Never........NEVER.......rely on an electrolytic as a bypass for RF
energy!!!!!! Use series resonance to your advantage. Chip, N6CA learned
that lesson from (deceased) Gary Frey, W6XJ, in the 1970's. The bypass
caps presented in his preamps are chosen to be self resonant at the
frequency of operation of each preamp. I've used the technique many times
over in home brewing. It's a well known fact among (most) design
engineers that tantalum capacitors are not much good above 500 kHz to a
couple of MHz. Most electrolytics are even worse. Go measure them on
your NANOs.
I just grabbed a 1000 ?F / 25 VDC cap from the parts bin and measured it on
a calibrated HP 8753C using the Smith Chart. Even at 1 MHz it measures 78
m¦¸ with a series reactance 78 nH. Sure, the DC portion is fine, but what
is 78 nH at 50 MHz?
X(L) = 2 x ¦Ð x f x L = 6.28 x [50 E 6] x [78 E-9] = 24.5 ohms
Sure, the +j24.5 is non-dissipative, but does not make a very good bypass
even at low VHF frequencies.
Take a capacitor better suited as a bypass at HF, a 450 pF dip mica. Same
setup at 10 MHz: 0.1 ohms at 462 pF. It goes self resonant just above 39
MHz.
Which makes a better bypass at HF.
Dave - W?LEV
On Sun, Feb 14, 2021 at 10:14 PM Manfred Mornhinweg <manfred@...>
wrote:
And what's the best bypass cap? Well, a single, plain, cheap aluminium
electrolytic!
Attached is the impedance plot for a 47?F, 25V electrolytic cap, measured
with lead lengths compatible with mounting it snugly on a PCB. Their narrow
pin spacing helps a lot in keeping their ESL low. I kept the same scale to
make comparison easy.
YES, a single 47?F electrolytic is a much better bypass cap than a
parallel combination of two ceramic caps of different values! Even in the
low VHF range!
The problems with electrolytic caps is that their ESR rises with age, and
rises much faster if they run hot, or if they have to carry large ripple
current. So they can't be applied in every situation. But in situations
that are kind to them, they are the cheapest and easiest way to get an
excellent wideband bypass.
--
*Dave - W?LEV*
*Just Let Darwin Work*
