Glider Electronics Experiment #1 - electrical crosstalk demonstrated

On Saturday, May 2, 2020 at 9:41:54 AM UTC-4, Richard Livingston wrote:
Nice demo of cross talk. Last year I fixed a problem in our glider where the microphone wires were not completely shielded, as in your demo, and were picking up signal from the radio speaker wires, also not completely shielded. The result was intermittently the radio would squeal during transmit due to feedback from the speaker back into the microphone. It was intermittent due to the wires shifting slightly every time the canopy opened/closed. Shielding both the speaker and microphone wires fixed the problem.

Rich L.

Such feedback could only occur if the radio had the (mis)feature of echoing the signal from the mic to the speaker. Kind of lets you know you've got the PTT pressed and it's all working, but not really necessary. With that happening, you may also get acoustic feedback, from the speaker back into the mic, even without crosstalk in the wiring. Maybe that feature can be turned off somewhere in the radio's settings?

At 21:32 02 May 2020, wrote:
On Saturday, May 2, 2020 at 9:41:54 AM UTC-4, Richard Livingston wrote:
Nice demo of cross talk. Last year I fixed a problem in our glider
where=
the microphone wires were not completely shielded, as in your demo, and
we=
re picking up signal from the radio speaker wires, also not completely
shie=
lded. The result was intermittently the radio would squeal during
transmit=
due to feedback from the speaker back into the microphone. It was
intermi=
ttent due to the wires shifting slightly every time the canopy
opened/close=
d. Shielding both the speaker and microphone wires fixed the problem.
=20
Rich L.

Such feedback could only occur if the radio had the (mis)feature of
echoing=
the signal from the mic to the speaker. Kind of lets you know you've
got
=
the PTT pressed and it's all working, but not really necessary. With
that
=
happening, you may also get acoustic feedback, from the speaker back into
t=
he mic, even without crosstalk in the wiring. Maybe that feature can be
tu=
rned off somewhere in the radio's settings?

Many radios have this foldback feature for use with headsets and intercoms.
But yes, it could cause acoustic feedback. But this does not satisfactorily
account for the above fix.

What would seem more likely is that the poor sheilding has allowed
cross-talk from the antenna cable to the speaker cables which has allowed
the RF to feed back into the radio box where it caused meyhem. The speaker
does not need to be active for this to happen.

The section on supressing cross-talk is going to be interesting because
there are so many possible causes.

Tim.

On Sunday, 3 May 2020 11:30:04 UTC+1, Tim Newport-Peace wrote:
The section on supressing cross-talk is going to be interesting because
there are so many possible causes.

Experiment #2 is just going to look at the effect of cable type on the type of noise seen in experiment #1.

Experiment #3 on-wards, if I manage to keep making them, will look at other types of noise.

Best regards,

Richard

Nicely done.

On Saturday, May 2, 2020 at 4:32:04 PM UTC-5, wrote:
On Saturday, May 2, 2020 at 9:41:54 AM UTC-4, Richard Livingston wrote:
Nice demo of cross talk. Last year I fixed a problem in our glider where the microphone wires were not completely shielded, as in your demo, and were picking up signal from the radio speaker wires, also not completely shielded. The result was intermittently the radio would squeal during transmit due to feedback from the speaker back into the microphone. It was intermittent due to the wires shifting slightly every time the canopy opened/closed. Shielding both the speaker and microphone wires fixed the problem.

Rich L.

Such feedback could only occur if the radio had the (mis)feature of echoing the signal from the mic to the speaker. Kind of lets you know you've got the PTT pressed and it's all working, but not really necessary. With that happening, you may also get acoustic feedback, from the speaker back into the mic, even without crosstalk in the wiring. Maybe that feature can be turned off somewhere in the radio's settings?

That's called the "sidetone", and many radios have an adjustment for that. It is also possible (likely even) that the feedback was from the RF back into the microphone wires. Connectors are notoriously leaky, and even coax can leak a little bit. If that RF signal gets into the microphone input it can be rectified by the transistors in the amplifier and produce feedback that will cause a squeal.

Rich L.

On Sunday, May 3, 2020 at 5:13:09 AM UTC-7, Richard Lancaster wrote:
On Sunday, 3 May 2020 11:30:04 UTC+1, Tim Newport-Peace wrote:
The section on supressing cross-talk is going to be interesting because
there are so many possible causes.

Experiment #2 is just going to look at the effect of cable type on the type of noise seen in experiment #1.

Experiment #3 on-wards, if I manage to keep making them, will look at other types of noise.

Best regards,

Richard

I would suggest one mo the effect of ground distribution, especially single-point commons, on noise.

Tom

I blame the noise in my cockpit on my choice of lunch.