Discussion:  Figure 1 shows an AET USB-S-1.84 RF Source comb generator (so called because its frequency spectrum looks like a comb on the display of a spectrum analyzer) and a small charger powered by two AA cells intended for charging mobile phones and USB devices. Although intended to charge USB devices until the batteries are exhausted, it makes a great 5 Volt USB power supply for many devices including the comb generator shown. During operation, the charger draws about 380 mA from the AA cells while powering the USB powered comb generator. That current drain could translate to 5 or more hours of operation per pair of AA cells.

The unit also seems to run well on a pair of AA nickel metal hydride batteries although I have not tried to see how long NiMH batteries will power the charger. To use NiMH batteries, a small plastic tab must be removed inside the battery compartment near the positive terminal of one of the batteries at the back of the charger (opposite end of the unit from power connector). The tab holds a metal piece in place that forms the series connection of the batteries. This tab hits the edge of an AA NiMH battery and causes the positive terminal to be held away from the metal piece, breaking the series connection. I used a small grinding tool to remove just enough of the tab to clear the battery. An alkaline AA cell has a taller positive terminal so the plastic tab is not a problem.

The test setup is shown in Figure 2. The charger is connected to the comb generator using a small cable furnished with the charger and the comb generator itself is directly coupled to the input of the scope to achieve the fastest possible edge rates by avoiding the dispersion and loss in coaxial cables which slows edge rates. A close-up of the test setup is shown in Figure 3.





Figure 4 shows the fast portion of the output of the comb generator. There are slower features on the output that can be seen on a longer time scale, but the part of the waveform shown is the source of most of the harmonics in the output of the generator. The pulse has about a 1 1/2 Volt amplitude with a rise time of a little less than a nanosecond.  Adequately powered from the charger, the comb generator seems to be working well.




I have seen this charger at Costco and Staples. It is widely available from other outlets as well. The charger would work well for field work where USB devices need to be powered, such as the comb generator RF source shown above.