Hi
Since Farhan's stated original intent in designing the BITX20 was to provide an inexpensive transceiver, maybe it would be appropriate to show BITX builders an inexpensive way to measure unmarked or questionable capacitors.? The following circuit came to me via Don Metzger - K8JWR, but I suspect the method has been around for quite some time.? The picture shown here is the relatively simple schematic for this tool.
Use a mains transformer that delivers a few ma. at about 15 volts RMS.? Connect a known .001 mfd capacitor at Cx and adjust the value of R1 ( for R1 I used a 250K trim pot salvaged from an old circuit card ) for an indication of 1.000 volts on your DVM.? Now you can measure capacitor values from a few pf up to 0.001 mfd by connecting them in the place of Cx.?
I used this method to "calibrate" the knob indicator on a 3X450 pf variable capacitor that I regularly use as part of my test equipment repertoire.? Now I just set the dip meter to the desired frequency, and use the variable capacitor to tune my inductor for a dip on that setting.? Once the coil is on-frequency I can read the capacitance from my calibrated variable capacitor and substitute that value of fixed capacitance into the circuit.
Part of the fun derived from building your own ham radio equipment is in devising ways to perform complex design and testing by use of relatively simple and inexpensive methods & equipment.? This idea fits into that category.
73's
Arv
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Before someone asks the question...No, this circuit will not measure electrolytic capacitors, but it will get you close enough for most of the small caps you will be using in designing resonant circuits for HF applications.
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