The Hatfield R.F. Bridge, Type LE 300/A is the modern development of the basic transformer ratio-arm bridge proposed by Blumlein and others and later developed by Mayo of the B.B.C. Research Department for use at radio frequencies. Further development by Hatfield Instruments Limited results in an extended frequency range over which the accuracy is well maintained, plus exceptional ease of operation. These features, plus the rugged construction, ensure that the Hatfield Bridge is equally suitable for industrial and laboratory use. Every effort has been made in placing the controls and providing comprehensive scaling to give direct and simple operation. To further this aim a legend plate is fitted to the top panel of the Bridge to provide the user with instant information without the need to refer to the Handbook. The Hatfield Bridge is calibrated directly in conductance, resistance, capacitance, inductance and reactance. Thus, at a glance the components of a complex impedance can be determined for two or three terminal networks. Plug-in terminals are featured and components, jigs, adaptors, etc. can be fitted in a fraction of the time taken with conventional screw terminals.

     A feature found only with the Hatfield Bridge is the built-in source and detector unit, Type LE 302. The source is a 1,592 Kc/s crystal controlled oscillator (2πf = l0⁷) whilst the detector is an A.G.C. tuned amplifier. The equipment is mains operated. The amplifier output is rectified and applied to the edgewise balance indicating meter on the front panel. The A.G.C. log characteristic clearly indicates the approach to balance since the pointer is never off the scale. At the frequency of the source and detector unit the Bridge is direct reading in terms of Inductance and Reactance. Where the source and detector has not been fitted as original equipment it may be fitted by the customer at a later date. In this event the source and detector assembly, Type LE 302, is supplied complete with the balance indicating meter. This can be fitted to the front panel following removal of the blanking-off plate.

     The Hatfield Bridge, Type LE 300/A can be supplied with or without source and detector equipment as required. The standard frequency is 1,592 Kc/s. Others can be supplied to order. Where the source and detector is not fitted or where it is desired to work at frequencies other than 1,592 Kc/s a signal generator with an output level of at least 100 mV and a receiver with a sensitivity of at least 5μV should be used.

This new bridge is a further development of the type LE 300/A and has the great advantage that measurements can be made with or without direct current or voltage polarisation of the object being measured. Furthermore, the source of voltage or current polarisation is not in series with the measuring circuit. Thus, the impedance of the external polarising source is eliminated from the balance equation. This desirable feature has the great virtue that the R.F. characteristics of inductors, varactors, diodes, transistors, resistors, transformers, etc., can be measured and variations of parameters noted with the application of a wide variation of direct current or voltage. For instance, semiconductor diodes can be measured in the forward direction with current up to 500 mA and in the reverse direction with voltage up to 100 volts. Transistors P.N.P. or N.P.N. can be measured over a wide range of D.C. conditions. Since the bridge will measure negative resistance, even Y 21 ' can be measured and transistor adaptors are not necessarily required providing the frequency of measurement is not too high.

     The new bridge is physically similar to the type LE/300 A and the only external difference is the provision of two additional sockets on the terminal board to which is connected the external polarising power supply. For most purposes dry batteries, a milliammeter and a suitable rheostat are all that is required for passing direct current through the object being measured. For voltage polarisation, dry batteries can again be used. Where a capacitor or reversed biased diode is being measured a voltmeter connected across the battery will indicate the voltage present across the component. Thus the component is free of any additional connections that could influence the measurement. The accuracy and range of measurements are the same as the type LE 300/A, but the frequency range is 100 kc/s—15 Mc/s.

1.0 GENERAL PRINCIPLES

     Most electronic engineers are familiar with the conventional universal impedance bridge, which will measure inductance, capacitance and resistance at an audio frequency, which is usually 1000 cycles or occasionally 1,592 cycles, so that ω = 10⁴.

     These bridges have resistive ratio arms, which are wire wound and it is this feature which limits the top frequency at which accurate measurements can be made. Furthermore, only two terminal measurements can be made. Various attempts have been made to design R.F. bridges using resistive ratio arms and one or two have appeared on the market. The first major step forward in design was made by the Research Department of the British Broadcasting Corporation during the last war, when the transformer ratio arm admittance bridge was developed.

     The principle of the tapped transformer ratio arm bridge is now well established and has been further developed by Hatfield Instruments Limited to a high degree of simplicity in use, together with exceptionally wide range and accuracy.

