The famous "Electronic Filter Design Handbook" by Williams and later editions by Williams and Taylor has an entire chapter on this with tables of pole and zero placements and passive and active circuits to implement them. This book is probably available in electronic format somewhere.
Can someone post a link or upload the quadrature phase shift chapter?
If I am wright you need constant phase shift of 90 degree in all frequency band. It can be realized with two chains consisting of allpass filters (op amp) that have constant phase diference of 90 degree. Here is example of such a circuit.
Digital implementation is quite simple , but you need 2 times more freq input to this circuit . Use 2 triggers to divide by 2 the input freq with differences that :
- first trigger will process on rising edge
- and second on falling edge of input freq