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

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

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. http://www.microwaves101.com/encyclo...ode-impedances). 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 microwaves101.com example:
C=3 'dB
C = 3
k=10^(-0.05*C)
k = 0.707945784

Z0=70.7
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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2. ## Re: Using coupling lines for DC block. How to choose coupling?

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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