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 2. Test Setup With Charger and Comb Generator
Close-up of Power Source and Comb Generator
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.
Figure 4. Plot of USB Powered USB Comb Generator Output
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.