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Khamis, Jun 27, 2013

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

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