Oscillators, Converters, Mixers [Transistor AF and RF Circuits (1965)]

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A single transistor is used in this audio-oscillator circuit, which can be used for code practice by inserting a key in series with a 12-volt source as shown.

------ Courtesy CBS Electronic Div., CBS Inc. Audio-oscillator circuit.

The output from the circuit is about 300 mw, which is sufficient to drive a small speaker; the variable resistor, as shown, is a tone control that can be adjusted for operator convenience. It is possible to add an L-pad in series with the speaker voice-coil lead to provide a volume control if desired.


A power oscillator suitable for use with transistor type 2Nl491, 2Nl492, or 2N1493 is shown. These transistors can operate with a collector-to-base voltage of 30 volts, 60 volts, or 100 volts, respectively.

By using a 2N1493 in the circuit shown, about 0.6 watt may be obtained, using a collector voltage of 50 volts and a collector current of 25 ma. In operation, the transistor case is clamped to a copper chassis to provide a low thermal resistance path to the chassis, and insulating mica is not required since the collector is connected to the chassis. The output load for this circuit is 50 ohms.

C,-5-50 mmf, variable C2, C1-7-100 mmf, variable C4, C5-feedthrough capacitor, 1500 mmf C6-0.01 mfd L-3½ turns No. 14 wire, 1/4" diameter, tapped at 2 turns RFC,, RFC2 -radio-frequency choke, 10 µh


Courtesy Radio Corporation of America. 70-megacycle power oscillator.


A low-power oscillator operating at 70 megacycles and using transistor type 2N1491 is shown. Applications for circuits such as this include local oscillators or variable-frequency oscillators as used in transmitters. In these application the output power is not required to be very high, and power output in the range from 25 to 50 milli watts is normally enough.

The circuit as shown has a typical power output of 35 milliwatts and a maximum power output of about 50 milliwatts. It is possible to use types 2N1492 or 2N1493 if a higher power output is required.

R1-200 ohms R2-9100 ohms R3- 1500 ohms C1-20 mmf C2-30 mmf C3-22 mmf c,-18 mmf Cs, C6-0.01 mfd 1-5 turns No. 20 wire close wound on CTC LS5 form, tapped at 3 turns (white dot core)

RFC1-radio-frequency choke, 10µh


Courtesy Radio Corporation of America. 70-megacycle Colpitts oscillator.

A Hartley oscillator of the type that can be used for local oscillators or variable-frequency oscillators in transmitters is shown using a type 2N1491 transistor. With a 20-volt supply and a current of 9 ma, this circuit will provide an output of about 35 milliwatts, with a maximum power output of about 50 milliwatts.

If a higher output is required, a type 2Nl492 or 2N1493 may be used. For the 2N1491, maximum collector-to-base voltage is 30 volts; for type 2Nl492 it is 60 volts; and for 2Nl493 it is 100 volts.

R1 -200 ohms R2-9100 ohms R3-1500 ohms C,-4-24 mmf, variable C2-12 mmf f ------- @ OUTPUT C2 l (LOAD=~ OHMS) C1-15 mmf C4 , C1 -0.01 mfd L-3 turns No. 18 wire,¾" diameter, tapped at 2 turns

------ Courtesy Radio Corporation of America 70—giga-cycle Hartley oscillator.


Crystal-controlled transistor oscillators are used where a stable signal source is required, as in a fixed-tuned transmitter or receiver, or in a signal generator.

The circuit shows a type 2N1491 transistor in a crystal-controlled circuit with a 20-volt supply. This circuit can use overtone crystals in the approximate frequency range from 25 to 30 Mhz. As indicated, using a 2N1491 with a load resistance in the 50- to 100-ohm range the power output will be about four milliwatts. Normally, for this application this is a sufficient power output; types 2N1492 or 2Nl493 may be substituted if a greater power output is required.

