I measured a slab of copperclad FR4 as having a capacitance tempco around +900 PPM/degC. I wonder what would be the temperature coefficient of delay for a microstrip or stripline on FR4. Not only does the dielectric constant change, but a board will expand in x/y/z too.
I guess I could measure it, which could be a nuisance.
--
John Larkin Highland Technology, Inc
picosecond timing precision measurement
jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com
To leading order, the effective epsilon of microstrip is (1+eps_FR4)/2. Delay is approximately proportional to the square root of that (i.e. the refractive index n), so one expects that
n ~ sqrt((1+eps))/2
You can obviously do that out at length, but for SWAG purposes, it's easier to do it numerically, with eps = 4 and d/dT(eps) = 0.0036/K.
sqrt((1+4.0036)/2)/sqrt((1+4)/2)-1 = 0.00036, i.e. 360 ppm/K.
The in-plane thermal expansion coefficient is less than this, so at this accuracy level it can probably be ignored.
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics
160 North State Road #203
Briarcliff Manor NY 10510
hobbs at electrooptical dot net
http://electrooptical.net
I just measured one the other day. I got 75% of that value. Nice approximation.
--
Best Regards,
ChesterW
+++
Dr Chester Wildey
Founder MRRA Inc.
Electronic and Optoelectronic Instruments
MRI Motion, fNIRS Brain Scanners, Counterfeit and Covert Marker Detection
Fort Worth, Texas, USA
www.mrrainc.com
wildey at mrrainc dot com
Thanks, it's good to pass the sanity check at least occasionally. ;)
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics
160 North State Road #203
Briarcliff Manor NY 10510
hobbs at electrooptical dot net
http://electrooptical.net
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