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Using coupling lines for DC block. How to choose coupling?

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Advanced Member level 3
Feb 25, 2012
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Parallel quarterwave sections often used as DC block in microstrip circuits. This sections are always slightly thinner than 50 Ohm lines. How to design such coupling? I have read about microstrip bandpass filters and coupled lines (odd/even impedance, etc. For filter Z0o and Z0e can be obtained from filter coefficients. But in this case all must be simpler. I think information from this link is enough, i just only to choose right "C = coupling in decibels". As i understand, C must be very small value, because C=30dB refers to weak coupling, while C=3dB is strong coupling. Knowing Z0o and Z0e it is possible to calculate line widths and gap, for example in QUCS line calculator menu. So how should i choose C value? Making it as small ass possible, or there is something else?

calculations in SpeqMath to check formulas from example:
C=3 'dB
C = 3
k = 0.707945784

Z0 = 70.7

Z0e =Z0 * ((1 + k)/(1 - k))^0.5
Z0e = 170.971890725

Z0o=Z0 * ((1 - k)/(1 + k))^0.5
Z0o = 29.235741494

Z0 = ( Z0* Z0e )^0.5
Z0 = 109.944134333

Also here we see wide sections between quarterwave parallel lines coupling:
I think it is quarterwave transforming from 50 Ohm to something like 25 Ohm, maybe if coupled lines are too thin in 50 Ohm system.
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You can design the DC coupler like quarter-wave fingers. MOre fingers give a better RF response and lower DC voltage allowed. The DC block impedance should be close to 50 Ohms if this is line impedance. You can simply take a longer line length and using a razor cut two or more fingers in it to block DC. Keep fingers length the quarter wave, it works well.

To extend DC block bandwidth, cut finger length a bit longer and shorter than the quarter wave length, and cut more than two fingers. For higher DC voltages, use precut quarter-wave lengths of micro-coax rigid or semirigid cable, or find a SMD capacitor good for your frequency range.
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