Harmonious Waves: Unveiling the Dynamics of the Pierce Oscillator Circuit

This article teaches us how to construct straightforward oscillator circuits utilising a single CMOS gate, such as the Hartley oscillator and Pierce oscillator. Both oscillators are examples of low-component-count oscillators that produce incredibly consistent and dependable frequency outputs.

Oscillator Pierce

A pierce oscillator circuit, like the one depicted in the following picture, can be readily constructed using a single CMOS gate and is based on a crystal oscillator architecture.

R1 is used to bias the single CMOS inverter to create a linear amplifier. Through the trimmer capacitor TCI, a crystal is visible linked to the piercing circuit's input and output.

The intended circuit is intended to function at the crystal's series resonant frequency. It goes without saying that no positive feedback has been applied in this instance between the circuit's input and output. This is a result of the input and output of the CMOS amplifier operating in the antiphase mode.

It may appear that the crystal is giving the amplifier negative feedback when there is serial resonance. Since C1 and C2 form a capactive centre tap around the crystal, where the centre tap is visible grounded, this may not actually be the case.

As a result, the crystal uses its two antiphase connections to function as a transformer while it is in the series resonance mode. As a result, we might observe a 180-degree phase change via the crystal, the two amplifiers, and positive feedback.

Although TC1 is included to adjust the oscillation frequency of the circuit to the nominal frequency of the crystal, this particular function is optional. Now that TC1 has been removed, the crystal can be connected straight across R1.

In this piercing circuit design, capacitors C1 and C2 can be shown to have values of 470 pF apiece. These parameters must allow the circuit to oscillate smoothly over a wide frequency range. In order to properly sustain the oscillation, you might want to reduce the relative values of C1 and C2 if the frequency is only a few MHz. As an alternative, you can choose slightly bigger C1 and C2 values when the frequencies are lower than a few hundred kHz.

The diagram shows an AND CMOS gate, but a buffer CMOS gate—like one from the IC 4050—can also be used.

Making Use of a FET

The Pierce crystal oscillator circuit has the advantage of not requiring any tuning modifications. A Pierce oscillator circuit built with a single 2N3823 (or 2N3821, 2N3822) field-effect transistor is shown in the accompanying figure.

The quartz crystal (XTAL) in this configuration is driven between the gate/input and drain/output phases of the FET. The purpose of the 2.5-mh RF choke (RFC1) is to keep RF energy away from the DC supply; it doesn't normally tune the circuit. The instant switch S1 is turned ON, the circuit begins to oscillate.

The capacitor C1 supplies the capacitive output coupling, making sure that the external load impedance is strong enough to prevent overloading the circuit and destroying the oscillations. The Pierce oscillator circuit uses about 2.3 ma of electricity from the 12 V DC supply while it oscillates at the crystal frequency.

In the circuit displayed, this was implemented at a frequency of 7 MHz. At this point, the amplitude of the RF output signal, without any load, is 6.2 volts RMS.

The crystal's fundamental frequency is where the Pierce circuit is intended to oscillate. Oscillation will therefore occur in the primary frequency of the crystal, rather than always in the designated (harmonic) frequency, if the crystal is of the harmonic type. Moreover, a very active crystal is needed for the Pierce oscillator.

Another Single FET Pierce Oscillator Circuit

The graphic above shows another simple Pierce crystal-controlled oscillator circuit. This circuit could be used as a marker generator to help with receiver alignment.

A single FET (field-effect transistor) BS170 is the active element that generates enough gain for the circuit to oscillate. Through the crystal, the input gate receives feedback from the drain (d) of the FET. The trimmer capacitor C1 adjusts the oscillator.

Oscillator Hartley

The Hartley oscillator, or simply LC oscillator, is a kind of frequency generating circuit in which the oscillation frequency is dependent upon a tuned circuit consisting of capacitors and inductors.

The widely used Hartley type oscillator can also be constructed with a single CMOS inverter. Compared to standard LC oscillators, this kind of Hartley oscillator circuit has a benefit in that the coil only needs one winding. Nevertheless, the winding of the coil must be centre tapped. The following figure shows the circuit diagram for a CMOS Hartley oscillator.

With the exception of using a center-tapped LC stage in place of a capactively center-tapped crystal, the ahrtley oscillator operates very similarly to the Pierce oscillator.

The circuit may function without a bias resistor thanks to the inductor L, which provides a D.C. path between the input and output of the CMOS inverter.

The circuit operates within a few hundred kHz to a maximum of 10 MHz of frequency. The values of L and C, which must be carefully chosen to fit the designated operating frequency range, will determine these frequencies.

If you would like the Hartley circuit to function as a variable frequency oscillator, you can adjust the value of capacitor C. Recall that the suggested tapping on coil L need not be precisely at the middle of the winding; for instance, the circuit may function properly even if L is replaced with the primary of an I.F. transformer.

Using a ferrite core and varying the number of turns, one can experiment with the coil L and measure the results using a frequency metre.

The Hartley diagram uses an AND CMOS gate; a buffer CMOS gate, like one from the IC 4050, can also be utilised.

Using a single Transistor

The following graphic shows an example of an oscillator single transistor circuit that is adjusted using a transformer and produces a sine wave (AF). This is actually a Hartley type oscillator circuit, in which the transformer's single center-tapped winding provides the tuning and feedback characteristics; the other winding functions as an output coupling coil as a result.

To build this Hartley circuit, you must first procure the small transformer T1 which is a 500 ohm to 30 ohm center tapped transformer. Meaning the circuit winding has 500 ohm and the output side winding has 30 ohm approximately.

The upper half section of the center-tapped primary winding of L1 acts like the base-input coil while the bottom half of the primary side of L1 act s like the collector output coil.

Capacitor C3 solely becomes responsible for tuning the oscillation on the primary side of the transformer. The frequency of the Hartley circuit is mainly established by capacitor C3 and the inductance of the total primary winding.

As indicated in the diagram if C3 value is 0.02 mfd, then the frequency will be roughly around 2 kHz. In order to raise the frequency, you may try lowering the C3 capacitance; to decrease the frequency simply increase the C3 capacitance.

To ensure that the circuit oscillates perfectly, the transformer winding should be correctly polarized as provided in the specs of the transformer by color dots.

Capacitor C2 does not have any role in the tuned circuit, yet it is positioned to prohibit the collector DC voltage appearing from the base of the transistor. The circuit provides an amplitude of 0.8 V RMS when the output is not loaded. The current consumption is 2 ma when a 6V Dc supply is used for the circuit

The figure above displays a contemporary RF Hartley transistor oscillator circuit. The values of L1 and C3 control the oscillator's working frequency. The placement of the tap on L1, which is typically between 1/5th and 1/4th of the total turns, determines the feed-back level.

This Hartley oscillator, for instance, will run at 5 MHz if the L/C combinations are as follows. L1 consisting of 20 turns of 18 SWG enameled copper wire, tightly wound over a 1 -inch plastic former. It has a tapping after five turns from the bottom.

C3 is a tiny adjustable capacitor which may have a highest capacitance value of 100 pF. The Hartley oscillator is a particularly well-liked circuit among circuit designers since it can function in the low audio range to UHF range with the appropriate L/C values.