- #1
Simon.T
- 15
- 0
Dear All
I am hoping for some insight into a transmission line problem I have been experiencing. I am quite inexperienced and unqualified in this area so if I am not clear please let me know.
In wiring loom assemblies my employer offers there is a twisted transmission pair. The conductors are terminated at either end into a soldered electrical insert. The assemblies come in various lengths (75, 50, 15 and 2.7m).
The specification of the un-terminated transmission wire (as quoted from the manufacturer data sheet) is <6dB/150m @ 20MHz. “Scaling” of this value seems particularly intuitive for determining testing limits for verification.
What we have noticed is that in the 75m loom, the measured attenuation is approximately half of the scaled manufacturer limit. In the shorter sections the measured attenuation is borderline or a failure; as I mentioned we are scaling the limits so that 15m testing limit is 1/5th of the 75m limit for example.
When we mocked up a very short loom (approximately 10cm) and terminated them into connectors, the loss through that assembly was MORE than the 2.7m terminated section.
It appears that the loss contributed by the connector inserts is not static. In fact, it seems the measured attenuation is a function of the ratio of “connector medium” to “transmission line medium”; the shorter the loom the more of an impact the connector has on the attenuation.
Is what we are seeing representative of typical transmission line characteristics? How can this behaviour be expressed/calculated?
From my studies I am familiar with the concept of standing waves, reflections and "ringing" due to impedance mismatching and I am aware there are a number of standard transmission line equations but I’m not sure how those apply here (see the first paragraph .
Absolutely any help would be appreciated. We are attempting to negotiate more lenient tolerances for the shorter looms that keep failing but we would like some basis for our recommendations.
Simon Thomas
I am hoping for some insight into a transmission line problem I have been experiencing. I am quite inexperienced and unqualified in this area so if I am not clear please let me know.
In wiring loom assemblies my employer offers there is a twisted transmission pair. The conductors are terminated at either end into a soldered electrical insert. The assemblies come in various lengths (75, 50, 15 and 2.7m).
The specification of the un-terminated transmission wire (as quoted from the manufacturer data sheet) is <6dB/150m @ 20MHz. “Scaling” of this value seems particularly intuitive for determining testing limits for verification.
What we have noticed is that in the 75m loom, the measured attenuation is approximately half of the scaled manufacturer limit. In the shorter sections the measured attenuation is borderline or a failure; as I mentioned we are scaling the limits so that 15m testing limit is 1/5th of the 75m limit for example.
When we mocked up a very short loom (approximately 10cm) and terminated them into connectors, the loss through that assembly was MORE than the 2.7m terminated section.
It appears that the loss contributed by the connector inserts is not static. In fact, it seems the measured attenuation is a function of the ratio of “connector medium” to “transmission line medium”; the shorter the loom the more of an impact the connector has on the attenuation.
Is what we are seeing representative of typical transmission line characteristics? How can this behaviour be expressed/calculated?
From my studies I am familiar with the concept of standing waves, reflections and "ringing" due to impedance mismatching and I am aware there are a number of standard transmission line equations but I’m not sure how those apply here (see the first paragraph .
Absolutely any help would be appreciated. We are attempting to negotiate more lenient tolerances for the shorter looms that keep failing but we would like some basis for our recommendations.
Simon Thomas