[time-nuts] Thermal effects on cables

REEVES Paul Paul.Reeves at uk.thalesgroup.com
Wed Jan 25 05:59:43 EST 2017

Hi Attila,

I quite agree, I should have remembered that.... but I tend to use fairly small coax anyway at those frequencies as a matter of course and it didn't register as something that needed saying. My fault!
It can be quite difficult to get people to notice the difference between SMA and 3.5mm - I have often had to repeatedly remind some of our test technicians of the difference as they tend to think they are interchangeable with the unfortunate results you describe :-(    It is just as well that the 3.5mm cal kit for the HP8510 (still a fine VNA, if lacking a few of the new bells and whistles) is a work of art with DTIs for checking the mechanical setup of the connectors - they are not just for show!


Paul Reeves

-----Original Message-----
From: time-nuts [mailto:time-nuts-bounces at febo.com] On Behalf Of Attila Kinali
Sent: 23 January 2017 19:33
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Thermal effects on cables

On Mon, 23 Jan 2017 15:13:39 +0000
REEVES Paul <Paul.Reeves at uk.thalesgroup.com> wrote:

> Surely the impedance of the cable is only affected by the ratio of the 
> inner conductor and outer conductor diameters modified by the internal 
> dielectric constant, nothing to do with the frequency of operation. 
> You might well have problems converting the larger diameters down to a 
> suitable size for the connectors at the higher frequencies though....

If you are only looking at the impedance, then yes. But once you get to high frequencies, you get also multi-mode behaviour of the coax cables and connectors, which leads to dispersion. That's why people hardly use N connectors for GHz frequencies, even though the connector itself would allow it. For VNAs where even small phase shifts/instabilities due to multi-mode behaviour/dispersion are a no go, the connectors are usually 3.5mm (basically a precise version of the SMA), 2.9mm, 2,4mm etc and go down to even 1.0mm which can be spec'ed up to 110GHz.

BTW: the 3.5mm connector is one that you will find on many instruments that go beyond 1-2GHz. Unfortunately it looks exactly the same as an SMA connector and will mate with one. Even more unfortunately, mating it with an SMA connector will scar the connector and most likely move it out of spec (ie degrade it to a simple SMA connector). If it's just an adapter, you've only lost a bit of money (in the order of a few €100).
If it was the connector of your VNA/oscilloscope/..., you might need to send it in for an expensive repair.

			Attila Kinali

You know, the very powerful and the very stupid have one thing in common.
They don't alters their views to fit the facts, they alter the facts to fit the views, which can be uncomfortable if you happen to be one of the facts that needs altering.  -- The Doctor _______________________________________________
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