SV: Re: [sdiy] BBD clock circuit

[Nicholas Gregorich]

On Aug 18, 2006, at 1:12 PM, harry bissell wrote:

> anthony wrote:
>> Hey Harry,
>> We all know you think BBD's suck, but WHY? And what do you use in  
>> place of them? Phasers, resonant filters and digital delays?
> LOL.  BBDs are a rather complex and somewhat unforgiving device.   
> At best, with the best design
> and all the tricks applied.... they rival a decent quality cassette  
> player in quality. At worst they are expensive
> and die very young.

<cut short>

Harry...erem Mr. BBD, sir.

Besides some Panasonic data sheets, I can't find much on BBDs.

Why do BBDs fail?  Looking the MN3004 data sheet I can't find the  
current draw or power specs, so I'm guessing they may consume lots  
thus dissipate lots of heat.  Hmmm, what happens to an overheated  
MOSFET anyhow?  Silicon dioxide burns up?

In my analog design course I was taught that capacitors take up a  
huge amount of die area.  Seeing that a BBD has several hundred to  
several thousand capacitors, is the die relatively large?  For  
example, I seem to recall that the compensation capacitor in the 741  
takes up a high percentage of die, maybe as high as 70%?

What causes BBDs to have such a poor signal to noise ratio?  My guess  
is the lack of negative feedback at least in the sense of a closed  
loop opamp versus open loop.  Other thoughts include any noise on the  
clock pin may leak to the signal through the high impedance gate of  
each of the transistors.

The THD-fcp plot is interesting that its "U" shaped.  The THD is at a  
minimum at 50kHz or so clock frequency, but goes up at both lower and  
higher frequencies.  At lower frequencies I would guess the circuit  
is acting like a low pass filter because the clock is running too  
slow and at higher frequencies perhaps the capacitors don't have  
enough time to charge.

Hope my questions don't have simple answers that I've overlooked.   
Not much SDIY or even engineering experience, but I guess I have to  
start somewhere.  Obviously not by designing with BBDs as per your  
advice, but I am interested in why such a seemingly simple device is  
such an art to work with.

Thanks.

Nick.