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Old October 24th 05, 04:26 AM
Joel Kolstad
 
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Default Wideband RF blocking

In a receiver, for wideband (10-150MHz) biasing of a MMIC amplifier using
inductors between the RF trace and the power rails, I'm finding that a single
inductor tends not to work so well due to (1) wanting a largish inductor (say,
one with 500 ohms of reactance at 10MHz in a 50 ohm system -- 8uH) but (2)
not going beyond the self-resonant frequency of the inductor, which of course
is smaller the larger the inductance (that 8uH inductor might typically have
an SRF of 50MHz, noticeably below the 150MHz I'm trying to achieve RF blocking
to!).

Is there a better means of providing wideband DC biasing/RF blocking than just
placing a large and small inductor in series? I've run SPICE simulations of
this, and -- just as when you stack multiple capacitors in parallel for wider
RF coupling -- there are significant anti-resonances that drop the overall
reactance of the pair of inductors to much less than 500 ohm (even less than
50 ohms!) unless you're very careful in the choice of individual inductor
SRFs, inductances, etc.

Thanks,
---Joel Kolstad


 
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