Negative output clock generator help...

Thread Starter

liquidair

Joined Oct 1, 2009
192
Hey guys, this is my first post and hopefully my problem has a very easy solution.

I'm trying to generate a complementary negative output clock to drive a BBD chip at high speed (up to 3MHz clock, 1.5MHz when divided). I plan to set up an LMC555 timer as a relaxation oscillator, then send that clock to a CD4013 to be split and divided down, then sent to a CD4049 to be buffered. That's fine and dandy when the clock output is +9V-0V, but I need 0V - -9V. So would this work:

Convert the output of the 555 to a negative clock using reverse series/shunt diodes on the output, then run the CD4013 and 4049 from a Vdd = Gnd and Vss = -9V??? What do you guys think? I really appreciate your help and time!
 

rjenkins

Joined Nov 6, 2005
1,013
Can't you build the whole clock circuit as single-supply, but actually run it as it's V+ being the BBD 0V and it's 'ground' being the -9V supply?

No level conversion needed.
 

Thread Starter

liquidair

Joined Oct 1, 2009
192
Well, originally I was going to do that but I read that the 555 timer's trigger has to be a positive voltage, unless I'm mistaken.
 

Wendy

Joined Mar 24, 2008
23,415
A CMOS TLC 555 is rated at 2Mhz, older 555s are 1Mhz or less. You'd be better going for either a transistor crystal oscillator, or make one with a gate.

What exactly do you mean by a negitive clock? Are you talking 2 square waves inverted from each other?
 

Thread Starter

liquidair

Joined Oct 1, 2009
192
Hey Bill, your 555 work was what inspired me to use the 555 timer!! I was planning on using the LMC555 which is rated at 3Mhz. I realize now the error in my reasoning about the trigger, supplied as RJenkins said the trigger would be a positive voltage (with respect to Vss = -9V).

Bill, with the 555 wired as a relaxation oscillator like you have written about, could you use an LFO on pin 5 (control) to slowly change the output frequency very very slightly so that you don't have a comb filter effect with a delay (bbd)?
 

Wendy

Joined Mar 24, 2008
23,415
As much as I like the 555, there are better VCO's out there. I assume that is what you're after.

You're after a monostable? I still don't understand what you're after exactly. A simple transistor can invert the signal, or a inverting gate.
 

Thread Starter

liquidair

Joined Oct 1, 2009
192
No, I'd like an astable. What I need is a variable frequency square-wave oscillator that is sweepable from roughly 160kHz to 3MHz. I need complementary outputs (like you'd get from a CD4047) and I need them oscillate between 0V (high) and -9V (low (roughly). This is to drive a BBD delay chip.
 

Thread Starter

liquidair

Joined Oct 1, 2009
192
Sorry, I made an error in that response....I need it sweepable from 80kHz to 1.5Mhz. My above frequency range is necessary only if I have to use the CD4013 to get the compl outputs.
 

Wendy

Joined Mar 24, 2008
23,415
OK, the output levels and complementary are no problem, the frequency is going to be a bit harder I suspect. I know there are some good VCO chips out there, but not off the top of my head. 80KHz to 1.5Mhz is a pretty wide range, you need the sweep to be continous (and linear)?
 

Thread Starter

liquidair

Joined Oct 1, 2009
192
Sorry Bill, I think my language isn't very clear. It's my lack of electronics knowledge that is causing me to be confusing I think. By "sweepable", I just meant variable with a pot.

I've looked at other chips but the only other thing that comes close to working (setting the frequency with a pot in that frequency range) is the CD4047, but I hear that it is pretty much useless above 100kHz or so.

Here's your 555 circuit I was going to use:
http://forum.allaboutcircuits.com/showthread.php?t=15342

Your above circuit however seems perfect. With the right values, C=17pf, a series 13.7k, and a 250k pot, the output frequencies are right. A CD4013 would then divide the clock into compl outputs, and a CD4049 would drive the 700pF load (using 3 sections per compl output).

My question really is how to get the output negative and if it's as simple as connecting Vdd to Gnd and Vss to a negative voltage (say -9/10 V)?
 
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