Oscillation for IIR Filter

You need to be careful of oscillation when you design multiple serial IIR's (more than four??).

The following is the case we design HPF by IIR, Fs=1000, Fc1=100, Number of IIRs=8, Data bit length=8, Coeffs bit length=12. If we input the frequency close to cut off, which is 90Hz with full scale, some internal signals exceed the limit so that, it's clipped by limiters. Therefore high frequencies are generated and those are magnified by internal HPFs. (The total gain of pass band is 0dB, but each IIR has each gain, which can be more than 0dB). The 90Hz, which needs to be eliminated, has some strange frequencies finally. (Simulator is ModelSim EE/Plus 5.2d by Model Technology Inc.)

If you design multiple serial IIRs, please check the characteristics of magnitude domain for the frequencies which is close to cut off. If you have this symptom, please increase the bit length or change the order to serialize IIRs.