In a recent post, Fred referred to the capacitance of a coaxial cable affecting a circuit under test, prompting me to restate:
“Every measurement will affect the circuit under test”.

I have previously spoken about voltage measurements where a 1000 ohm per volt meter is specified and the differences that can show up if a meter with different sensitivity is used.

A similar problem can arise with oscilloscope use, where the capacitance of the test lead can affect the test under way. If a coax cable is used to connect the scope, the stray capacity can be in the tens of pf and can cause detuning of any resonant circuit. This is why a variety of probes are available to minimise this effect. A typical passive probe will include a resistor to divide the scale by 10:1, but perhaps more importantly, will reduce the stray capacitance to 4-5 pf in a top brand (expensive) probe but more typically 10-12 pf in a more generally available probe.

Say for instance, you are looking at the anode of a mixer or IF amplifier and the IFT capacitor is 100 pf. By connecting the probe, you are effectively putting another 10 pf across the circuit, so if you try tuning that IFT, it will change when you remove the probe.

The task to remember is that this effect will occur and to take steps to minimise or avoid it. In the above example, if you want to tune the mixer anode transformer coil, you could put the scope on the IF amplifier or detector, so that it is isolated from the section you are adjusting.

Harold

Hi Gandhn,

Excellent post (as usual) ... I can add a few comments ...

Scope Probes ... sometimes the Coax of the Scope probe is deliberately made with a centre conductor of high impedance wire, hence it forms part of the voltage divider to achieve 10:1 etc.. So changing the coax or even its length can change the 10:1 ratio.

I usually listen to the radio or amplifier while probing with scope probe and if volume drops, then I know that I am detuning/loading the IF or Oscillator etc..

When tuning a radio, I disconnect the AVC/AGC and probe the negative grid voltage.

"Excellent post---" Aye, I second that.

Many years ago I discovered this effect the hard way, after spending far too long trying to track down a troublesome chroma fault in a PYE CTV. The colours would lose sync after many hours of operation, i.e. the 4.43 MHz oscillator would drift when hot.

I struggled with this fault and my boss was getting leaned on hard from the customer. This was a CRO only job, but that led me up that famous creek over and over----- The very act of placing the probe into the oscillator would bring out the fault. Remove the CRO probe and the fault would disappear. Again and again this happened. Eventually I pinned down the culprit (ceramic pF cap) but the CRO did hinder my fault finding overly.