2.0 NOTES ON DESIGN

     Since the bridges measures the unknown as an admittance, the measured values will be given as a conductance in parallel with a susceptance, that is, G + jB and the dials should be calibrated in millimhos. However, this result is not in a form suitable for every day use, particularly in the case of susceptance, which is dependent on frequency. For this reason the dial which measures the susceptance is actually calibrated directly in capacitance and can be switched to read either Positive or Negative values. It will readily be seen that a negative capacitance is in fact an inductance and if the frequency of the source to the bridge is fixed at a suitable value the dial can also be calibrated in inductance and reactance. In order to further increase the usefulness of the bridge, the conductance dial is also calibrated in resistance. Thus, when an unknown is measured, the bridge directly gives at once the conductance, resistance, reactance and either capacitance or inductance according to which parameter obtains.

3.0 ADDITIONAL EQUIPMENT REQUIRED.

     The bridge requires a signal source having an output of at least 100mV into 75Ω. It should be well screened and the stray field should be less than one microvolt , Good class signal generators fulfil this requirement. A well screened receiver can be used as a detector and it should have a sensitivity of better than five microvolts. The generator is used modulated and telephones, loudspeaker, or output meter used to obtain a null balance.

     Hatfield Instruments Limited can supply a very compact source and detector having a crystal controlled fixed frequency of 1,592 Kc/s. An output meter is also supplied and the complete equipment is mains operated. The fixed frequency is so chosen that ω = 10⁷. The inductance calibration of the LE 300 bridge is correct for this frequency. A full specification is given at the end of this handbook.

4.0 CONTROLS

     The large dial on the left of the instrument is directly calibrated in conductance in black and resistance in red. Adjacent to it at the edge of the panel is its associated set zero control. Above and to the right of this dial is its associated range change switch which has six positions marked X 0.1, X 1, X 10 positive, and X 0.1, X 1, X 10 negative. These factors multiply the conductance dial reading. IT SHOULD BE NOTED THAN FOR RESISTANCE THESE FACTORS MUST BE REVERSED, THAT IS, FOR 0.1 READ X 10 AND FOR X 10 READ X 0.1

     The large dial on the right on the instrument is directly calibrated in capacitance, in black, and inductance in red. Adjacent to it are its associated set zero and range change control. The latter is a six position switch with three positions marked X 0.1, X1, X10 in each of two groups marked Positive and Negative. This arrangement allows both capacitance and inductance to be measured.

     Two twin sockets are provided to connect the source and detector. Because the bridge circuit is a linear passive network their function can be interchanged without loss of accuracy, when measuring passive unknowns.

     The left hand socket is engraved S and B and the right hand socket is engraved B and D. The sockets marked B are connected to the bridge. When the bridge is supplied with the built-in source and detector, Type LE 302, the sockets marked S and D are connected to the source and detector respectively. In this event two twin plugs, having internal shorting links are also provided. When these plugs are inserted into the sockets the internal source and detector is automatically connected to the bridge.

5.0 TERMINALS

The terminal panel is conveniently located on the upper face of the bridge panel, and has six sockets built into it. Three of the sockets are connected to intermediate tappings on the transformer ratio arms and provide additional multiplying factors of X 0.1, X 1 and X 10 for conductance and capacitance. FOR RESISTANCE AND INDUCTANCE -THESE FACTORS MUST BE REVERSED, THAT IS, READ X 10 FOR 0.1 AND X 0.1 FOR X 10. The other three sockets are marked N for Neutral, C for Common and E for Earth. Their use will be described in later sections. Into these sockets are plugged the special spring loaded terminals supplied with the bridge. A peg is also supplied which is intended for use with the 0.1 terminals, when switched to read 0 - 2.5 pF full scale. On this range the metal body of the plug which has capacitance to earth, causes the dial to read about 0.7 pF. The peg, which is an insulator, has a tapered flat on one side and will, therefore, lock a wire into the socket. The socket panel is very well suited for special plug-in jigs adapted for production testing. Additional plugs modified for such jigs are available on request.

6.0 RANGE OF MEASUREMENT

6.1.     The conductance dial is calibrated from 0 to 1 millimhos. By means of the range switch the reading can be divided or multiplied by a factor of ten. A further factor of ten in both directions is obtained by the use of the appropriate terminal. Thus conductance can be measured from 0 - 0.01 millimhos full-scale to 0 - 100 millimhos. full-scale. Where resistance is being measured the switch and terminal factors are reversed. Thus resistance can be measured from 10Ω to 10MΩ.

