Linear vs. Audio taper pot w/frequency gen.

[Brian218]

Hi,

I'm in the process of designing/breadboarding a signal generator with the XR-2206 Function generator chip. The data-sheet is on this link: **broken link removed**

Eventually, I plan to interface it with a micro-controller to provide a LCD display, frequency counter function and misc. other functions, but that's a little ways down the road...

The frequency of the EX-2206 is determined from the RC time constant.

The ideal frequency range would be 'full range' .01 Hz to 1 MHZ, or at least close to full range. I initially plan to use 4 timing caps (.001:mu:F to 1:mu:F), which will be selected in any combination by a 4 pin DIP switch. The timing resistor will be a 1 Meg pot, in series with a 10k-100K pot for fine adjustment. All RC values are preliminary and may be changed for optimal output.

Most of my frequeny projects are in the audio range, but a few of them are at higher frequencies.

My main questions are:
1. I'm considering an option that will allow selection between audio and linear taper pots for the timing resistors. Would this be a worthwhile option, and if so, how much difference is there between an Audio and linear taper pot? I don't have any audio pots laying around, so I can't really swap one with a linear to see what the difference might be.

2. Has anyone else built a function generator with the XR-2206 (or similar) chip, and do you have any advice or recomendations on this project?


Thanks in advanced!

 

[crutschow]

My main questions are:
1. I'm considering an option that will allow selection between audio and linear taper pots for the timing resistors. Would this be a worthwhile option, and if so, how much difference is there between an Audio and linear taper pot? I don't have any audio pots laying around, so I can't really swap one with a linear to see what the difference might be.

Audio taper pots have a logarithmic change of resistance with rotation. The resistance from the counter clockwise end to center is about 10% of the total and the resistance from the clockwise end to the center is about 90%. This would give a much finer control of the resistance from the counter clockwise end of the pot which, in your case, would be the low frequencies ( I assume you would tie the wiper to the CCW end to give an increase of frequency with a CW rotation). Whether this is desirable may have to be determined by experiment.

There are some pots available with a reverse audio taper which has the lower resistance from the CW end. That would give a finer adjustment for the high frequencies which may be preferable for your application.

A way to approximate a non-linear pot is to put a resistor from the wiper to one end. For example, if the resistor is equal to the pot resistance, than the resistance at the center position would be 1/3 of the maximum instead of 1/2. Of course, the maximum resistance is also reduced by 1/2, so you would need a pot of a larger value (double in this case) to give the same max. resistance as the pot with no resistor.

 

[Brian218]

Thanks Crutschow!

I appreceate you response, I'll try your suggestion and see what works best for my application.
B-