PCM Speech CODEC

[clueless84]

Hi All, I'm currently doing a project for college, Design of Switched capacitor filters
for a PCM Speech CODEC.  I am to use a biphase clock to drive the switches on the capacitor. This is where my problems begin. I do not know how to design a biphase clock, I was told to use a johnson counter, but when I implemented this it just gave me 2 signals in phase.
Any help would be appreciated.

 

[audioguru2]

Hi Clueless, Welcome to our forum.
Aren't you allowed to use a switched-capacitor filter IC?

 

[Kevin Weddle]

What's this, problems with the clock. You shift your phase back upstream where you have the sine wave oscillator. A series RC will give you the phase difference. Then the sine wave oscillator is converted to a square wave. This is the method of choice anyhow.

 

[clueless84]

No we have to built everything, well I think we can buy certain parts if we're really stuck, not sure though. How do you shift the phase back upstream? As the name would suggest, I really am quite clueless.

 

[audioguru2]

Hi Clueless,
Switched capacitor filter ICs use a "non-overlapping clock generator". I don't know how to make one easily using a Johnson Counter like a CD4018, maybe it needs two cascaded CD4018 counters. Please post your Johnson Counter defective bi-phase clock circuit so we can troubleshoot it.

View attachment 36836

 

[clueless84]

I'm using a 2 D type flip flop's. I dunno if my diagram will appear, hopefully it will, i'm new to all this. The chip i'm using is a 74LS74, i'm using a 10K square wave pulse. If my diagram posts, I was originally using the  output C1 from between Q1 and D2. I have now changed it bit I think my power supply has decided to act up so i'll get back to you when I figure it out.