Hi If you head down to your local big box store, they will happily sell you a thousand foot spool of RG-6 coax for next to nothing. If their prices are still to high, the auction sites will sell it for even less. It has a 75 ohm impedance and a bandwidth of several GHz. The rather convent formula of RT = 0.35 / BW then comes in. A 3.5 GHz cable will limit you to a 100 ps rise time. In all likelihood, you will be unable to generate a signal with this fast a rise time.
You also will have some loss effects in the cable that are frequency dependent. The calculation above assumes you have done a few tricks to take care of this. If not, to get a 10 ns rise time, you need to maintain a 35 MHz bandwidth. That works fine if you have a buffer every 500 feet. No tricks, just a CMOS buffer chip. As noted by others, it *is* coax. You need to drive it and terminate it with 75 ohms. At 35 MHz, a cheap 75 ohm resistor will do the trick just fine. At 3.5 GHz you may need to get a bit more careful. So is the 500’ limit an issue? I’d suggest that it’s not. Consider chopping up the spool in a binary series of 400, 200,100,50,25,12.5, 6.5, 3.25 feet. You now have a set of buffered lines that can be arranged to give you a nice set of 256 time steps. Yes, the delay of the buffers will get in the way a bit. The actual line lengths will be a bit shorter as the lengths drop. So how much delay do you get from a 400’ line? Velocity factor comes in here. Best guess is that your foam RG-6 has a 0.78 velocity factor. The "speed of light” in the coax is 78% of the speed of light in vacuum. Your 400 foot coax has about a 520 ns delay. Your stack comes out just a bit over 1 us. Bob > On Nov 24, 2015, at 9:04 AM, Thomas Allgeier <[email protected]> wrote: > > Hello, > > > > I have an ACAM GP22 TDC chip and evaluation board which I am looking at for > “work” purposes – I work for a company active in the weighing and force > measurement world. > > > > I should say from the start that I am new to time and frequency measurements > and not even an electronics engineer – but then I have been exposed to > high-precision electronics for the last 25 years hence have picked up some > dangerous degree of half-knowledge. > > > > We want to use this chip to measure the period of a square wave, of around 13 > kHz i.e. in the 70 µs range. As the application is potentially high-accuracy > we need to know the period to within 1 ns or better. > > > > In order to evaluate the chip I was planning to replicate John A’s experiment > with the coaxial delay line from the HP5370b – but as my interest is in > “measuring range 2” of the GP22 I need a delay of 500 ns or more (actually 1 > µs sounds a better start). This is the equivalent of a 200 m length of cable. > I fear trouble with this: Am I not getting unwanted inductivities if I use a > coil of that size? > > > > So, to come to the point: Am I pushing the concept of a coax delay too far > with 1 µs and are there other (simple/reliable) ways to achieve this kind of > delay? I have tried it with a shorter piece of cable (around 2 ns which is > measured in “range 1”), there I seem to get consistency virtually to within > 100 ps. But I need to know if the device sticks to this level of performance > when the periods are much longer, and thus measured in “range 2”. > > > > Thanks and best regards, > > Thomas. > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
