Thank you for your input. I did some more tests today and below is what I found out. Basically, the punch line is that Wien bridge oscillators are apparently horribly unstable in terms of voltage, that or my breadboards are haunted...
Firstly, C9 was never included in any of these measurements/circuits, I soldered it off my PCB to test this circuit ..
Secondly, what do mean by R30?
Although my original question was regarding my breadboard (BB) vs PCB I decided to eliminate all the variables and breadboard a whole new oscillator from scratch. I even took the BAS16 SMD diodes and mounted them on break outboards.
As you can see from the next two pictures, the same circuit, made with exactly the same through hole components (and the LMV751 on a breakout board), acts relatively the same using BAS16 or 1N4148 diodes. This is surprising, since they are what I thought would have the largest impact on the amplitude, since they are what cut it off technically. The only observable difference is that the BAS16 are quicker to ramp up.
New breadboard with BAS16
New BB with 1N4148
As you can see, both these breadboards give me much higher Vpp (1.2V) than my original BB but not as high as the PCB (1.8V)
The next picture is old BB that was giving issues (posted again to appease Nyquist and the rest of you
Old BB with 1N4148
The old BB was measured again with another LMV751 chip and it acted the same as before (Vpp=680), furthermore, the LMV751 chip from the old BB was put on the new BB and the new BB acted the same (Vpp=1.2).
At this point I had two IDENTICAL BB, checked over 3 times by myself, as well as a lab tech with 30+ years of experience at Phillips and neither of us could explain the readings we observed..the only physical difference was that one BB was slightly smaller than the other, and a slightly different shade of white !!
However, touching the components here and there, I noticed that if I touch either side of the 200k resistor, the amplitude of the osculations increased dramatically (~2Vpp). This was replicated on both the old and new BBs. Assuming this was my stray capacitance, I added a 3nF capacitor from the node between D2 and R17 to ground and this confirmed my belief. The cap increased the amplitude of the oscillations.
To see if this was the cause of the increase on my PCB (since I have a large ground plane under my oscillator circuit) I measured the capacitance from that node to ground and got a reading of 3pF... no dice!
I gave up at this point, and while touching the circuit some more, I realized that touching the ceramic part of C14 slowly (~10mV per sec) kills the signal while touching the ceramic part of C13 increases the signal. This turned out to be due not to my capacitance but due to temperature.. Apparently the 1% through hole capacitors I bought are garbage at Temp sensitivity and change their capacitance even from the heat of ones finger..
Long story short, unless any of you have something to add that I missed (I really really hope you do!!) let this be word of caution for those trying to get a predictable amplitudes from a diode controlled wien bridge..
Now, onto my actual question, how to deal with this, since the rest of my PCB cant be changed and was designed for an output around 600mVpp (based on the old BB)...
One idea I had, was to use a shotkey diode since it has the same footprint and should technically cut out the oscillations quicker, since they have a smaller forward voltage.
Another was to use thermistors, however I doubt the tiny amount of current this thing drives will heat it up significantly.
Basically, I need to generate a sine wave that has a constant amplitude over time, and which cannot ever go above 600mVpp. Does anyone have any suggestions for a good voltage throttle circuit, or where I can start looking?
Also, since the wien bridge seems to be pretty rubbish in the reliability sector, what other oscillator designs would you recommend? I dont necessarily need to run at 20kHz, I can probably go up to 100k, so maybe a wien bridge using a watch crystal would be a solution.
Another is to output the clock signal from my ATtiny45 micro controller and lowpass filter that into a sine wave, would that work?
What do you think of those solutions?
Thank you again for all your help, and even if this never gets figured out completely, hopefully it will be useful for anyone trying make a signal generator this in the future...
