Full resolution (JPEG) - On this page / på denna sida - 1958, H. 8 - The Transformer Ratio-arm Bridge, by Raymond Calvert
<< prev. page << föreg. sida << >> nästa sida >> next page >>
Below is the raw OCR text
from the above scanned image.
Do you see an error? Proofread the page now!
Här nedan syns maskintolkade texten från faksimilbilden ovan.
Ser du något fel? Korrekturläs sidan nu!
This page has never been proofread. / Denna sida har aldrig korrekturlästs.
Fig. 6. Arrangement for a continuously variable reactance
control.
Fig. 7. Arrangement for a continuously variable
conductance control.
ease of operation is to use a single decade and a
continuously variable control for accuracies of the
order of 1 %. A second decade need only be
introduced when it is necessary to increase the accuracy
to 0.1 %, or when it is required to obtain very high
discrimination at balance to measure small changes
in impedance.
Correction of Standards
Since only one fixed standard per decade is
required, it is a simple matter to make each one
effectively pure. The technique adopted is illustrated
in fig. 8, applied to a capacitance standard.
At a given frequency a capacitor can be represented
by a pure capacitance Cs shunted by a resistance
Rl. The problem is to remove the effect of Rl leaving
only the pure quadrature component Cs. It is
therefore necessary to cancel the ampere-turns produced
by Rl in the current transformer. This is done by
feeding a current through a fixed trimming resistor
Rt into the opposite side of the transformer. The
current is adjusted to give exact cancellation by
adjusting the voltage at the wiper of the pre-set
potentiometer P. If the voltage applied to the capacitor
is E and a fraction of this, kE, is applied to the
trimming resistor, the effect of Rl is completely
removed when
, = N* Rt_
Nt’ Rl
Standards Multipliers
To keep the number of standards to a minimum
the in-phase and quadrature standards are chosen
to cover approximately the same range of impedance
at the operating frequency of the bridge.
Occasionally, it is required to measure a complex
impedance the in-phase and quadrature components of
which are of quite different orders. It is therefore
convenient to be able to shift the effective
impedance of one standard with respect to the other.
Since the voltage transformer is tapped to provide
the decade adjustment of each standard the current
transformer must be tapped to shift the relative ranges
of the standards. This is shown diagrammatically in
fig. 9. A pair of taps at, say, 10 turns and 100 turns
will give a 10 : 1 shift.
This facility must however be used with caution.
If a measurement were made where the quadrature
currents are in the ratio of say 1 000 : 1, perfect
standards would be necessary and there would have to
be no losses or stray capacitances associated with
the wiring. Any small out-of-phase current
introduced by the major component standard could give
rise to a considerable error in the measurement of
the minor component of the unknown impedance.
This will be dealt with in more detail under the
subject of three-terminal measurements.
(6)
The adjustment is independent of the voltage, and
therefore holds for all positions of the tapping
switch.
The effect of a reactance term associated with the
conductance standard can be removed in the same
way. At a given frequency the standard can be
regarded as a pure conductance shunted by a reactance. The
reactance causes an unwanted quadrature current to
flow in the current transformer. This can be
cancelled by a quadrature current of opposite sign, the
ampere-turns being made equal. It is generally
convenient to use a capacitor for the trimming control
and to give it the correct sense by connecting it
either to the unknown or standard side of the
transformer, according to the sign of the spurious
reactance.
Range Multipliers
It has been shown how the decade steps are
obtained by suitably tapping the standards winding of the
voltage transformer. The range of the bridge can
now be extended above and below that of the
standards by means of taps on both the voltage and
current transformers; as shown in fig. 10.
In the case of capacitance measurements a
convenient standard is 0—0.01 microfarads connected
between the 100 turn and 10 turn windings
respectively. Tapping the unknown windings at 1 000, 100,
10 and 1 turns gives the ranges shown in the table.
Voltage Transformer Tap Current Transformer Tap Range
1 000 1 000 0— 10 pF
100 1 000 0— 100 pF
100 100 0—1 000 pF
10 100 0—0.01 uF
10 10 0—0.1 \iF
1 10 0—1.0 pF
1 1 0—10 uF
Fig. 8. Technique for correction of standards.
1 112 ELTEKN I K 1958
<< prev. page << föreg. sida << >> nästa sida >> next page >>
