I’ve been wanting an LCR meter for a lot of years now but have refrained from buying one due to my skepticism of their accuracy. A voice in the back of my head is telling me that I could probably measure the value of a small-value capacitor a number of times and get a number of different results.
The variation would be due to factors like overall meter accuracy & resolution, lead capacitance, physical arrangment of the leads, body capacitance as the measurement is made, etc. But maybe I’m wrong, as often is the case…
In reading over the manual for the new Ft. Tuthill 80m kit, it appears that there are ten 0.1uF capacitors that should be matched. If not, the rig stands a small chance of breaking into self-oscillation.
From the assembly manual:
The kit will work best if the 0.1 uF capacitors of the filter section are matched. This kit has a total of 40 0.1 uF capacitors, and 10 of them are in the filter section. These are marked “104”. It is strongly recommended that all 40 capacitors are measured and the closest 10 caps in value are used as the filter capacitor parts.
The matching process involves getting a blank piece of paper, measuring each 0.1 uF capacitor, placing the part on the paper next to the recorded value. I suggest recording three digits if the measured value is over 0.1 uF (0.1xx uF) or two digits (0.09x) if the value is less than 0.1 uF.
Matching is not strictly necessary, but there is the outside chance that a filter section could go unstable and oscillate if 0.1 uF caps are used that happen to be far apart in value. Matching the 0.1 uF caps produces a very nice filter response with a relatively flat pass band.
LCR meters are cheap enough – I guess I’ll see if they’re of any real value in measuring component values we normally deal with in QRP construction projects. But the price of the kit just went from $50 to a bit over $100…… make that $65 thanks to Bob’s suggestion in the comments section – a decent enough price for a transceiver and a piece of test equipment.