS where " TDR" software resides help me and point me in the right direction.
YES, I can download all and then I can search but if somebody already knows where to look , let's share that info, please...
and thanks
...
