Butler Oscillator

Fig. 4, Symbols for FET's and MOS FETs, e-sopf e-sopf m

Butler Oscillator Design

Fig. 5, Collins 100 kc calibrator.

Fig. 5, Collins 100 kc calibrator.

a tod a tod rn

Ftg. 6. Triode equivalent of Coffins 100 kc call brotor shown in Fig, 5.

Ftg. 6. Triode equivalent of Coffins 100 kc call brotor shown in Fig, 5.

SOO'KDOO p'

BhSC

SOO'KDOO p'

BhSC

Butler Oscillator Design

LI - SIEMENS Mil íaL,) «OONÍ2 MATERIAL .J>e T Pf?iN5AHY. 4 J. SECONOART

Fig. 8. Isolation amplifier added to Fig» 7,

LI - SIEMENS Mil íaL,) «OONÍ2 MATERIAL .J>e T Pf?iN5AHY. 4 J. SECONOART

Fig. 8. Isolation amplifier added to Fig» 7,

Butler Oscillator

Fig. 9. P-channel FET oscillator and isolation amplifier.

Fig. 9. P-channel FET oscillator and isolation amplifier.

the control electrode and source of current carriers. Fig, 4 shows the representations of FETs along with a tube symbol to demonstrate the likeness.

' lie \-channel is most like the vacuum tube because one applies positive voltage to its drain and negative bias voltage to its gate for amplifier operation. The P-channel types are the other way around with negative voltage on the drain and positive bias. (The MOS types—which stands for Metal Oxide Siliconare still rather expensive for amateur use.)

To illustrate how one can go directly from a w ell-established tube circuit to an equivalent FET circuit, lei's steal the 100 kc crystal calibrator from the Collins 75S1. The original is shown in Fig. 5, and its triode equivalent in Fig. C (we must draw the triode equivalent since we are going to replace the tube with a triode device). The only difference between these two circuits is that the output coupling |>I (lie original (electron coupling) has been left out. Let us now, again, redraw the circuit substituting an FET and lower the "plate'* voltage a bit to suit the Ej«.,, of the transistor as in

A parallel resonant circuit for the desired oscillator frequency is placed across the

"plate" load to allow sufficient dc to flow and to make a low impedance tap available for output.

The 8-50 pf trimmer capacitor can be adjusted a bit for each crystal to bring the oscillation frequency right to that which is stamped on the crystal can. In most cases, the output frequency will fail within the crystal tolerance with no adjustment necessary.

Since we had to dispense with the electron-coupling method of deriving the oscillators output, it would be well to add some output isolation to this FET equivalent This can be done by adding an amplifier stage, using a conventional silicon transistor, as in Fig. 8,

In the above example, the FET used was a Texas Instrument TIX-882, a germanium FE'I which was available for about $3.00 several years ago. This type (N channel) was used to

Butler Oscillator
Fig. 10. A Butler oscillator as analysed by sections.
Buttler Oscillator Follower
Fig. 11. In this circuit for crystals designed for series mode operation, an inductonce is added in parallel ta the crystal to resonate the holder capacitance ond prevent spurious oscillation.

allow the tube-circuit-conversion example to use a positive supply, for illustration purposes only. The TIX-882 is no longer available, but a silicon N channel FET could be substituted.

Since it is the P channel devices that are now available inexpensively (U146 and U147), Fig. 9 shows the circuit (again redrawn) for one of these.

The main intent in the foregoing section was to demonstrate how one can use FT 171, FT 241, FT 243, and similar surplus crystals with FET oscillators so that they oscillate as marked. However, in the more recently available surplus, there are some crystals which are designed for series-mode operation. Military types in the HClO/U (coaxial) case are for series operation: CR9 and CR 24. Also, many of the crystals in the HC6/U (hermetically-sealed metal cases with spaced, 0.050" pins) are for series operation: CRI9, 23, 25, 26, 28, 30, 32, 35, 45, 51, 52, 53, 54^ 65, and 75.

Some of these crystal units were designed for tube type oscillators of the ''Butler" type, but will work well in the type of oscillators shown in Fig. 2. The Butler oscillator achieves the low impedance driving source and low impedance input by use of a cathode-follower and a grounded grid stage respectively. The Butler oscillator is shown in Fig. 10, divided into its component sections. As can be seen, V, is a Colpitis Oscillator whose capacitive-tap does not return directly to the cathode but drives V2, a cathode follower. V2 drives the cathode of V1 through the low series-resonance resistance of X.

This method of utilizing the series-resonance frequency of a crystal with a tube circuit is the "hard way," although an equivalent FET Butler oscillator could be built. It is far easier to use a single bipolar transistor oscillator like one of those in Fig. 2. An actual circuit is shown in Fig. II, for a 24 mc crystal oscillator using a CR 24 coaxial crystal.

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