R1 -200 ohms

R2-680 ohms 13 -9100 ohms c,-22 mmf

C2-20 mmf

C3, C4-0.01 mfd

CR1 -27 Mhz crystal

L1-15 turns No. 22 wire close wound on CTC

LS5 form (white dot core)

L 2 -2 turns No. 18 wire wound over cold end of L1


Courtesy Radio Corporation of America. 27-megacycle crystal oscillator.


This circuit shows a 100- khz crystal-controlled oscillator using a 2N1613 transistor. Both a square-wave and a sine-wave output are produced with excellent frequency stability.

This circuit is similar to a Hartley oscillator where positive feed back is obtained from collector to base using phase reversal in the tapped tank circuit of L1, L2, and C2. The oscillator frequency depends upon the resonant frequency of the series L-C network in the feedback loop. This network includes capacitor C3 and the quartz crystal operating in a parallel mode.

Capacitor C3 is made variable to act as a fine frequency control adjustment. There is a square-wave output from jack J2 since the collector is driven from cutoff to saturation, and a sine-wave output is available at jack J1 across the tank circuit made up of L1 and C1 .

------------------- Courtesy Fairchild Semiconductor Corp. 100- khz crystal-controlled oscillator.


This circuit shows a 2-meter converter using five transistors. This is a double-conversion 2-meter amateur radio band converter with an output frequency on 7 megacycles to be used with a communications receiver. There is an r-f stage, a first mixer, a crystal-controlled oscillator, a second mixer, and a second oscillator which is tunable.

Q1 is an r-f stage using a common emitter with the input and out put coils broad enough to pass the entire 4-megacycle-wide band.

It is tuned to 146 megacycles, which is the center of the band. Link coupling from 01 is fed to the base of mixer 02, operating as a common emitter. Oscillator injection is fed to the emitter of 02 from oscillator Q4, which is a crystal-controlled first-conversion oscillator.

This oscillator is in a common-emitter circuit with feedback through the crystal to the base. Output of Q2 is at 30 megacycles, and it is fed through coil L4, whose bandwidth must be 4 megacycles. L4 is capacitively coupled to transistor Q3, which is the second mixer stage, while Q1, provides a variable oscillator-injection voltage covering the range from 21 to 25 megacycles. This converts the incoming signal to 7 megacycles. Output of the converter is designed to operate into the base of the first mixer of a communications receiver.


L 1-5T #18 Buss 3/8" O.D. ½" Long 1T Input Tap

1 ½T Base Tap L 2-4½T #18 Buss%" O.D. ½"

Long 2T Collector Tap Secondary 2T #25 Soldereze Bifllar at Cold End L3-4T #18 Buss%" O.D. ½" Long

¼ T Tap for Injection

Secondary 1 ½ T #25 Soldereze Bifllar at Cold End L 4-8T #21 Soldereze on ¼" Form ~ White Slug L 1 -28T 10/44 Litz on ¼" O.D.

Form ~ White Slug Secondary 3T #28 Soldereze Bifllar at Cold End 15 Turn Collector Tap L 6 -16T #24 S.P.U. on ¼" O.D. Form ~ Red Slug 2T Tap for Injection L4 , Ls, and L 6-Cambridge Thermionic



A type 2N1493 is shown in this circuit as a self-excited oscillator operating at 250 megacycles. This circuit can provide an output of 150 milliwatts with an efficiency of from 15 to 20 percent at 250 megacycles. The output matching circuit is of the pi type which will reduce harmonics in the load.

c,-3-15 mmf

C2 -4-50 mmf

C4-0.6-5.5 mmf

C5-0.002 mfd

R1-400 ohms

RFC,, RFC2 -radio-frequency choke, 0.82 µ.h L-1 turn No. 14 wire, 1-inch diameter

C7, C1 -feedthrough capacitor, 1000 mmf


Courtesy Radio Corporation of America. 250-megacycle oscillator.

