Sorry, but I must ask you:sk.shawn said:Dear all,
...I have tested all individual blocks separately and they are all working (opamp, FVC, open loop VCO). I feel that it could be just some minor thing that I need to get it figured out... Shawn
karesz said:do you have pls at your OpAmp resistors too?
You are operating your opamp in open loop integrating configuration, which means it might have a DC gain of 80 or even 100dB, i.e. an input change in the order of only -10µV .. -100µV can cause your required output change from 1.2 to 2.2V.sk.shawn said:... Ultimately, the range of voltage that I would like to have at the opamp output is 1.2V to 2.2V
erikl said:Yes & Thnx Erikl!sk.shawn said:... In order to find the correct operating point (by adjusting Vp @ V+), you should first decrease the DC gain of your opamp by using a feedback resistor in parallel to the integration capacitor C2. (I guess this is the reason for karesz' question above.) The value of this resistor depends on the output resistance at the V- output of your FVC, but a value in the order of 100kΩ .. 1MΩ should be fine for the beginning.
You know-my english is some time more to deshifring _worser as possible...:-( Sorry!
K.
erikl said:Hi shawn,
with your schematic and the paper it's much clearer now, thank you! Seeing now that your FVC together with the opamp is a proper switched-capacitor circuit, you probably don't need a feedback resistor. And as it's not a full chip design (as I reckoned before, because most of the threads in this forum are), you have no problem to inject a (small) negative DC voltage at the Vin input, hence no operating point adjustment at the Vp=V+ input is necessary. So I suggest the following procedure to adjust the operating point resp. range:
During the adjustment, keep the VCO in open loop, i.e. break the loop between the opAmp output and the VCO input. Control the VCO for the desired frequencies by inserting an appropriate DC input voltage.
1. Put the VCO to the lower corner frequency. Measure the necessary DC control voltage - let's say it's 1.2V .
2. Then - at the Vin input - without applying a sinus ac voltage - adjust a (probably small) negative DC voltage which generates the required opamp output voltage of 1.2V . This calibration procedure has to be done only once or just for a few iterations (s. below). Then this voltage should be fix forever.
3. Now push the VCO output by an appropriate DC control voltage at its Vin input to the desired upper corner frequency. Let's say the necessary control voltage now is 2.2V.
4. Now measure the opamp output voltage at this upper corner frequency. If it's also close to 2.2V, you're already done. If it's too for away, you can adjust it by changing the Csw/Cint ratio, which defines the ac gain of the opamp. If your opamp output is too high, reduce Csw, if too low, increase it.
5. If you had to adjust Csw, you possibly must readjust the -Vin voltage, i.e. iterate from point 1 to 4 above.
When your operating range adjustment is ok, you can close the loop and it should work. Applying an additional (capacitively coupled!) ac voltage at the -Vin input shouldn't change the operating point.
Hope this helps. Good luck! erikl
Yes, perfectly.sk.shawn said:
Still correct, as I suggested.sk.shawn said:Assuming it to be, I proceeded on by feeding Vctrl (i.e. the explicit input now to the VCO) the lower value of my intended range (i.e. 1.2V). This in turn generates an oscillating signal whose frequency is about 90MHz. Now the next step, if I didn't misunderstand your explanation is to start adjusting Vin which is a small negative DC voltage in this case. I did the adjustment until I saw a 1.2V appearing at the opamp output and the Vin value found is 9.8uV.
No! I didn't suggest to readjust -Vin ! The old value must be kept! Keeping the old -Vin value, you should just measure the new opAmp output voltage resulting from the new 77MHz frequency controlled by the open loop 2.2V input voltage to the VCO. Thoroughly re-read my point 4. above, and - if necessary - make the iteration steps as mentioned in point 5.sk.shawn said:And next, I did the exact same thing for the upper value of my intended range (i.e 2.2V). Again, I fed 2.2 to Vctrl which generated an oscillating signal whose frequency is about 77MHz. After which, I did the same thing of adjusting Vin again until I saw a 2.2V appearing at the opamp output and this Vin value now is 341uV.
