determining cable capacitance

[powerplayj]

I have made some DIY cables and some seem to be soldered better than others from what I noticed with the presence / loss of higher frequencies.

Is there an easy way to determine cable capacitance even though my meter only shows nf?  Most cable capacitance values would be in the pf range so I was wandering what other options would I have?  Would a PC oscilloscope be sensitive enough to show upper frequency loss in higher capacitance cables?

[Paul Perry (Frostwave)]

The DMM on my desk here has lowest cap scale of 4nF.. but, it measures down to .001, using that scale, that is one pf.
So it is probably the case that your meter will do.

[powerplayj]

unfortunately mine does go below 1 nf

[R.G.]

Most cables are in the range of 10 to 50 pF per foot of cable. A long cable can get to 1nF. But if the finest resolution you can get is 1nF being the lowest order count, you're right, your meter can't measure it accurately.

So, can you think of other ways to measure the capacitance of your cable? If you have a 555, you could hook it up as an oscillator, use a known capacitance and measure that frequency, then sub in the cable for the known cap. The frequency will then be different. The ratios of frequencies would tell you the capacitance.

You could also do a capacitive divider. Run a signal into two capacitors in series. Measure the signal at the junction of the caps. The signal level divides by the ratio of the capacitors' impedances, just like with resistors. Then the signal level tells you the relative ratio of known to unknown cap.

Hook up an inductor to it. Measure the frequency of resonance by tuning an oscillator until it resonates. Then knowing the inductance, you can calculate the capacitance at that frequency.