about the waveform(lacy help me!)

[vick123y]

hi,all.especially lacy. I have a question to ask again. As what you have told to do,I got the circuit to run. As well,I got the right waveform at 0.8MHz.But unfortunately,I didn't got it at6.4MHz. I think there must be somthing wrong with my circuit or some of the settings. I hope youcan help me again. THANK U!! I put the circuit and the waveform.

[lacy]

I have to ask a question before I can determine exactly what is happening here. You are creating a frequency by division and the running it to a low pass filter. Are you trying to have the output filter to eliminate this frequency or do you what the filter to pass this frequency? If you are trying to elimate this frequency then it appears to be doing that on my simulations of the circuit.

One thing I will tell you is that this IC has it's ouput bandwidth locked at 10 MHz. I f am correct in my math and my asumption about this IC (based on my reading of the datasheet) then you are multiplying 6Mhz by 6Mhz which would equal 36MHz and the you are dividing this by 2 for a frequency of 18MHz. This would put you over the bandwidth limit on the IC and could be what is giving you incorrect results at your 6MHz input. This is just my theory.

Please let me know what you are expecting at the output of the filter and then we will proceed from that point.

[vick123y]

I want to use the filter to let the lower frequency to pass.I have tried lots of time.When I let the both voltages(6.4 MHz,6.402MHz) at 1V,it can give me the right result.But when I give a lower voltage(you konw,there is no such a high voltage in industry),such as 10mV(6.4MHz)and6.402MHz,it cannot work as I expected.The waveform you can see in my first message.

AD734 is exactly a multiplier.But what I have put in is the cosine,so you can not work out just as a multiplier.The formula is cos(WxT)*cos(WyT)=cos((Wx-Wy)T )+cos((Wx+Wy)T).So I can get the correct frequency 2KMHz.

[lacy]

I want to understand what is trying to be accomplished and you will have overlook any misunderstanding I may have on what you are wanting this circuit to do. From your last post it seems to me that you are attempting to modulate one frequency with the other. Is this correct? The reason I ask is that in my results I only see one output frequency which is a combo of the input frequencies . What I did was to test at the IC before the filter. The frequency I came up with was 12Mhz. There was no other frequency there so the ouptut filter was just eliminating that one frequency at it's output.

I am continuing to look at this. If I find anything that will help I will post it for you.

[lacy]

O.K. My apologies for my misunderstandings here, but I do see now what you are referring to. I just didn't have the scope set up right in my tests and that is why I was seeing a flat line on the output of the filter.

Unfortunately at this time I do not have an answer for you. I will continue investingating this a see what I can find. It appears that your circuit configuration is correct as it was previously so we can elimnate that as a cause. I am curous if this chip in simulator requires a certain amplitude of voltage on it's input in order to function correctly. I have run into that on occasion where a chip wouldn't work in the  simulator until it's input was above a certain level. This was mainly on digital chips. I will look into all this and let you know whether I come up with something that will help.

[lacy]

Unless there is something not right about the model for this IC, I can't seem to find why it will not work below 500mV. This is the lowest voltage that give the ouput your are looking for. I tried various simulator settings and came up with nothing.

Maybe Max, Nestor or other NI Team member can help. They may be able to see something that I don't or have information I am not aware of.