Marconi Instruments TF 2700 - 2.7 CONNECTION OF EXTERNAL A.C. SOURCE

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2.7 CONNECTION OF EXTERNAL A.C. SOURCE

The bridge may be energized from an external a. c. supplyat freq-

uencies between 20 cl s and 20 kcl s. The external source is connected to

the bridge via a jack plug inserted into the EXT A. C. socket; the action of

inserting the plug switches off the internaI oscillator.

Ideally the a. f. supply should be connected via an isolating transfor-

mer, such as Marconi type TM 7120 which is available as an optional acce s -

sory. The use of a transformer designed specifically for the purpose

eliminates the possibility of a false balance due to capacitive coupling when

measuring low impedances at high frequencies. The arrangement of con- .

nections with transformer TM 7120 is shown in Fig. 2.15. The earthy side

of the A. F. Oscillator output should be connected to the INPUT terminaI on

TM 7120 adjacent to the SCREEN terminaI. These two terminals should

then be linked to the -VE (chassis) terminaI of the bridge.

'"-

A.f. ' [::f:::~~ ~

-"1AT8 : ---

--o

I ---EXtAL

I SOCI£T

Fig. 2. 15 Connection of External A. F. Source

via Isolating T ransformer

The TM 7120 transformer has been designed to match an oscillator

output impedance of 600 a but may be used far other impedances provided

that for lower impedances the input power is limited to t;W. The lo ad on

the secondary will vary from 2.5 a on the lowest range of the bridge to about

1 1(1 on the highe st depending upon the function selected and the BALANCE

and LOSS BALANCE settings.

li a TM 7120 isolating transformer is not available, the next best

alternative is to use a battery powered audio oscillator isolated from true

earth. The case or earthy output of such an oscillator should be connected

to the sleeve of the jack plug, and the oscillator spaced about lO in (25 cm)

from the bridge. li only an a. c. mains power oscillator is available then

proceed as detailed above, piace the bridge clear of any earthed metalwork.

avoid handling its case while making the measurement and guard against

hum pick up from the test component.