Transmitter Circuit

gmcwilliams

Joined Dec 12, 2006
10
I have been designing a Wien-bridge oscillator using limiting diodes for amplitude stabilization.

General Circuit can be viewed here.

So I am trying to get this circuit to oscillate at 300 kHz. I was originally using a LM318 op-amp as my oscillator but I decided that I wanted to have the ability to increase my frequency if I so desired. I upgraded to the AD812AN high speed video op amp. My problem is that my calculated frequency is not even close when I use the AD812 to get an oscillation.

F = 1 / (2*pi*R*C) R = 4.7k C = 100pF

F ~= 340kHz

In actuality I am getting an oscillation > 6 MHz. Is this because I am using a video op amp? Is there something I am missing here that would allow me to design for the oscillation frequency instead of randomly swapping resistors and caps until I get where I want to be? Thanks for the help.

Greg

gmcwilliams

Joined Dec 12, 2006
10
Another curious note: I just changed the capacitors in the circuit from 100 pF to 1500pF and got zero change in the oscillator's frequency. I am going to change the resistors from 4.7k to 200k to see if that changes the frequency at all.

Any suggestions as to why my components are not effecting the oscillation? The sine wave is very clean and looks very good.

Dave_

Joined Mar 22, 2007
28
Are you building it on breadboard? Perhaps one of your capacitors is not connected properly and/or a track is broken so there is an open circuit, then the stray capacitance of the breadboard would come into effect, as wika says this is about 2 - 25pF which would cause a frequency of 6MHz+.

gmcwilliams

Joined Dec 12, 2006
10
Thanks Dave,

I went through and rechecked all of my connections and actually did find an incorrect trace.

I am having another problem that I am finding very peculiar: I am able to very clean oscillations at 255 kHz using a 100 pF +/- 2% tolerance and a 4700 +/-5% resistor. If I change either the resistor or the cap I lose oscillations. I wanted to bring the oscillations a little higher so I attempted to put in a lower resistance and it didn't oscillate. I put the 4700 back in and it sprang back to life. I just finished swapping out an 82 pF capacitor for the 100 pF and the oscillations stopped. I put the 100 pF back in and they came right back, clean and crisp as could be. I even have amplitude control due to using pots on the output.

I'm just curious as to why the oscillations would stop if I am attempting to increase oscillations. The AD812 has a gain bandwidth product of over 120 MHz so that shouldn't be a factor. Additionally the slew rate is more than enough for the voltages I am attempting to produce. Any suggestions on things I could test as being the reason why I can't increase oscillation frequency? I would like to be able to go from 250kHz to 3MHz using the same design except swapping out caps/resistors.

Thanks for all the help,
Greg