     Table '1' shows the switch and terminal positions for measuring conductance:-

6.3     It will be observed that in both the tables and in tables 3, 4 and 5 to follow that there are other combinations of terminals and switch positions, which can be used.

     For instance, where terminal X 1 is used with switch position X1 it would be possible to use terminal X 10 in conjunction with switch position X 0.1 to obtain the same range of measurement. However, in such a case the accuracy of the minor component would be reduced. This is explained more fully in the section dealing with the measurement of capacitance.

6.4.    The capacitance dial is calibrated 0 to 250 pF. The associated range switch multiplies or divide the reading by a factor of ten. Again a further factor of ten in both directions is obtained by the use of the appropriate terminal. Thus capacitance can be measured from 0 - 2.5 pF full-scale to 0 - 25 000 pF full-scale; either Positive or Negative.

     Table '3' shows the switch and terminal positions for measuring capacitance:-

For other combinations of terminals and switch positions see sub-section 6.3

     It will be remembered, however, that the upper limit of susceptance, which the bridge will measure, is nominally fixed at 100 millimhos. A capacitance of 25 000 pF reaches this value at a frequency of approximately 670 Kc/s. Therefore, above this frequency the capacitance that can be measured is proportionately reduced. There is a further restriction in measurement above 1.5 Mc/s and this is explained more fully in the section, Measurement of Capacitance.

6.5.    Adjacent to the capacitance calibration is the red scale calibrated 0 to 1000. This scale is used for the measurement of both inductance and reactance. It will be observed that the product of the two scales anywhere under the cursor is always 10⁴. Thus 40 × 250 = 10⁴. This value is for microhenries and picofarads and for fundamental units is 10^-14. Since ω²LC = 1 and we have seen that LC = 10^-14 it follows that ω² must be 10¹⁴ or ω = 10⁷. This value of ω gives a frequency of 1592 Kc/s and, therefore, if the frequency of the source is adjusted to this value the red scale will read directly in inductance. Table '4' shows the switch and terminal positions for measuring inductance:-

For other combinations of terminals and switch positions, see sub-section 6.3.

     It will be observed from table '5' that reactance can be measured down to 4Ω for ω = 10⁷. This value of reactance corresponds to a susceptance of 250 millimhos and will have an error of approximately 5%. This error can be corrected and is fully shown in the section, Measurement of Reactance.

7.0 OPERATING INSTRUCTIONS

7.1 Choice of Terminal

     The value of the unknown being measured is obtained by multiplying the dial reading at balanced by the appropriate switch range and the terminal factor. The errors in the bridge are a minimum when using the X 1 terminal and the range switches are set to X 1. It will be obvious that a X 1 factor could be obtained by using the 0.1 terminal and setting the range switch X 10. Generally the 0.1 terminal should be used for high impedance, and the X 10 terminals for low impedance. When complex impedances are being measured the major component should determine the terminals to be used.

7.2. Earth Terminal

     The earth terminal is connected internally to the chassis and case. Where unbalanced impedances, having appreciable capacitance to earth, are being measured the earth terminal should be connected to the common terminal. The unknown should then have the earthy side connected to the common terminal.

7.3 Neutral Terminal

     The bridge network is centred on a neutral plane which has no D.C. connection to earth. The neutral plane is brought out to the neutral terminal. For many purposes it is convenient to earth the neutral terminal. This will occur when measurements are desired on three terminal networks, for which see section 12.1

7.4. Common Terminal

     For all measurements one side of the unknown must be connected to the common terminal, whether the measurement be on a two or three terminal network. When measuring an unbalanced two terminal unknown, connect the common terminal to the earth terminal. When measuring a balanced two terminal unknown, neither the common or neutral terminal is earthed.

7.5 Initial Balance

     Before any measurements are made and before the unknown is connected, the bridge must first be balanced. The correct procedure is as follows:-

(a) Connect earth terminal to neutral or common terminal if required.

(b) Adjust range switches to appropriate settings.

(c) Set both dials to zero

(d) Now, by using the set zero controls, balance the bridge for a null. It will be found an advantage to reduce : the output from the source (or reduce the R.F, gain of the detector), while searching for the null, since it is easily masked by overload of the detector.

(e) Connect the unknown and adjust the main dials for balance.

     If it is found that the choice of switched range was incorrect and that the range has to be changed, disconnect the unknown and rebalance the bridge again before making the final measurement. For a given position of the range switches, the set zero controls will not change appreciably with the frequency from 15 Kc/s to 5 Mc/s and only slightly up to 15 Mc/s.

8.0 THE MEASUREMENT OF RESISTANCE AND CONDUCTANCE

8.1.     Where the resistor to be measured is a small component the earth terminal is not used. As an example consider the measurement of a small carbon resistor known to be somewhere between 50 and 100 Ω at a frequency of 1 Mc/s.

8.2.     After balancing the bridge and making measurements, the dials in conjunction with the range switches, give a direct reading of the unknown. In our example let the conductance range switch be set to X 10, the X 10 terminal is used and the conductance dial reads 0.14 millimhos. The combined range factor of the related switch position and terminal will be one hundred. Thus 100 × 0.14 = 14 millimhos. Since the dial is also calibrated in resistance the reciprocal can be instantly read which in this case is 71.5Ω. In order to obtain balance, the reactance dial will have been adjusted and since such a small resistor would have a capacitance of only 0.5 pF approximately, the reactance switch will have been set to X 1 capacitive and the capacitance dial will read very nearly zero.

8.3.    Small carbon resistors from about 50 to 2000 Ω do not change appreciably with frequency and have sensibly constant values up to approximately 15 Mc/s. Higher values will decrease as the frequency rises. This is characteristic of carbon resistors and is greater for moulded types than for film types. A moulded resistor of 470K may only measure about 300 K at 15 Mc/s, whereas a 470K high stability resistor will measure about 400 - 420 K. On the other hand wire wound resistors increase in value with frequency. Some vitreous enamelled wire wound resistors increase over three times in value between 1 and 15 Mc/s. Most resistors will exhibit a minor component which is capacitive and usually only 0.5 to 2 pF. When measuring a resistive component which is lower than 20Ω at frequencies in excess of 10 Mc/s, the residual inductance of the bridge at the X 10 terminal will begin to become noticeable. A ten ohm resistor measured at 15 Mc/s will appear inductive and connecting leads of only 1/8" will add to the error.

9.0. MEASUREMENT OF CAPACITANCE

9.1.    Since the measurement of capacitance will always be associated with resistance it will be necessary to choose a suitable range factor for the resistive component. In the measurement of capacitance it will usually suffice to set the conductance range switch at X1. The switch and terminal positions, to obtain a desired range of capacitance, have been shown in table '3' sub section 6.4. It will be observed that there are other combinations of terminals and switch positions, which can be used. For instance, where terminal X1 is used with switch position X1 it would be possible to use terminal X10 in conjunction with switch position X 0.1 to obtain the same range of measurement. This can be done if so desired at frequencies up to approximately 1.5 Mc/s. depending on the magnitude of the capacitance. However, above this frequency the accuracy of the minor component (resistive) rapidly decreases.

9.2     When the unknown is the low loss capacitance such as a silver mica or air condenser, the power factor will be better than 0.001 which is nearly the limit of the measurement in the bridge. A power factor of 0.001 corresponds to a phase shift of only 3 seconds, an exceedingly small angle of measurement. Furthermore, the difference in magnitude between the sine and cosine flux in the output transformer would then be only one part in two million. For an air condenser having a power factor of say, 0.0001, the difference would only be one part in two hundred million. When using the combination of range and terminal referred to earlier in this handbook, the phase shift varies from two to three seconds, which may be positive or negative. Other combinations of range and terminal cause much larger phase shifts both positive and negative and are not recommended where the measurement of the minor components is important.

     The specified limit on Tanδ is ±0.0015. This value corresponds to a shunt resistance of ± 75 000Ω across a condenser of 0.01μF at 300 Kc/s, or a similar shunt resistance across as condenser of 200 pF at 15 Mc/s.

     When measuring low loss capacitors it may be necessary to adjust the set zero conductance control to obtain a null, should the conductance appear to be slightly negative.

9.3.     In sub-section 6.3 it was pointed out that the upper limit of susceptance that the bridge will measure is nominally fixed at 100 millimhos. A capacitance of 25 000 pF reaches this value at 670 Kc/s. Therefore, above this frequency the capacitance that can be measured is proportionately reduced. Ideally the maximum value that could then be measured would decrease with frequency until at 15 Mc/s the capacitance is 1000 pF.

     However, there are further limitations due to (a) the self inductance of the unknown and (b) the residual inductance of the bridge. Looking in at the common and X 10 terminal this residual is 0.02 μH. At the X1 and 0.1 terminals the values are slightly higher but produce no significant error because the capacitance to be measured on them should not exceed those shown in table '3'. When using the X 10 terminal the-measured values of capacitance will be high by a fractional amount ω²LC, where L = 0.02 μH.

     It is convenient to rearrange this expression to read, percentage error = 8f²C/10⁵ where f is the measurement frequency in megacycles and C = the measured capacitance in pF. A few examples will now illustrate the order of correction. For f = 5 Mc/s and C = 400 pF deduct 0.8%. For f = 15 Mc/s and C = 100 pF deduct 1.8%. Thus for a susceptance of 100 millimhos (1000 pF) at 15 Mc/s one must deduct 18%. These corrections do not take into account the self inductance in the measured specimen, which may be significant at the higher frequencies.

10.0. THE MEASUREMENT OF INDUCTANCE

10.1     Since the bridge measures the unknown as an admittance, an inductor will be presented as a conductance in parallel with a negative capacitance, i.e. a value of capacitance which has an equal reactance to the unknown at the frequency of measurement. In order to arrive at the value of inductance, the frequency must be known. If the frequency is so adjusted that ω = 10⁷ the red scale on the capacitance dial will read directly in microhenries. This is fully explained in sub-section 6.4. Reference to Table '4' in that section will show-the switch and terminal positions for the desired range of measurement.

10.2     The measurement of inductance is perfectly straightforward and the only corrections to be made are (a) when using the X 10 terminal and (b) the correction for self capacitance of the unknown. When using the X 10 terminal it is only necessary to deduct 0.02μH from the measured value (for ω = 10⁷). The correction for the self capacitance is given in Appendix '1' which will be found at the end of the book. Where the self capacitance is known the true inductance can quickly be found from the dial reading as follows:- for ω = 10⁷ the frequency of measurement is 1592 Kc/s. At this frequency the capacitance which tunes the inductor to resonance is clearly the sum of the self capacitance and the reading on the capacitance dial immediately above the value of inductance. An example will make this clear. Let the measured value of inductance be 80 μH. Above this value read 125 pF. Now assuming that the self capacitance is 10 pF the total tuning capacitance is 125 + 10 = 135 pF. Below this value on the capacitance dial read 74 μH which is the true inductance of the unknown. It will now be obvious that the self capacitance must be a small fraction of the tuning capacitance to prevent serious error, if there is no correction. Single layer solenoids do not usually have a self capacitance greater than 1 pF approximately. Small wave wound coils may be 3 to 10 pF. Where the self capacitance is an appreciable fraction of the tuning capacitance greater accuracy can be obtained by measurement at a lower frequency. In this event it will be convenient to make the frequency of measurement either 502 Kc/s or 159.2 Kc/s. In the former case the inductance calibration is multiplied by 10 and in the latter case it is multiplied by 100. Thus the scales are still direct reading. At any other frequency f2 the inductance calibration is adjusted by the factor (f1/f2)² where fl = 1592.2 Kc/s. Alternatively, the inductance can be computed from the equation L = 1/(ω²C) where C is the measured value of negative capacitance.
It must be emphasized again that the value of inductance measured is the parallel value, which is substantially the same as the series value provided that Q is greater than 10.

10.3     The measurement and effect of the resistance of an inductor will now be described,. Two quite different cases will be illustrated; the first, a small R.F, coil and the second, the admittance of a loaded transformer. Let the frequency of measurement be 1592 Kc/s. The conductance switch is set to X 0.1, the capacity range switch to X1 negative and the X1 terminal is used. The R.F. coil is measured and the conductance dial reads 0.055 millimhos. Below this value read l8KΩ. Since the range factor for conductance is 0.1 the conductance is 0.0055 millimhos, conversely the range factor for resistance is X10. Hence the resistance is l80KΩ. The capacitance dial reads 110pF. Below this value read 90μH. It is assumed that the coil self capacitance is not greater than 1pF. Thus the R.F, coil is measured as 90μH in parallel with 180KΩ. The series values can easily be obtained from the equation in Fig, '3'. To find these values the reactance must be known. This is easily done since the reactance can be read directly from the inductance scale. For f = 1592 Kc/s, the reactance is ten times the inductance. Hence, 90μH has a reactance of 900Ω. Further, since Q = Rp/Xp we have
l80 000 / 900 and the Q of the measured coil is 200.

10.4     Now consider the admittance of a loaded transformer. Let the frequency of measurement be 10 Mc/s. The conductance switch is set to X 10, the capacitance switch to X1 negative and the X 10 terminal is used. The transformer is bulky and unbalanced, so will have appreciable capacitance to earth. Therefore, it is necessary to connect the common terminal to the earth terminal before taking-measurements. The bridge is balanced and the conductance dial reads 0.14 millimhos. Since the range factor is 100 the conductance is 14 millimhos or 71.5Ω. The capacitance dial reads 10 pF negative. Since the capacitance range factor is X 10 the capacitance is 100 pF negative. The value of inductance is calculated from the expression L = 1/(ω²C) and is found to be 2.55 μH. In this case the Q is very low
and the series values can be obtained from the expressions in Fig '3'. These have been calculated and are as follows:- Rs = 59.5Ω and Xs = 26.5Ω. At 10 Mc/s this reactance is 0.424 μH. Thus, the transformer has an admittance comprising a conductance of 14 millimhos shunted by a negative capacitance of 100 pF. Alternatively, it can be considered as an inductance of 0.424 μH in series with 59.5Ω. All these values of course only obtain at the frequency of 10 Mc/s.

11.0. MEASUREMENT OF REACTANCE

11.1     It has already been explained in sub-section 6.5 and subsequently in sub-section 6.6 that if the frequency of measurement is so adjusted that ω = 10⁷ the red scale on the capacitance dial will read directly in inductance and reactance. It will be remembered that the inductance scale is multiplied by ten to give the value of reactance and further the values of reactance are identical for either capacitance or inductance. This very useful feature is further amplified and clarified by the following example. With the range switch positive a capacitor is measured and found to be 150 pF. The inductance that will resonate with it at 1592 Kc/s is on the red scale immediately below i.e. 66.5μH. Multiply this value by ten and we have also the reactance of the inductance and the capacitor i.e. 665Ω. The reactance will be directly proportional to fl / f2 where fl = 1592 Kc/s. Note, however, that if the range switch had been set at negative an inductance would have been measured and the reactance at any other frequency f2 would be proportional to f2 / fl .

11.2     In table '5' it will be noticed that on the X 10 terminal the reactance scale reads 4 to 100Ω. Since the bridge has a residual inductance of 0.02μH on the X 10 terminal it has a residual reactance of 0.2Ω at ω = 10⁷. Where positive capacitive reactance is being measured add 0.2Ω to the value read on the dial. Thus to a dial reading of 5Ω reactance add 0.2 making the true value 5.2Ω. However, where negative capacitive reactance (inductive) is being measured deduct 0.2Ω from the dial reading to obtain the true value. It will be obvious that for reactance greater than 20Ω the correction need not be applied since the error is then less than 1%.

12.0 THE MEASUREMENT OF A THREE TERMINAL NETWORK.

12.1     To illustrate the measurement of a three terminal impedance consider the common case of a condenser in a metal box. Reference to Fig. 2a shows such a condenser where it is desired to measure Cl at terminals tl and t2. It will be obvious that without use of the neutral terminal, the result will be Cl + C2 C3 / (C2+C3).

     If, however, the case is connected to neutral as at Fig. 2b C2 and C3 do not enter into the measurement. Fig 2c shows that C2 shunts the input transformer, and C3 shunts the output transformer. Since both transformers are of a very low impedance, these shunt capacitors are of no significance and do not affect the balance of the bridge.

     This new R.F.Bridge has the great advantage that measurements can be made with or without direct current or voltage polarisation of the object being measured. Furthermore, the source of voltage or current polarisation is not in series with the measuring circuit. Thus, the impedance of the external polarising source is eliminated from the balance equation. This desirable feature has the great virtue that the R.F. characteristics of inductors, varactors, diodes resistors, transformers, etc. can be measured and variations of parameters noted with the application of a wide variation of direct current or voltage. For instance, semiconductor diodes can be measured in the forward direction with current up to 500 mA and in the reverse direction with voltage up to 100 volts.

     The new bridge has the same specification and range of measurements as Type LE 300/A. The only external difference is the provision of two additional sockets on the terminal board to which is connected the external polarising power supply. For most purposes dry batteries a milliammeter and a suitable rheostat are all that is required for passing direct current through the object being measured. For voltage polarisation, dry batteries can again be used. Where a capacitor or reversed biased diode is being measured a voltmeter connected across the battery will indicate the voltage present across the component. Thus, the component is free of any additional connections that could influence the measurement.

     It will be appreciated that it is a serious disadvantage to insert D.C. into the unknown from an external source. The series impedance of the supply will, in such a case, be in series with the unknown. At radio frequencies, a capacitor can be placed in parallel with the source resistance, but its impedance is not likely to be less than a few Ω and in the measurement of semi conductors this could well be prohibitive. It will be observed from the circuit diagram supplied with this Handbook that the direct current or voltage is introduced in the neutral line. Therefore, no external connections have to be made to the unknown to introduce polarisation. By the use of blocking condensers the polarising current can only flow through the unknown and, therefore, the polarising current supplied by the D.C. source is, in fact, the current flowing through the object under test. Since the unknown is directly connected to the bridge and the polarising source is injected to the neutral line the balance equation is not in any way affected.

INSTRUCTIONS FOR USE

The bridge is used and set up exactly as described in the Handbook for the Type LE 300/A.

Measurement of Inductors

     Connect the inductors to the bridge and connect a suitable D.C. source, which is metered, to the sockets marked D.C.. In many cases a simple rheostat battery and meter is all that is required. The inductor is then measured in the usual way. The D.C. supply can then be switched on and as the current is increased in required steps any change in inductance is immediately noted when the bridge is re-balanced.

Measurement of Semiconductor Diodes.

     If it is desired to measure a diode in the forward direction, connect the metered D.C. source to the D.C. sockets. Connect the diode to the bridge in the usual way and make the measurement. Switch on the D.C. supply and set to desired current. The bridge is re-balanced and the measurements noted. If it is desired to measure the characteristics in the reverse direction, connect a suitable voltage source to the D.C. sockets. It should be noted here that the sockets have no polarity. The source can be connected either way which will then change the sign of the voltage applied to the diode. Successive measurements can then be made for different voltages. The maximum voltage which may be applied is 100 V. This facility is particularly useful in the measurement of varactors. The maximum current that can be passed into the D.C. sockets depends on the sockets used on the terminal panel which is located on the top of the bridge. The maximum permissible currents are detailed below:-

GENERAL

     Measurements can be conducted over the full frequency range i.e. 15 Kc/s to 15 Mc/s with or without the direction application of D.C. At any time during r.f. measurements one can always determine the effect of polarisation by merely applying a polarising source to the D.C. sockets. It will also be appreciated that it is possible if required to measure the unknown at one frequency say 1 Mc/s and inject A.C. to the D.C. sockets at some other frequency. It might be required for instance, to measure the inductance of a ferrous cored inductor at 1 Mc/s with 1 Amp at 50 c/s also flowing through the inductor. This could easily be done since it is only necessary to apply an external 50 c/s source to the D.C. sockets. In such cases the non-linearity due to the ferrous core would cause the inductance to change as a function of the 50 c/s A.C. field. This would tend in some measure to obscure the balance. In the case of ferrite materials the non-linearity does not cause the balance to be unduly flat.

The Measurement of Negative Resistance.

In order to further increase the versatility of the R.F. Bridge, Type LE 300/A the design now incorporates a provision for the measurement of negative resistance, The conductance range switch now has six positions, three of which are positive and three are negative. The instructions in the Handbook from 6.0 to 6.3 now apply for both negative and positive values of either resistance or conductance.

     It will be found that when using the Transistor Adaptors particularly for the measurement of Y 21' the measurement of negative resistance is essential, and it was this requirement in particular that prompted the re-design of the R.F. Bridge, Type LE 300/A.

	CAPACITANCE
	Positive	Negative
X 0.1	A → (c)	A → (d)
	D → (d)	D → (c)
X 1	A → (b)	A → (e)
	D → (e)	D → (b)
X 10	A → (a)	A → (f)
	D → (f)	D → (a)

C0 = 

N = 

C1 = 

C2 =