[sdiy] I have found that SSM2210 transistor arrays give better performance in my latest oscillator design.

[Ian Fritz]

Ray --

With a 100 pF integrating cap and low frequencies you are working at very 
low current levels.  A bit of 60 Hz pickup wouldn't be too surprising.  You 
might try 4700 pF for your 2.5 V amplitude saw generator.  This is about 
twice the typical value of 2200 pF used for a 5 V core, so you would be in 
the more usual current range.

   Ian


At 07:01 AM 6/25/2006, Ray Wilson wrote:
>The jitter seems most prevalent at lower frequencys when I use a value 
>below 330pF (integrator cap). So instead of getting a nice hollow sounding 
>square wave I get a square with what seems to be a very very slight but 
>audible 60 Hz jitter. I believe it may be induced from the fluorescent 
>lights above my bench (4 feet away). When I grounded the metal part of my 
>work table (one of those folding table types with the particle board top, 
>metal frame and folding legs) it was reduced to inaudibility. I haven't 
>seen it with my scope. That was with 100pF. When I use 330pF or above I 
>get the same level of inaudibility without grounding the metal parts of 
>the table. I know that sounds weird but it's true. It is not a problem at 
>higher frequencies (> 150 Hz) I'll try some of your suggestions and see 
>what happens. My bench power supply is capable of 1 Amp at +/-12V (and 1 
>Amp at +/-15V) and I'm only using 30 to 40 mA in the circuit. I use 
>LM7812/LM7912 and LM7815/LM7915 pairs with National's suggested bypassing.
>
>Thank you.
>
>Ray
>
>
>
>----- Original Message ----- From: "Ian Fritz" <ijfritz at comcast.net>
>To: "Ray Wilson" <raywilson at comcast.net>
>Sent: Saturday, June 24, 2006 9:42 PM
>Subject: Re: [sdiy] I have found that SSM2210 transistor arrays give 
>better performance in my latest oscillator design.
>
>
>>At 03:18 PM 6/24/2006, you wrote:
>>
>>>All I've done so far is remove one of the LM394s and replace it with an 
>>>SSM-2210 and then adjust the scale pot while inputting 0 through 7 
>>>volts. If I try to use a smaller cap (and I assume lower the current at 
>>>10KHz) I tend to get a fair amount of jitter. Is there something you 
>>>would suggest I try to lower the current other than reducing the 
>>>integrator cap value? Or is there something that could reduce jitter at 
>>>low cap values? I appreciate your input. I have observed that you are 
>>>way more into the math and theory than I am. I'm more of a technician 
>>>(try this and that until it works kind of guy) than an electronics engineer.
>>
>>Well, Ray, I've always leaned very heavily on the "Edisonian" approach 
>>myself.  Especially in the rarified atmosphere of the fine points of VCO 
>>tracking. The theory explanations tend to come after the data and the 
>>bloody work of tweaking up the circuit.  And I very much appreciate the 
>>high level of performance you are getting from relatively simple and 
>>inexpensive designs.
>>
>>As far as jitter, I really don't have any ideas.  I haven't ever looked 
>>too closely at that ... never seemed to be much of an issue.  I guess the 
>>first thing I would look at is possible HF oscillations in the converter. 
>>After that, maybe noise pickup on the supply lines to the converter. 
>>Ferrite bead decoupling between the switching section and the converter 
>>might be something to try.  Also be careful of switching spikes coupling 
>>radiatively into the converter.  How bad is the jitter?  Is it audibly 
>>annoying, or just something you see on a scope?
>>
>>   Ian