I just tried my method of measuring L and C and indeed it does not work very well (at least not when measuring with audio frequencies). I have two 4.5 foot lengths of cable, one RG-58C/U (50 ohms) and the other RG-59B/U (75 ohms), both terminated at both ends with BNC connectors. For the 50 ohm cable, I measured C=3D145.2pF and L=3D1.05uH at 10kHz, using a bench top high quality digital LCR meter (Danbridge CT-20). The C indicated a high Q while the L had such a poor Q that the meter auto-detected it as a resistance with parasitic series inductance. The far end was shorted with a shorted female BNC connector for the L measurement. For the 75 ohm cable, I measured C=3D96.8pF and L=3D1.3uH. sqrt(L/C)=3D 85 ohms for the 50 ohm cable and 116 ohms for the 75 ohm cable. Correct direction (75 ohm is larger), but not even the correct ratio of impedances. The problem is the L measurement. The theoretical C per foot for RG-58C/U is 30.8 pF/ft, which would be 138.6pF for this 4.5 foot length. My measurement is only 5% off. For the RG-59B/U, the value should be 20.5pF/ft or 92.3pF for this cable. I was once again about 5% high. This is no doubt partially due to the BNC connectors and parasitics in the leads from the test jig to the BNC connector. I tried to obtain a value for the inductance external to the cable by connecting my BNC "short" directly to the leads going to the test jig and I got 0.6uH, which gives you an idea of the magnitude of the parasitics involved here. Subtracting 0.6uH from each of the two above L values and re-computing Zo for each gives 55 ohms for the 50 ohm cable and 85 ohms for the 75 ohm cable. Much closer but still very hand-wavy when I have only one (partially) significant digit in my L measurement after subtracting-out the stray inductance. I bet that I could do this much more accurately with my 1MHz HP4271A LCR meter, but I have not yet gotten around to making a test jig for it so I am not even sure if it works properly :( Sean On Tue, Nov 29, 2011 at 4:43 PM, Richard Prosser wrot= e: > To get the sort of accuracy you're asking for, you really need a > network analyser of some sort. The impedance will vary with frequency > - especially from audio frequencies through to VHF etc. =A0The main > component of this is skin effect, where the inductance changes > slightly as the centre conductor moves into its RF region. The outer > conductor (screen) impedance is also likely to change somewhat. There > is also an effect known as "structural return loss" > which is due to manufacturing variations in the cable. =A0Where these > variations correspond to a 1/2 wavelength the cable impedance can > change markedly as the cable starts to operate as a filter. Obviously > manufacturers try to minimise this but it can be s real problem with > the cheaper grades of coax. > > For an overall appraisal the TDR method is probably going to be the > easiest to implement with the cable terminated in it's nominal > impedance. But you may get different results when using it at RF. > > What sort of frequency rang is involved? =A0- is it used for data (i.e a > TDR method may be more appropriate anyway) or RF, in which case some > sort of network analyser is really needed. If it's to be used over a > narrow frequency band you may be able to get an idea with a bridge > and a signal generator but it's not going to be easy. > > RP > > > > On 30 November 2011 05:22, Kerry Wentworth = wrote: >> Harold Hallikainen wrote: >>>> I have a customer who wants to measure the characteristic impedance of= a >>>> length of coax. Actually, we know what it is supposed to be according = to >>>> the >>>> manufacturer, but they want to verify it. >>>> >>>> Test equipment available: high accuracy multimeters (5-1/2 digit?), ch= eap >>>> analog oscilloscope, relatively low frequency signal generator (sine, >>>> square, triangle) (top end is maybe 1Mhz), frequency counter, bench >>>> supply. >>>> >>>> Willing to buy high-precision R's or C's if needed. >>>> >>>> Accuracy required: hopefully a few percent, 10% is some help. >>>> >>>> I am really a digital guy, so this is a bit out of what I do. >>>> >>>> Any ideas? >>>> >>> >>> How about this? >>> >>> Drive the coax with a square wave through a resistor that is the same a= s >>> the suspected characteristic impedance (the driving resistance is not >>> critical here). Watch the coax input with a scope. Vary the resistance = at >>> the far end of the coax until the scope shows a square wave. With anyth= ing >>> other than the characteristic impedance terminating the coax, you shoul= d >>> see the reflection. If the driving impedance is the same as the >>> characteristic impedance, you should see only one reflection. In that >>> case, the square wave will either have a step up or down before settlin= g >>> at the proper voltage depending on whether the terminating impedance is >>> low or high. >>> >>> Good luck! >>> >>> Harold >>> >>> >>> >>> >> Also, the longer the cable being tested, the larger the reflection for a >> given rise time / mismatch. >> >> Kerry >> >> >> -- >> http://www.piclist.com PIC/SX FAQ & list archive >> View/change your membership options at >> http://mailman.mit.edu/mailman/listinfo/piclist > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .