Project - A 2Terminal Test Oscillator
Peter Cole DA1PE describes a 2-terminal test oscillator for inductance measurement
Anyone who is interested in home construction will at some time or other have the need to measure the inductance of small coils. This can be a difficult problem, as commercially made equipment of sufficient accuracy is often far too expensive for occasional use in an amateur workshop. The simple LCR bridges that are affordable generally only Work at audio frequency and are notoriously inaccurate on their Low inductance ranges. The result of this is that coil construction is often so hit-and-miss that for many applications the home constructor has to use expensive ready made coils, which could have been wound at home from materials costing a few pence if test facilities had been available.
The method of measurement described here is a inexpensive solution to the problem that I have been using for over 40 years. It uses the coil under test in parallel with a known 1% capacitor to form the tuned circuit for a 2-terminal test oscillator. After making allowance for circuit strays, the total circuit capacity and the oscillating frequency can be entered in the standard resonant circuit formula to calculate the inductance .of the coil accurately enough for all normal purposes.
Using this method has the advantages that a) coils can be tested in a practical circuit at the operating frequency and b) measurements are just as easy to make on toroids and sealed coils such as IF transformers as they are on open coils. An additional bonus is that the circuit can be used for a variety of other purposes such as a signal generator, a dip meter or to measure small capacitors.
The 2Terminal Oscillator
Fig 1a
Fig.la shows the basic circuit of an FET source coupled 2terminal oscillator, which is the semiconductor version of a valve circuit that was popular in the 1950s. This circuit, which was derived from an early radar pulse generator called a cathode-coupled multivibrator, forms a very active tuneable oscillator that can be made to work reliably from Low audio frequencies up to the VHF and UHF ranges.
The great advantage of this circuit, and the thing that makes it so useful as a test oscillator, is that it doesn't need any sort of capacitive tap, inductive tap, or feedback winding to make it work. Simply connect a parallel tuned circuit across the two test terminals, apply power, and away it goes. The only criteria are that the tuned circuit has a realistic LC ratio (a minimum of 1-2 pF per metre of operating wavelength is a good rule of thumb for the capacitor value), and is made from good quality components.
Operation of the circuit itself is quite straightforward. TR2 in Fig.la is an FET source follower that is directly coupled to the common gate FET amplifier TR1 by way of their shared load resistor R2. C1 provides a positive feedback path from the drain of TR1 to the gate of TR2 and this maintains oscillation at the resonant frequency of LC so long as the loop gain of TR1,TR2 is enough to overcome circuit losses.
Fig 1b
Fig lb is the same circuit rearranged to allow R1 to be increased in value from 10k to 2M2. This reduces damping of the tuned circuit which in turn extends the frequency range and gives more consistent operation with Iow-Q coils. The improvement given by this simple change is quite significant, as with cheap general purpose 2N3819 FETs my original circuit oscillated up to 15MHz whereas the modified one worked to over 25MHz.
The BF244A FETs suggested in the parts list are suitable to 30MHz or more, but operation at higher frequencies than this calls for VHF type FETs. Out of several readily available types that I tested, the best were J310s (listed as VHF/UHF oscillators/amplifiers) with which the oscillator worked from a few hundred Hz (with a homemade ferrite pot-cored coil plus lpF) up to 60MHz with a small hairpin loop tuned by 25pF (the stray capacity of my test oscillator).
Practical Circuit
Fig 2
Fig.2 gives the practical version of the tester, where TR1 and TR2 are connected as in the oscillator circuit of Fig lb. VR1 is a[ amplitude level control, that has been added mainly to allow the circuit to be used as a dip meter as explained later.
Output from the oscillator is taken from across the common source resistor R2, via C2, to the source follower TR3. This is used as a buffer amplifier to feed the output and monitoring sockets SKT1, SKT2 and the RF level metering circuit formed by D1 and D2. R7 is an isolating resistor included to minimize the effect on the metering circuit if a Low impedance load is connected to SKTI: The value of R7 is not critical as long as it is at least twice the value of RS. Likewise the value of C9 is not important and this can be varied to set the level of signal fed to the frequency counter, if one is used.
Construction
The prototype was built on a 0.1 inch matrix board, and this was used to produce the PC board layout shown. Thus it should be a simple task to copy the layout back onto matrix board if you don't want the bother of etching a PCB.
Construction of the test oscillator is quite straightforward, as there is nothing critical about the components or layout except for the wiring to the tuning capacitor and the collector of TR1. These connections must be of thick wire or braid, and kept as short as possible so as to reduce the chance of parasitic oscillations which can give false readings on a frequency counter.
Transistor holders are recommended for the FETs. This makes it possible to experiment with different types and also allows the circuit to be used as a simple tester for small-signal N-channel FETs. This is easy to do by replacing TR1 with the FET to be tested and checking for oscillation. But don't forget that the PCB shown is designed for use with BF244A FETs, and look up the leadouts of other types before you plug them in (e.g. the gate and source of the J310 are reversed to those of the BF244A).
The oscillator itself should be built into a well shielded enclosure, e.g. a small diecast box as suggested in the parts list. This makes a compact, rigid unit so that when used as a dip meter the coil can be coupled to circuits in 'hard to reach' places.
Choosing the best value for the variable capacitor (VC1) is a tradeoff between providing wide frequency coverage for use as a tuneable oscillator, and good bandspread for other measurements, These factors are also influenced to some degree by the quality of the tuning dial, but the suggested 10-75pF has been found to be a fair compromise when used with an 8:1 vernier reduction drive.
Coil and test sockets are very much a matter of availability and personal preference. I use a BnG valve holder and a pair of 2mm wander sockets only because they suit existing coils. Otherwise I would have chosen standard terminal posts which are more versatile as they accept wires, spade terminals or wander plugs.
M1 is a small edgewise VU meter of about 250pA FSD (mine was rescued from a scrapped cassette recorder). As it is not used to make any critical measurement’s the exact FSD and scale calibration of the meter are not too important. What is important though is that the pointer moves freely without sticking and it is wise to check this point carefully before cutting holes in an expensive diecast box to fit a salvaged meter.
Following on from last month. To test capacities with drive level. the oscillator it is best to use a ,.coil/capacitor combination that will tune to the Low or middle part of the HF range. Again, the exact values aren't at all important, but a coil of 50 turns of enamelled wire close wound on a 2cm diameter former (about 33pH) and a 100pF 1% capacitor make a useful test standard.
Start by setting VR1 to its minimum resistance, and the wiper of VR2 to about half way. Then with a scope and counter connected to SKT1 and SKT2 respectively, switch on. If all is well reset VR2 to give a full scale reading on the meter and then check that the meter level changes as VR1 is altered. Don't be surprised to find that the output waveform from SKT1 is very distorted and varies in shape with the setting of the level control. This is quite normal. At the same time the frequency will shift slightly due to changes in the FET internal capacities with drive level
Frequency Measurement
The easiest way to measure oscillating frequency is to use a' counter connected to SKT2. Hi accuracy is not necessary, but sure that the reading is stable as some counters (like my simple homemade one) are quite. temperamental when reading distorted asymmetrical like those generated by this oscillator. Another useful check is to see if the counter reads the with different settings, of the time base switch, it is not uncommon to find frequency doubling on ranges with non-sinusoidal in If you don't have a counter find out the frequency by searching for the oscillator signal on a coverage receiver. To do this the normal receiving aerial with a few cm of wire (to attenuate external signals) and operate the test oscillator alongside the receiver. Starting from a much higher frequency than you expect, search for a strong unmodulated carrier. When you find one identify it as the test oscillator either by switching on and or by moving your hand near to the coil to cause the frequency to shift slightly. Having found the oscillator signal on your receiver, listen also on the harmonics (2f, 3f etc) and sub harmonics (f/2, f/3 etc.) of this frequency to find out if you really are listening to the fundamental and not to an harmonic or an image signal.
Measuring Inductance
Once the oscillator is operating ) correctly, it is a simple matter to find the inductance of a coil. Put a convenient 1% capacitor (Ct) in parallel with the coil to be measured, connect this tuned circuit to the test terminal with the shortest possible leads, and measure the oscillating frequency. A reasonable estimate for the circuit stray capacity is 10pF plus the minimum value of the tuning capacitor. This must be added to the value of the fixed 1% capacitor to give the total capacity to be used to calculate the inductance. For example, with circuit strays of 25 pF and a fixed 1% capacitor of 100pF, the inductance is calculated using a 125pF in formula 3 of Appendix 1.
Greater accuracy
Whilst the above is good enough for most purposes, the accuracy can be improved by actually measuring the circuit strays. This is very easy to do as it entails only one extra measurement and a calculation using the procedure summarized in Appendix 2. Start by connecting a test coil to the oscillator, and with the tuning capacitor set to minimum, note the frequency (Fl). Next, connect a 1% capacitor (Ct) across the coil, measure the new (lower). frequency (F2) and substitute values of Fl, F2 and Ct in formula I of Appendix 2. This gives the total distributed capacity in the oscillator with the variable capacitor at its minimum setting.
By repeating this routine with different settings of the variable capacitor, the tuning scale may be calibrated in terms of capacity. However if you do want to do this, it is easier to use the procedure that is described later.
Computer program
Although the formulae given in the appendices can be solved easily enough on a pocket calculator, it is 'worthwhile using a computer if a lot of coils are going to be tested. Listing 1 is a program to find the inductance of a coil, and the circuit distributed capacity from frequency measurements using formula I of Appendix 2. Run the program and enter the values requested at the prompts: 1. Value of the added 1% tuning capacitor (Ct). 2. Oscillating frequency without Ct - (Fl). 3. Oscillating frequency with Ct added - (F2).
The result of running this program is a printout giving the inductance of the coil; the total distributed capacity, and a table of resonant frequencies for the coil with capacitors from 20 - 100pF in 5pF steps. If desired, these steps may be altered by changing the values in the loop between lines 280-320. In line 280 'X = 0 to 16' controls the number of steps while in line 290 '(2E-11)' is the 20pF starting point and '(5E-12)' gives the 5pF steps.
Measuring small capacitors
As well as being a useful indication of the tuning range of the coil, the data in the printout table may be used to calibrate the tuning scale of VC1 in terms of total circuit capacity. Do this by setting the tuning dial to obtain the frequencies corresponding to the capacity values in the printout: e.g. with VC1 set for oscillation at 5.028 MHz, circuit capacity = 30pF; at 3.894 MHz, capacity = 50pF and so on.
After this is done, it becomes a simple matter to measure small capacitors: 1. With a test coil connected to the oscillator terminals, set VC1 to a convenient capacity, say 50pF. 2. Measure and note down the frequency of oscillation. 3. Connect the unknown capacitor across the test coil 4. Reset tuning to give the frequency in 2. and note the new capacity reading. 5. Unknown capacitor is 50pF minus the capacity value in 4.
Alternatively, capacitors can be measured by using formula 2 in Appendix 2 which is also presented as Program Listing 2
Use as a dip meter
Even though the GDO / dip meter is highly regarded by radio amateurs, it is hardly ever used in a professional workshop or laboratory. This is largely because it needs skilled handling and careful interpretation of results if the measurements made .are to be relied on. Much of t, he difficulty arises because reliable oscillation, over the wide frequency range , expected of a dip meter needs a very active oscillator circuit, i.e. one with a large amount of positive feedback. ‘Unfortunately the high level of feedback tends to compensate for the power sucked out from the dip oscillator by the circuit being measured. This masks the dip so much that, even with greatly increased coupling, only the tiniest flicker of the meter needle may be noticed.
The 2-terminal oscillator is no different in this respect. Indeed, over most of the operating range it oscillates so strongly that a discernable dip can be found only when the oscillator and the tuned circuit under test are so' overcoupled that a double-humped tuning response results. This causes severe pulling of the oscillator tuning and makes it impossible to find the true dip frequency.
To overcome this problem, RV1 is used to reduce the loop gain of the circuit so that oscillations are only just maintained. For best results, you. may need to experiment with the settings of VR1. But I find that, after using the setting-up procedure described earlier, there is a clear dip with loose coupling when the meter is set just below mid-scale by VR1.
Conclusion
Although the principal function of this test oscillator is to measure the inductance of small coils for use at HF, it does have a lot of other uses. I've described some of them here, and a little brainstorming should reveal others. But even if it doesn't, the instrument will still be invaluable around the shack. Build one, and you will wonder how you have ever managed without it.
2-Terminal Test Oscillator: Program Listing 1.
This program is written to run with QBASIC On a PC compatible. Although line numbers are not needed for QBASIC they have been used to make it easier to convert the program to other Basics. PRINT '#######.###' is a formatting statement that can simple PRINT statement in earlier Basics.
100 CLS
110 PRINT
120 'CL-test.bas - P C Cole March 1993
130 'To find distributed capacity of 2-terminal oscillator:
140 PRINT "CL tests with 2-terminal test oscillator."
150 PRINT
160 PRINT "Tuning capacity(pF) ";: INPUT Ct
170 Ct = Ct * 1E-12
180 PRINT "Frequency at minimum C(MHz) ";: INPUT F1
190 F1 = F1 * 1000000
200 PRINT "Frequency at maximum C(MHz) ";: INPUT F2
210 F2 = F2 * 1000000
220 Cd = Ct / (iF1/F2) ^2 - 1)
230 L = (1 / (2 * 3.142 * F2 * SQR(Cd + Ct)) ^ 2)
240 PRINT USING "Inductance (uH): ########.###"; L * 1000000
250 PRINT USING "Cd(pF) : ########.###"; (Cd) * 1E+12
260 PRINT "Resonanc6 with
this coil:"
270 PRINT "C (pF)", "F(MHz)"
280 FOR x -- 0 TO 16
290 C = (2E-11) + (5E-12 * x)
300 F -- I / (2 * 3.142 * SQR(L * C))
310 PRINT USING "#######.###"; C * 1E+12; F * .000001
320 NEXT x
330 INPUT "Do you want another calculation ? ", AS
340 IF AS = "N" OR AS = "n" THEN GOTO 350 ELSE GOTO 150
350 END
A typical printout from running program 1
CL tests with 2-terminal test
Tuning capacity (pF) ? 100
Frequency at minimum C( Mhz) ? 5.83
Frequency at maximum C( Mhz) ? 2.49
Inductance (pH): 33.393
Resonance with this coil:
Cd (pF) F(MHz)
20.000 6.158
25.000 5.508
30.000 5.028
35.000 4.655
40.000 4.354
45.000 4.105
50.00O 3.894
55.000 3.713
60.000 3.555
65.000 3.416
70.000 3.291
75.000 3.180
80.000 3.079
85.000 2.987
90.000 2.903
95.000 2.825
100.000 2.754