Actually - in my first posting to your thread - I reckoned you'd design a full chip system which has to get along with one single power supply. This of course is also possible with your discreetly built system: Just provide the operating point calibration at the V+ input of your opAmp, as I adviced you in my first posting. In this case you may use a single supply (of 3.3V) for all your subcircuits; no negative supply is necessary for the opAmp (in case you use an opAmp with a Common Input Voltage Range including the GND rail, as the LM324, which you probably use, I guess. The LM324's min. single supply voltage is 3.0V).sk.shawn said:P.S An important thing that I left out initially. For just testing now, I'm powering my opamp with dual supplies (i.e 2.5V and -2.5V) while the rest of the circuits (FVC and VCO) are operating in single supply with a value of 3.3V. In fact, the entire circuit should be just operating in 3.3V supply where I will eventually power the opamp also using a single 3.3V supply. But since, I'm at this stage of figuring out things, I just wanted to get the idea first. Will this supply thing be an issue? Just to let you know, the reason for powering the opamp using dual supplies initially is, to me, for simplicity sake since Vp is grounded. I reckon having dual supplies would be an easy way to get around the biasing of the input transistors of the opamp. Please let me know if there are any issues with this?
FvM said:Seems like you don't want to verify the operation of the feedback circuit? I don't understand why.
If you finally succeed to operate the circuit, can you kindly report, if one of my guesses about it's failure have been correct.
erikl said:Still correct, as I suggested.
However, this voltage is very small, and probably much too sensible to be fixed forever (and that's what it should be!). In this case, I'd really suggest to use a feedback resistor (as I suggested before). I'd recommend to use a value Rfb which results in a dc gain of 10 .. 100, i.e. Rfb = 1.5 .. 15MΩ. This will shift the necessary -Vin (or +Vin, s. my last point below) input voltage from 9.8µV to a value in the order of 10 .. 100mV, which is much easier to be stabilized and kept. See my point 2. above: "Then this voltage should be fix forever."
Now the next point is where you misunderstood me:
No! I didn't suggest to readjust -Vin ! The old value must be kept! Keeping the old -Vin value, you should just measure the new opAmp output voltage resulting from the new 77MHz frequency controlled by the open loop 2.2V input voltage to the VCO. Thoroughly re-read my point 4. above, and - if necessary - make the iteration steps as mentioned in point 5.
PS: Exchange "lower" and "upper" frequency corners in my instructions above - I couldn't know that there exists an inversion in your FVC.
Yes, I think so. And anyway, an SC based FVC should output (resp. inject into the Vn=V- input of the opAmp) a signal which rises with frequency, as I anticipated in my 2nd posting to you (cf. "lower" and "upper" frequency corners in my posting from Tue, 05 Jan 2010 0:25).sk.shawn said:So, basically from this and from what FvM had speculated much earlier, the FVC is not working properly. Am I right?
Yes, please!sk.shawn said:I will keep updating the status.
IMHO this SC circuit actually isn't really a FVC: I don't see how it could produce a frequency-dependent output voltage. What you state above, I much more consider as a 2nd order effect (not only because it's so tiny, but generally by studying the circuit). Probably this small increase just results from different duty cycles at the 2 frequencies used.sk.shawn said:... as the frequency increases, the signal generated by the FVC will increase too which is true for my case although the increase is not very large but still there is an increase. So now the FVC is like working to me ...
Yes, I think this one is ok.sk.shawn said:Also attached is the frequency response of the opamp which is alright to me.
Actually, I cannot understand (from the original paper) how this FVC is supposed to work. In order to produce a changing DC control voltage for the VCO, the input of your opAmp also must receive a frequency-dependent DC voltage, and I can't see how this should be produced by your SC circuit, sorry.sk.shawn said:In addition, I seriously don't understand the purpose of having an integrator capacitor there when Vin is going to be a DC input which will cause the capacitor to open and hence no negative feedback in return. In that case, should Vin be a DC signal at all. I'm really confused. Also, if a feedback resistor was required then why didn't the author of the paper (provided earlier here) mention anything about it?
Hi Shawn (and FvM),FvM said:A negative Vref (if possible) would be solution to invert the feedback loop operation.
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