450 Mhz-TO-105 Mhz CONVERTER

This is a converter designed to convert a 450 -mhz radio-frequency input signal to an i-f output of 105 Mhz. The circuit includes a two stage amplifier, a 450-megacycle to 105-megacycle mixer stage and a 345-megacycle local oscillator.

There is a two-stage r-f amplifier using 2N2415 transistors with a power gain of 20 db, a noise figure of 4.5 db, and a bandwidth of 10 megacycles. The third stage is a 2N2415 mixer with a conversion gain of about 12 db; this circuit is fed from the 2N1407 local oscillator, as shown. The noise figure of this circuit overall is 5.0 db, and there is a gain from input to output of 32 db.


Courtesy Texas Instruments Inc. 450 mhz-to-105 Mhz converter.


This circuit shows three 2N2188 transistors used in a 50-mega-cycle converter. The input is to inductor L1, which is tuned to the signal frequency. The secondary of L1 is connected to the base of the common-emitter r-f amplifier ( Q1), and the output of this stage is coupled to the base of the common-emitter mixer stage (Q2). The oscillator is crystal controlled in a common-base circuit; it provides an injection signal to the emitter of Q2. The output of Q2 is developed across inductor L4. Because the oscillator is crystal controlled, the broadcast receiver working with this converter must be tunable. Since the entire broadcast band is only one megacycle wide,


… the crystal-controlled oscillator will allow only a one-megacycle portion of the 50-megacycle band to be covered. With a crystal frequency of 49.550 megacycles, there will be reception only from 50.1 to 51.1 megacycles in the 50-megacycle band. With a converter of this type it is necessary to use individual, different crystals to allow different 1-megacycle segments of the band to be received.

30-TO-5.5- Mhz MIXER

This circuit uses a 2N2188 transistor to mix a 30-megacycle input with a 35.5-megacycle oscillator frequency to provide a 5.5-megacycle i-f signal. A 50-ohm input is required for the 30-megacycle signal, and a 50-ohm input to T2 is required for the 35.5-megacycle oscillator signal. A 200-ohm load is required for the 5.5-megacycle out put i-f signal. The capacitor used across the output eliminates most of the 30- and 35.5-megacycle signals.

------------ 30- to-5.5-megacycle mixer. Courtesy Texas Instruments Inc.


This circuit shows a transistor type A-1383 (Amperex Electronic Corporation) used in a crystal oscillator operating at 76.8 Mhz. A power source of 11.5 volts is required, and the collector current is approximately 4.5 milliamps. The power output is approximately 2 milliwatts at the operating frequency of the crystal. Coil L1 is 8 turns of No. 18 wire wound on a ¼ -inch-diameter coil form.

------------ Courtesy Amperex Electronic Corp. 76 -mhz crystal oscillator.


This circuit shows a 2N2415 pop diffused-base germanium transistor operating as a 500-megacycle oscillator with a power output of 10 milliwatts from a power source of 15 volts. Capacitor C4 is adjusted for the proper frequency of operation, while capacitor C5 is used to adjust the power input.

----------------- Courtesy Texas Instruments Inc. 500 -mhz oscillator circuit.

250 Mhz-T0-60 Mhz CONVERTER

This circuit shows a six-transistor converter for conversion from 250 to 60 megacycles. There is an r-f amplifier, a mixer, a separate local oscillator with a buffer, and a two-stage i-f amplifier. Transistor type 2N2865 is used in all of these stages. Individual chassis were used in the construction of this circuit.

The 250-mhz r-f amplifier has a single-tuned input and output with about 12-db gain and a bandwidth of 13 Mhz. The 310 -mhz local oscillator is a Colpitts driving a buffer stage, which is a common emitter.

A matching network is used to connect the buffer stage to the mixer.

Conversion gain of the mixer is approximately 11 db. A single tuned network is used to match the 50-ohm cable to the base, while a double-tuned network at the output is used to provide a transformation to the 50-ohm cable driving the succeeding stage.

There are two neutralized, double-tuned stages in the 60-megacycle i-f amplifier with about 40-db power gain and a bandwidth of 10 Mhz.

