[sdiy] VCO unstable in Spice, stable in real world

[Rutger Vlek]

Hi guys,

That's a great idea, thanks! Initially I was investigating a way to trigger an additional discharge transistor in the slave VCO with a saw wave from the master oscillator that was fed into a pulse shaping circuit. But I couldn't get the pulses to be as nice and tight as the comparator output. So that's why I really like this alternative solution of bringing the master's comparator output directly to an additional discharge transistor in the slave VCO. For buffering I have a couple of ideas (sorry, spending 1/6th of a 4069 is such a waste):

1) I need some sort of voltage controlled switch in the sync-path to be able to turn on/off the synchronization from a CPU. Could an analog switch fulfill the role of buffer at the same time? And what would happen with the 'floating' sync input, if the switch is open? Would I need a SPDT switch to normalize it to ground? I guess since it's the current that switches the transistor it doesn't matter?

2) I have a few spare 4011 NAND gates, so I could probably wire these up to buffer the master's comparator output as well. But then I still need to fit voltage controlled switch somewhere. Maybe with some clever wiring the NAND gates could be the buffer and the switch at the same time? If one of the NAND inputs takes the CV to turn it on/off and the other takes the comparator output?

Best,

Rutger

On 4 feb 2011, at 21:45, David G. Dixon wrote:

>> Bringing the reset comparator output uhh... 'out' can be troublesome. You
>> can easily inject noise onto the comparator output from the outside world
>> and screw up the VCO reset.
> 
> Yes.
> 
>> I'd buffer that output if possible, a 4000 series CMOS gate would do well
>> and the propagation delay of 50ns (max for a 4069 on 15V supply) is 
>> unlikely to be a problem.
> 
> Awesome suggestion!  I suppose you could invert the comparator output with
> CMOS as well without adding significant delay, if you needed the inverted
> version for whatever reason.  The only problem is that you've then got five
> spare buffers on the 4069 chip.
> 
>> Of course, don't forget to add a 1K series resistor on that output :^)
>