June 13, 2012

The Syscomp Electronic Design Ltd. WGM-201 Advanced Waveform Generator provides sine wave output up to 10 MHz as well as arbitrary programmable waveform shapes up to 3 MHz. The WGM-201 is specified as having an output resistance of 150 ohm. A typical usage of the sine output of this generator would be to measure frequency response of circuits up to the 10 MHz limit. For reasonably accurate measurements requiring calibration, it is useful to measure the precise output-level of the generator up to 10 MHz.

It is important to understand that above ~ 1 MHz, small circuit capacitance values can shunt the output of the generator simply from the complex voltage divider network formed by the 150 ohm internal impedance and the capacitative reactance. In order to minimize the relative effects of shunt capacitance at higher frequencies, the generator output can be terminated with lower values of resistance. For example, with a 50 ohm termination at the generator output, the effective (Thevenin) resistance of the generator is the parallel combination of 150 and 50 ohm or 37.5 ohm. Of course the effective Thevenin output voltage is also lowered (by 1/4) from the nominal value. To demonstrate the importance of shunt capacitance, the following 2 graphs show the effect of load capacitance on the normalized voltage output (assuming a load with much higher resistance than the 150 ohm generator resistance). The first set of plots show the dramatic filtering effect above 1 MHz as shunt capacitance is varied, a simple result of the complex voltage divider in this single-pole network. The second set of plots, with a termination resistance of 50ohm (or Rth=37.5ohm) demonstrates the cutoff moving to higher frequencies:

The LM6172 is a high-speed 100 MHz GBW op-amp with unity-gain stability and a very high slew-rate of 3000 V/μs configured as a high-speed follower. This op-amp can easily drive the half-wave rectifier circuit at its output. Since this op-amp has a fairly high input bias current of 1.5uA, a 1 kohm +input termination resistor was used to ensure negligible output voltage offset driving the half-wave rectifier output. The input high-pass filter removes any DC and has a cutoff of ~ 700 Hz and therefore is intended to be used above ~ 10kHz. This op-amp circuit can therefore can be used as a standard 1kohm load with small capacitance load to measure other op-amp circuit outputs. The LM6172 output drives a 1N34a half-wave peak detector circuit with a 20 Mohm load and 0.01 uF filter capacitor. The 20 Mohm resistance consists of a 10 Mohm resistor and a standard 10 Mohm input DVM measuring DC voltage. The accuracy of the DVM ranges was checked using a 0.1% precision DC voltage reference IC. For a fixed input voltage level, the rectified output DC voltage level is constant from 10 kHz to at least 10 MHz with negligible ripple. This was confirmed with a circuit simulator using the complete diode characteristic. Amplitude calibration from input to output was performed at a lower frequency and compensates for the diode voltage drop in the rectifier circuit. The detector circuit and the entire measuring configuration used for all the measurements above are shown below:

Typical measured stray capacitance values for various connections on a 3"x2" protoboard are shown below. Note that the capacitance across the center channel is quite low (0.4pF) while the capacitance between adjacent tracks, corresponding to adjacent pins of DIP pinouts, is much higher at 2pF. This has obvious implications in feedback stray capacitance (from inverting input to output) for single versus dual op-amp DIP configurations when measured on these types of breadboards: