On Fri, Apr 8, 2011 at 4:48 PM, W7RIS <w7ris.arrl@softgoals.com> wrote:
> There are 7 capacitors required to build this kit. Yes DigiKey lists the
> parts but half of the capacitors required for the kit show '0' stock and
> none on backorder. This includes looking for substitutes of any voltage or
> tolerance. I know back 10+ years ago I didn't have any problems finding
> these parts. But not so in today's economy.
Might it just be that the demand for the form factor of parts that industry
needs has changed completely and we haven't re-worked our designs and
expectations? I wonder what the performance hit (if any) would be if you
were to use completely standard ceramic surface-mount chip capacitors in the
AM BCB design? I suspect there would be no problem at all, and they might
actually be better.
You can get every necessary value for the BC band-reject filter from Mouser
(and I'm sure, Digikey) in 100V 5% ceramic surface mount type. I forgot to
check if all were available in C0G, but I know some were, and I think all
are... it's very standard and the correct choice for RF applications.
Prices are low... 40 to 60 cents in Qty. 1 for some spot checks.
>From what I can find the HF Q of the standard chip caps at least 600-1000
(dissipation factor of at most 0.0015 is quoted many places, which is Q=667,
some data sheets say Q>1000). And I think all this is probably more or less
the same for C0G ceramic disks vs. C0G ceramic chips if dielectric
dissipation is at fault.
Intuition tells me that anything a leaded ceramic disc cap could do, a
surface mount chip cap can do better with the exception of handling
continuous RF currents and dissipating the resulting heat... not because
they're more lossy (they may be less) but because they are physically
smaller. For receiving filters this should not be an issue. For things
like QRP low pass/bandpass filters, etc, they might. (For QRO filters you'd
probably be looking for mica caps anyway. But even there, surface mount
versions seem to be more available and a bit cheaper ($4-$5 in Qty.1) at
least in new parts )
It's not too hard to use surface-mount chip caps in dead-bug construction as
long as you have a decent pair of tweezers.
73
Dan
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