ESR Project Build - Part 2 - Oscillator

The basis of the tester starts with a 100 kHz oscillator circuit. The two designs are similar, but I'm building based upon the W2AEW circuit:

The oscillator consists of a pair of CD4049 inverters biased to oscillate. The remaining four hex inverters are used to drive the signal into a 10 ohm resistor. This produces the signal sent to the Device Under Test (DUT) with enough power, but low enough in voltage to avoid silicon diode conduction.

The scope trace in Figure 1, shows the oscillator signal when the circuit was powered from about 6 Volts. The rise and fall times are 80 ns and the duty cycle was about 55%.

Figure 1. Oscillator output (before drivers)

Figure 2 illustrates the output signal out of the driver stages into the 10 ohm resistor. The voltage drops to about 355 mVpp and the frequency rose to about 119 kHz. With the driver attached, there was less load upon the oscillator stage. At these lower voltage levels, you can see more of an influence from the inductance side of things in the trace output.

Figure 2. Driver stage output into 10 ohms.

Getting the oscillator to work was fairly painless. Because I am prototyping this, I always run the risk of solder blob shorts and this time was no exception. After elimination of a supply side short and then another solder blob affecting the oscillator circuit, the circuit took off with no trouble.

In the next part, we'll look at the receiving side of the DUT probes.