Epson Oscillators

Epson Programmable Oscillators


Data sheets in pdf format can be downloaded directly from Epson at:

http://www.eea.epson.com/products/qd/qdcrystalosc.htm

The parts are available in surface mount, 14-pin or 8-pin DIP package footprints (the packages actually have only 4 pins). It wasn’t clear from the data sheets just how closely you can specify the frequency, although if you follow the links on the Epson web site, there is a place where you can enter a desired frequency and it will tell you if it is valid. But there were still a lot of questions, like how close do they actually come to the specified frequency? Are they stable enough to put out a “clean” CW note on HF, or are they useful only as clock oscillators for digital circuits? How much phase noise? Since they are only $3.33 apiece, it doesn’t cost much to find out (except that Digi-Key has a $5 service charge on orders under $25).

I ordered two oscillators, both with 8-pin DIP footprints, CMOS type, 50 ppm stability. One of the four pins on the package is either an “output enable” or “standby” pin, depending upon which option you specify. I ordered an 18.573000 kHz oscillator with the output enable option, and a 1.703915 MHz oscillator with the standby option. The 18-MHz part could be used with a 74HC390 dual decade counter to provide a square wave output at 185.730 kHz, which was chosen because it falls midway between 60-Hz harmonics. Although I don’t have any plans to get back into MedFER operation, the 1.7 MHz part could be used to directly drive a MedFER final. The frequency of 1.703915 was chosen to be near the top of the expanded AM broadcast band, and is not a multiple of any common reference frequency like 10, 100 or 1000 Hz. The idea was to see if the oscillator could be programmed to the nearest 1 Hz increment. When I called Digi-Key, the person who took my order contacted the person who does the oscillator programming on another line, and verified that both frequencies were OK. The parts were shipped out the same day.

The elaborate fixture shown below was used to test the oscillators. People with soldering anxieties should find it comforting that the oscillators work fine on a plug-in protoboard, at least up to the 18 MHz region.

OSCILLATOR TEST FIXTURE

The DC supply voltage was applied to the points marked V+ and GND, and a scope/counter lead was connected to the OUT point, which is the oscillator output coupled through a 0.1 uF capacitor. Another 0.1 uF capacitor is used as a bypass between V+ and GND. If the OE/ST pin is left floating, the oscillator runs continuously. When it is grounded, the oscillator is either placed in STANDBY or OUTPUT DISABLE mode, depending on the chip option. The difference is that chips with the STANDBY option will turn off completely when OE/ST is grounded. In chips with the OUTPUT ENABLE option, the oscillator keeps running but the output stage is turned off. There apparently is an internal pullup resistor of a couple of megohms on the OE/ST line, because the current from the pin is only about 2 microamps when it is grounded. If you want to be able to shut off the oscillator with a logic-level signal and minimize power consumption while in the disabled mode, you would want a chip with the STANDBY option. For rapid on/off switching, as in a CW transmitter, you’d need the OUTPUT ENABLE option.

Here are the results of frequency versus supply voltage tests at room temperature.

Oscillator 1: Digi-Key Part No. SG-8002DC-PHB (8-pin DIP, output enable option, CMOS, +/- 50 ppm)
Specified frequency: 1.703915 MHz

Oscillator 2: Digi-Key Part No. SG-8002DC-SHB (8-pin DIP, standby option, CMOS, +/- 50 ppm)
Specified frequency: 18.573000 MHz

Watch Crystal Oscillator

Free Example Electronic Circuit Design 

32.768 KHz Oscillator Circuit Diagram

Below are a couple circuits you can use to produce a 32.768 KHz square wave from a common watch crystal. The output can be fed to a 15 stage binary counter to obtain a 1 second square wave. The circuit on the left using the 4069 inverter is recommended over the transistor circuit and produces a better waveform. The single transistor circuit produces more of a ramping waveform but the output swings the full supply voltage range so it will easily drive the input to a CMOS binary counter.

GTO Sawtooth Wave Generator

A simple sawtooth generator using a GTO and a Zener diode is shown in figure.

GTO Sawtooth Generator

When the supply is switched on, the GTO will turn on, resulting in the short-circuit equivalent from anode to cathode. As a consequence the capacitor C will begin to charge toward the supply voltage, as illustrated in figure. As the voltage across the capacitor C exceeds the Zener potential, there will be a reversal in gate-to-cathode voltage resulting in a reversal of gate current. Eventually, the negative gate current will be large enough to turn the GTO off. Now the capacitor C will begin to discharge through resistor R„ because turning off of GTO results in the open-circuit equivalent from anode to cathode. The discharge time will be determined by the circuit time constant T = CR₂. The proper choice of R₂ and C will result in the sawtooth waveform shown in figure. Once the output voltage V_out drops below Zener voltage V_Z the GTO will turn on and the process will repeat.

555 Timer Oscillator

A voltage-controlled oscillator (VCO) using the timer 555 is shown in figure.

555-timer-voltage-controlled-oscillator

The circuit is sometimes called a voltage-to-frequency converter because the output frequency can be changed by changing the input voltage.

As discussed in previous blog posts, pin 5 terminal is voltage control terminal and its function is  to control the threshold and trigger levels. Normally, the control voltage is ++2/3V_CC because of the internal voltage divider. However, an external voltage can be applied to this terminal directly or through a pot, as illustrated in figure, and by adjusting the pot, control voltage can be varied. Voltage across the timing capacitor is depicted in figure, which varies between +V_control and ½ V_control. If control voltage is increased, the capacitor takes a longer to charge and discharge; the frequency, therefore, decreases. Thus the fre­quency can be changed by changing the control volt­age. Incidentally, the control voltage may be made available through a pot, or it may be output of a transistor circuit, op-amp, or some other device.