The overall power gain of this converter is 69 db with a 5 -mhz band width and a 7-db noise figure. The sensitivity is 3.5 microvolts, and image rejection is greater than 40 db.




This circuit shows an 80- through 6-meter mobile converter using four transistors. It is designed to be used with an ordinary broadcast radio. Circuit parameters for 6, 10, 15, 20, 40, and 80 meters are shown in the circuit.

The converter uses a 2N2188 r-f amplifier, 2N2188 local oscillator, 2N2188 mixer stage, and 2N2188 beat-frequency oscillator. There is provision for the converter antenna, a 12-volt source, a broadcasting antenna, and the audio radio input.

The r-f amplifier (Q1) is operated as a common emitter, with a 50K resistor in the emitter circuit used as a gain control. The output of this stage is coupled from a tap to the base of the common-emitter mixer stage (Q2). Q3 is the crystal-controlled conversion oscillator operating in a common-base stage, grounded at the crystal frequency. It provides injection voltage to the base of Q2.

The output frequency from transistor Q2 is always in the broadcast band, and it is fed through the antenna terminal of the broadcast radio. In this way, the broadcast receiver is used as a tunable i-f stage so that the entire frequency of all the amateur bands below 10 meters may be tuned in, but only a little over 1 megacycle of the 10-meter and 6-meter bands can be covered. Transistor Q4 is a beat frequency oscillator to cover the entire broadcast band.


All coils and transformers wound on 9/32" coil forms. Cores are powdered iron.

Taps are referred to "cold" end of coil.

80 Meter coils are universal wound. All others are single layer close wound.

Mixer transformer L = 125 mh Primary 100 turns universal wound Secondary 60 turns Bifilar wound from start.

S1 Band Switch: shown in 80 meter position.

S2 Junction Switch: shown in off position.

BFO Coil 52 turns #40 wire universal wound Emitter tap at 12 turns Output tap at 5 turns

----------- Courtesy Texas Instruments Inc. 80-to-6-meter converter.

450 Mhz-to-105 Mhz CONVERTER

This circuit shows the six-transistor converter with a 50-ohm input and a 50-ohm output. Power requirements are 12 volts at 42 ma.

The input r-f amplifier (Q1) is designed for low-noise operation.

In the common-base circuit shown, there is a typical power gain of 13 db with a noise figure of 5.9 db. R3 is used for stability. If desired, a 2N2998 transistor can be used to replace the 2N2996 transistor in the r-f amplifier input. This will produce a 3-db increase in power gain.

The local oscillator (Q3) operates at 345 mhz, using a Colpitts circuit with 5 mw of output power. In this circuit, because a buffer (Q4) is used, less than 1 mw of power is necessary, and the buffer provides a constant load for the oscillator.

Q2 is the mixer where the 105-mhz i-f signal is produced. The impedance at the base is kept low for maximum performance. There are two stages of i-f amplification. The 105 -mhz i-f signal is coupled to the first i-f amplifier, which is a common-emitter circuit having a stable power gain of 20 db and a noise figure of 2.5 db. The second i-f amplifier is exactly like the first, and its output is fed to a 50 ohm load.

In this circuit, as shown, the 30-db bandwidth is 20 megacycles, while the 3-db bandwidth is 6.5 megacycles; the noise figure is 7 db with a power gain of 63 db.



----------- Courtesy General Electric Co. Code-practice oscillators.


Fig. A shows a code-practice oscillator using two 2N107 transistors and a 1.5-volt source. The frequency of the tone can be varied by means of the variable resistor, and either a pair of earphones or a PM speaker can be used in the circuit as shown.

Fig. B shows a 2N2160 unijunction transistor used in a different code-practice oscillator circuit. The voltage source is a 45-volt battery. A tone control for varying the audio frequency and a volume control are provided. A key is inserted in the jack in the circuit as shown; the three-position switch selects the phones, phones and speaker, or the speaker.

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