I have designed a circuit with two oscillators that need to be mixed. I know the low pass filter of the circuit attached works properly. I wanted the oscialltors to run at 100Khz. That isn't a must. The osciallator and mixer circuit isn't operating properly.
You gonna have to redesign your oscillators because those 0.1 µ
base-emitter caps short the signals to ground.
And . . . both oscillator signals aren't mixed but added therefore the
output of the low-pass filter will always be zero.
Those oscillators are Colpitts, and there is a voltage (about 100 mV) across the 0.1uF caps. [edit]If you want more output on the bases, make the collector and base resistors more equal. (To maintain frequency, you'll have to change the values.)[\edit]
Point about the mixing is well taken; you need a mixer to combine the two signals. [edit] Look for single balanced mixers, you can do it with two or three transistors.[\edit]
Got the mixer curcuit working and the low pass filter. The osciallators don't seem to work or they only work for the first 5ms or so. Any advice. Is there any other designs of oscillators?
I'll give that a try. Realized it wasn't the osciallator I think that is the problem but that Pspice doesn't read the oscialltion properly. The oscialltion looks as it is attenuated. Then if you change step size and slower time. You can see the oscialltion. Has anyone had this problem before or know what is up with this?
You can also use a single transistor. But doing so usually reguires a great level difference between the two input frequencies. Mixing requires operating in a non-linear mode. With a balanced mixer the output is generally only the sum and difference of the input frequencies. Using a conventional mixer(unbalanced) the output has four frequencies. The sum and difference and the two input frequencies. The low pass filter only passes the difference which I assume in this case is going to be an audio frequency. A good balanced mixer is the SA602 or the SA612.