Pierce

The low profile CX miniature surface-mount crystal is ideal for small, battery operated portable products. The CX crystal designed in a Pierce oscillator (single inverter) circuit has a very low current consumption with high stability.

An economical and commercially available quartz temperature sensor, Y1 and IC1, an LTC-485 RS485 transceiver in transmitter mode, form a Pierce crystal oscillator. The sensor, an Epson HTS-206, presents a nominal frequency of 40 kHz at 25°C and a temperature coefficient of –29.6/ppm/°C (Reference 3). The transceiver's differential-line-driver outputs deliver a frequency-coded temperature signal over a twisted-pair cable at distances as far as 1000 ft.

The Pierce oscillator is one of the more popular circuits, and is the foundation for almost all single gate oscillators in use today. In this circuit, Figure 2, the signal from the input to the output of the amplifier is phase shifted 180 degrees. The crystal appears as a large inductor since it is operating in the parallel mode, and in conjunction with CA and CB, forms a pi network that provides an additional 180 degrees of phase shift from output to the input.

The Pierce oscillator shown above is essentially a common emitter amplifier with a tuned circuit for a collector load and a quartz crystal as a feedback element. In order to determine whether the Barkhausen criteria is satisfied, loop gain must be determined at the frequency of oscillation. This is accomplished by drawing the AC equivalent circuit of the Pierce Oscillator as shown below.

The Pierce oscillator shown above is essentially a common emitter amplifier with a tuned circuit for a collector load and a quartz crystal as a feedback element. In order to determine whether the Barkhausen criteria is satisfied, loop gain must be determined at the frequency of oscillation. This is accomplished by drawing the AC equivalent circuit of the Pierce Oscillator as shown below.

A simplified schematic of the oscillator circuit used in Chrontel products is shown in Figure 1. Note that the typical 2-pin crystal has been replaced by its equivalent circuit model. • Co is the pin-to-pin capacitance. Its value is associated with the crystal electrode design and the crystal holder. • Rs is the motion resistance. Its value is specified by the crystal manufacturer. • Cs is the motion capacitance and Ls is the motion inductance, which are not specified, and are functions of the crystal frequency.

The Pierce-gate oscillator of Figure 1 is well recognized by most designers, but few understand how to specify the crystal correctly. The crystal used in the topology of Figure 1 can be either a fundamental AT-CUT or BT-CUT. A BT-CUT crystal has poor frequency stability over temperature compared to an AT-CUT. This topology uses a parallel crystal and not a series crystal. When a parallel crystal is specified, the crystal manufacturer will also require that you specify a load capacitance.

The crystal used in the topology of Figure 1 can be either a fundamental AT-CUT or BT-CUT. A BT-CUT crystal has poor frequency stability over temperature compared to an AT-CUT. This topology uses a parallel crystal and not a series crystal.

The oscillator is un-tuned so the only tuning procedure required is to select the band (coil tapping) and peak it for maximum RF output. The valves used were selected because they had B7G bases and were the first I put my hand on when I reached into my junk-box. Virtually any pentode valves will work in this application. An EL84, for example, will deliver over ten watts in this circuit but the valves quoted will deliver seven watts from 3.5 to 30 MHz. If you should use another tube then don't forget to check the screen voltage. If it is less than 250v then you will need a resistor in the screen supply to the valve.