500 kHz-1.5 kHz in three steps
0.25 - 8 V p-p
|This instrument is useful for measuring coil Q
and for measuring antenna characteristics.
When I build equipment that sees relatively little use compared to, say, a meter or scope, I position the batteries outboard. This practice does away with having to unscrew the bottom and it avoids the necessity of a power switch.
RF requires short leads and good bypassing. To minimize the time spend debugging, I chose to build the circuit "Manhattan Style."
|Manhattan style consists of using a ground plane
instead of a single ground point. Connections are made by soldering components
to little islands or pads insulated from the ground plane.
I use an ordinary notebook paper hole punch to punch my pads from .015 pcb material I got from an Electronic Goldmine assortment. Each pad measures about 4 pf to ground. It takes a drop of superglue and a few seconds to mount each pad.
Soldering leads to ground is a breeze!
|The light bulb does not go incandescent, instead
serving as a classic Wien bridge amplitude stabilizer. The 1K pot
adjusts waveform purity; it should be set and left alone.
The selector switch to the left of the circuit board is the frequency selector. It switches between the 1% resistor pairs for range control. (It's not shown on the schematic.)
Why didn't I use a dual pot that would provide continuously variable output to well beyond 2 mHz? With a selector switch, I need not worry about getting the setting exactly the same each time. Also, dual pots with accurate tracking are expensive and probably wouldn't be accurate enough keep waveform distortion low.
Another way to achieve continuously variable frequency is to substitute a dual gang variable capacitor for the 100 pF capacitors. In this case, no selector switch is needed for R1 & R2, which must be exactly equal in value. Also, the positions of R1 and 100 pF cap would need be reversed from what is shown in the schematic. This provides the 100 pF caps with a common connection point on the variable capacitor. Waveform purity can suffer, producing some nasty harmonics if the varicap sections are not closely matched in capacitance.
This instrument is intended
for testing purposes only. Any radiation must comply with Part 15
of the FCC code. The second AD811(Analog Devices) serves as a buffer
containing an attenuator, helping ensure the operator is in compliance
with federal law.
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