Am 23.08.2018 22:04, schrieb Chris Wilson:
Hi Chris,
I added a 470pF cheap disc ceramic across each
of the 2000pF polystyrene caps, and indeed the filter moved lower, to
pretty much the optimum spot (thank you Stefan). But I see a lot more
attenuation with these two caps added. I presume disc ceramics are not
ideal?
The Q of them should not be critical but things like temperature
coefficients... I prefer to use styroflex caps for such RX filters but
others can be fine too.
Two caps? There should be 3 in that circuit. If you just tune two of the
circuits to 137 kHz then the filter curve will be wider and have more
attenuation. Actually there could be two resonance peaks close together,
which you may not see due to the low Q.
Just try to add a 3rd C. I would expect this improves things. It is good
to play arround anyway, then you learn intuitively what causes what result.
The inductors are cheapo pre made ones (look like 1/4 W
resistors), again off Ebay so probably far from optimal. I may need to
look at raising the Q of this thing! Are there high Q commercially
made inductors in 680 and 2.2 uH or do I need to wind my own?
Wind your own! I built those:
http://www.iup.uni-heidelberg.de/schaefer_vlf/LF/LF_RX_pot%20_cores.jpg
How sub
optimal is using disc ceramics for parallelling the branded poly caps?
Quite, but still good for a first test.
73, Stefan
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