04-05-2012, 12:39 PM
I think that the reason for the screened lead when measuring d.c is because the VTVM has a very high input impedance: an unscreened lead will pick up stray a.c. / r.f. fields and thus produce misleading indications on the meter. However, when a.c. / r.f. is being measured, that screening will become a capacitor - and thus shunt some of the signal away (the magnitude of which will depend on the cable: length and permittivity of its core insulation, and frequency of the signal) - again, producing misleading indications.
The use of different sockets for a.c. and d.c. is simply a way of simplifying the otherwise necessary complicated switching arrangements when changing between these two modes of measurement.
As for the 1 meg-ohm resistor being needed on d.c. but not a.c., I can only assume that the intrinsic design of the VTVM produces an 'ohms per volt' sensitivity on d.c. that is far higher than that when on a.c. - and this could be due to the necessary rectification internal to the VTVM when measuring a.c. / r.f. As I am sure you are aware, 'ordinary' moving coil multi-meters always have a lower ohms/volt sensitivity on a.c. than when on a d.c. measurement range: the inclusion of the rectifier circuit is usually at the core of this difference.
Al.
The use of different sockets for a.c. and d.c. is simply a way of simplifying the otherwise necessary complicated switching arrangements when changing between these two modes of measurement.
As for the 1 meg-ohm resistor being needed on d.c. but not a.c., I can only assume that the intrinsic design of the VTVM produces an 'ohms per volt' sensitivity on d.c. that is far higher than that when on a.c. - and this could be due to the necessary rectification internal to the VTVM when measuring a.c. / r.f. As I am sure you are aware, 'ordinary' moving coil multi-meters always have a lower ohms/volt sensitivity on a.c. than when on a d.c. measurement range: the inclusion of the rectifier circuit is usually at the core of this difference.
Al.
