Nuts & Volts - September 2005

error may be due to the instability of the RC frequency. Approximately 30 minutes elapsed between the first and second measurements.

The frequency response, unfortunately, is highly non-linear (see Graph 1). This may not be a problem if your design is a closed-loop. In this case, you simply increment or decrement the voltage until a proper response is obtained. However, if you want to output a particular voltage, say 2.52 volts, it’s not effortless.

The crude way to obtain frequency response is to build a look-up table based on values you actually measure on a breadboard. But this is problematic, as resistors and, especially, capacitors have values that vary by several percentage points. So, there will be differences in the output voltage from unit to unit. You could tailor your uC by inputting specific values during program loading for specific components, but this is cumbersome. A better way is to use the built-in A/D that most uCs have.

During start-up or self-test (a self-test routine is crucial), you could measure the actual voltage with known applied frequencies and build a look-up table automatically. This limits the resolution to the available A/D resolution, but this may be adequate for many applications. Most uC A/D converters specify a 10K or less analog signal impedance. This is not a problem because C2 is very large relative to the A/D capacitor.

Your look-up table will probably contain about 20-40 entries, and you will interpolate between the values. This procedure reduces the memory requirements and usually provides adequate results. When you do this, you are really converting the curve to 20-40 straight-line segments. Generally, the difference between the curve and the straight line are minimal. Of course you can, and should, calculate the difference to be sure it is suitable for your application.

PHOTO 1. A POOR ATTEMPT AT AN FM SINE WAVE. HOWEVER, THE SIGNAL IS NICE AND CLEAN WITH NO TRACE OF DIGITAL NOISE.

voltage is created (Figure 2a). If the diode’s anode is connected to a positive voltage (typically VCC or + 5 volts) the output voltage can be about 50% greater than that (Figure 2b). This gives you great flexibility and is something that ordinary D/A converters cannot accomplish. You can output any voltage from - 3 to + 3 volts and from + 4. 5 to + 7. 5 volts with a standard 5-volt power supply. (There may also be enough current to supply the negative voltage requirements for an op-amp or two.)

These circuits will work with any digital input. However, if you are using a uC, you can select whatever output you desire. All that is needed are a couple of free I/O pins and another diode. Figure 3 illustrates how this can be done. The key concept is to realize that an output pin in a high-

FIGURE 2. BY CHANGING THE DIODE CONNECTIONS, DIFFERENT OUTPUT VOLTAGES CAN BE GENERATED.

Frequency Modulation

DC voltages are nice, but nothing is fixed. Naturally, you’ll want to change it from time to time. The question is how quickly the DC level can change. I breadboarded a quick and dirty modulator that swept the frequency from 3,400 Hz to 175 KHz. It was supposed to be a sine wave, but if you look at Photo 1 you’ll see that the falling edge has a ledge in it. The FM frequency was about 10 Hz and the resultant signal amplitude was about 0.75 volts. You can change the values of the resistors and C2 to change the response of the circuit. You can probably get up to a useful frequency of 60 Hz, but I wouldn’t expect to go much higher.