Post by thread:[OT] balanced filter stuff

17/9/'99 Hello all, I have to design some filters for my course (Low pass), and some of them are balanced, and are to be considered way above ground (Some are not). If I am to design this filter, do I do it as a normal unbalanced type and then put the same inductance in the other leg and 1/2 the shunt capacitance?, or is this seem as a virtual ground point and the capacitance value does not change? Should I be using some balance caps to ground too? Filter unbalanced 1MHz 15uH 15uH in -------------/\/\/\/\/------/\/\/\/\/----------- out | | | --- --- --- --- --- --- 1200pF | 4700pf | 1200pF | | | | GND GND GND Zin = Zout = 50R Filter balanced 1MHz 15uH 15uH in -------------/\/\/\/\/------/\/\/\/\/----------- out | | | --- --- --- --- --- --- 1200pF | 4700pf | 1200pF | | | | | 15uH | 15uH | in -------------/\/\/\/\/------/\/\/\/\/----------- out Zin = Zout = 50R or should it be:- GND | | --- --- 4700pF at suitable voltage | | in -------------/\/\/\/\/------/\/\/\/\/----------- out | | --- --- --- --- 1200pF | 1200pF | | | | 15uH 15uH | in -------------/\/\/\/\/------/\/\/\/\/----------- out | | --- --- 4700pf | | GND As you may have gussed I am not very good at this, help would be nice Dennis

The ones that are to be considered to be way above ground, how are they being driven? Transformer? If it is a transformer and it has a centre tap you could use the centre tap as an earth. For the ones where the earth is usable I'd first try using your unbalanced filter referenced to earth and put it in both legs of the balanced line, after all you could consider it as being two unbalanced circuits 180 degres out of phase with each other I'm not sure how this is going to affect the input and output impedances though. Filter unbalanced 1MHz Zin = Zout = 50R ? 15uH 15uH HOT in -------------/\/\/\/\/------/\/\/\/\/----------- out------------------ | | | --- --- --- --- --- --- 1200pF | 4700pf | 1200pF | | | | GND | | | GND ----------------|-----------------------|-----------------------|--------------- ---- | | | | | | --- --- --- --- --- --- 1200pF | 4700pf | 1200pF | | | | Cold in -------------/\/\/\/\/------/\/\/\/\/----------- out---------------- 15uH 15uH Regards Stuart Dennis Plunkett wrote: {Quote hidden}> > 17/9/'99 > > Hello all, > > I have to design some filters for my course (Low pass), and some of them > are balanced, and are to be considered way above ground (Some are not). If > I am to design this filter, do I do it as a normal unbalanced type and then > put the same inductance in the other leg and 1/2 the shunt capacitance?, or > is this seem as a virtual ground point and the capacitance value does not > change? Should I be using some balance caps to ground too? > > Filter unbalanced 1MHz > 15uH 15uH > in -------------/\/\/\/\/------/\/\/\/\/----------- out > | | | > --- --- --- > --- --- --- > 1200pF | 4700pf | 1200pF | > | | | > GND GND GND > > Zin = Zout = 50R > > Filter balanced 1MHz > 15uH 15uH > in -------------/\/\/\/\/------/\/\/\/\/----------- out > | | | > --- --- --- > --- --- --- > 1200pF | 4700pf | 1200pF | > | | | > | 15uH | 15uH | > in -------------/\/\/\/\/------/\/\/\/\/----------- out > > Zin = Zout = 50R > > or should it be:- > > GND > | > | > --- > --- 4700pF at suitable voltage > | > | > in -------------/\/\/\/\/------/\/\/\/\/----------- out > | | > --- --- > --- --- > 1200pF | 1200pF | > | | > | 15uH 15uH | > in -------------/\/\/\/\/------/\/\/\/\/----------- out > | > | > --- > --- 4700pf > | > | > GND > > As you may have gussed I am not very good at this, help would be nice > > Dennis

I *meant* to answer at the time! Dennis Plunkett wrote: > I have to design some filters for my course (Low pass), and some of > them are balanced, and are to be considered way above ground (Some are > not). If I am to design this filter, do I do it as a normal > unbalanced type and then put the same inductance in the other leg and > 1/2 the shunt capacitance? > Filter unbalanced 1MHz 15µH 15µH 1200pF 4700pf 1200pF GND GND GND OK, if you want this balanced at the *same* impedance, you have to re-distribute the inductances symmetrically. This is performed by taking half of each inductance and putting it in the other side: 7.5µH 7.5µH 1200pF 4700pf 1200pF 7.5µH 7.5µH If in addition you want to "center-tap" the capacitances, each must be substituted by two in series of double the capacity: 7.5µH 7.5µH 2400pF 9400pf 2400pF GND GND GND 2400pF 9400pf 2400pF 7.5µH 7.5µH If you wanted to do much the same thing, but wanted twice the characteristic impedance, you would just mirror the original circuit: 15µH 15µH 1200pF 4700pf 1200pF GND GND GND 1200pF 4700pf 1200pF 15µH 15µH For four times the impedance (as if fed by a 2:1 balun), you follow the same "rule" and double inductances, halve capacitances: 30µH 30µH 600pF 2350pf 600pF GND GND GND 600pF 2350pf 600pF 30µH 30µH And again, if you didn't want the earth reference: 30µH 30µH 300pF 1175pf 300pF 30µH 30µH I think that's how it works... -- Cheers, Paul B.