[time-nuts] Measuring coax temperature coefficient with a TICC
Dr. David Kirkby (Kirkby Microwave Ltd)
drkirkby at kirkbymicrowave.co.uk
Wed Apr 19 06:10:05 EDT 2017
On 15 April 2017 at 02:34, Mark Sims <holrum at hotmail.com> wrote:
> I finally got around to using a TICC to measure the temperature
> coefficient of 100 feet of generic RG-58 coax using a TICC. The TICC was
> clocked by a HP 5071A 10 MHz output. The 1PPS output was connected to the
> input of the coax and the TICC chB input. The TICC chA input was connected
> to the coax output via an inline terminator. The TICC was set to "debug"
> mode and Lady Heather plotted the chB-chA timestamp difference (hence the
> negative cable delay values).
> The coax had been chilled down for 2 hours in a 5 degrees F in a freezer,
> connected to the TICC, and left to warm up in a 75 degree F room. Over
> the 10F to 70F temperature range (measured with an IR thermometer) the coax
> delay spanned around 300 ps... so figure around 5 ps per degree F (10 ps
> per degree C) for 100 feet of cable.
> I'm adding currently adding the ability for Heather to use an external
> temperature sensor...
I would not assume that a reel of coax that is coiled up will behave the
same as when used in in a lab environment in the usual way. So whilst this
might be an interesting experiment, I believe some caution would need to be
applied before assuming that such a measurement is representative of how
coax is normally used. When not on a reel, a heated coax is free to expand
radially with no external pressure force applied, apart from that due to
air pressure. When on a reel, that's not the case, as the coax is the
middle of the reel is going to have forces applied that are much greater
due to the mass of the coax. To take an extreme example, if you use a foam
dielectric coax, when on a real, the airgaps in the foam are likely to
become smaller as the cable will experience mechanical forces as it tries
to expand, constrained by the coax around it.
Also, when the coax tries to expand under heat, it is likely to deform to
take up the space between the turns on the reel, so possibly become more
Unfortunately cable is likely to behave very differently at 10 MHz than 10
GHz, so it not necessarily useful to repeat a measurement with a small
piece of coax at 10 GHz, where its phase change could easily be measured on
a vector network analyzer.
Another spanner in the works is that the impedance of coax (usually 50
Ohms), is given by the equation
Z=sqrt( (R + 2 pi f L)/(G + 2 Pi f C) )
where R is the resistance per unit length, L is the inductance per unit
length, G is the conductance per unit length and C is the capacitance per
unit length. The high frequency approximation is that 2 pi f L >> R, and 2
Pi f C >> G, so it simplifies to sqrt(L/C). Those two assumptions become
less valid at low frequencies.
Overall, what you are doing seems interesting, but I would question how
much the results will relate to real-world use of coax, where its not
normal to have great real of it.
Dr. David Kirkby Ph.D CEng MIET
Kirkby Microwave Ltd
Registered office: Stokes Hall Lodge, Burnham Rd, Althorne, Essex, CM3 6DT,
Registered in England and Wales, company number 08914892.
Tel: 07910 441670 / +44 7910 441670 (0900 to 2100 GMT only please)
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