Friday, September 23, 2011

Taking a look at basic ring oscillators

Just playing around with ring oscillators cobbled up using a number of basic CMOS gates. Measured VDD of the linear power supply = 5.07 to 5.09V for all circuits. Circuits were assembled on a solderless breadboard. Interconnect wires varied in length. And the relatively long interconnects--compared to the short traces had these been fabricated on a PCB--almost surely affected performance and the characteristics of the oscillators.

I. First up is the Fairchild CD40106BCN hex Schmitt inverter. According to the datasheet its maximum propagation delay = 400ns at 25°C and VDD = 5.0V. Its maximum rise/fall time (transition time) = 200ns at 25°C and VDD = 5.0V. This oscillator has the lowest frequency among those tested. But its output is also the closest to looking like a square wave. Numbers in the schematics refer to IC pin numbers

I measured the output at the various pins to see how transition time would be affected--wanted to decrease it to make the wave as square as possible. The most significant effect occurs with the very first inverter after the oscillator. However, as you can see below, having inverters to "clean up" output after the oscillator has an impact on frequency as well.

Output at pin 8:

At pin 2:

At pin 4:

At pin 6:

II. Motorola MC14049UBCP hex inverting buffer. After discovering how lousy the output of a 3-inverter configuration was I tried a 5-inverter one. As you will see there is improvement but it isn't that remarkable, even if the frequency is almost halved.

At room temperature and VDD = 5.0V, maximum rise time of the 4049 is 160ns, while maximum fall time is 60ns. Maximum propagation delay for low to high transition = 120ns. For high to low max propagation delay = 60ns.

A. Using 3 inverters:

Output at pin 15:

At pin 6:

At pin 4:

At pin 2:

B. Using 5 inverters:

Output pin 4:

At pin 2:

III. If I had them in my inventory I would've tested a 74AC hex inverter and Schmitt trigger, but the only part I have that comes close is a National Semiconductor MM74HC02N quad 2-input NOR gate. Datasheet says it has a maximum propagation delay of 18ns at 25°C and VDD = 4.5V. Maximum rise and fall time = 15ns at 25°C and VDD = 4.5V.  With those specs, it's easy to see that the 74HC line is ultrafast compared to the jurassic 4000 series. However, its transition time to propagation delay time ratio isn't good enough to produce square waves. Actually the output is close to a sine wave. In fact, as can been seen in the scope readings, its rise and fall times aren't even short enough for the output to reach VDD or ground.

At pin 10:

At pin 13:

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