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enter image description here

I was designing an oscillator for HF range. Although both the circuits are same, the first circuit doesnt work while the second one does, despite the fact that the varicap used has 17 pF at 20 V.
( Since it is my first encounter with varicap, I think I am not using it correctly)

Where am I going wrong ?

Also the output ( of the second one) is centered at around 15 V rather than 0 V. How to correct that ?

EDIT 1 :

Looks like there is a serious flaw in Multisim. I redesigned the circuits to be identical. But in Multisim, no matter what, the output is 41 KHz ( Yes, i simulated circuit 2 in multisim). I checked all the wirings etc, they are fine. LTSpice gives results close to the theorotical ones.

EDIT 2 :

I still do not understand where is the error. Here is the most recent circuit, following the suggestions : enter image description here

Again here, the circuit might seem to work. But thats not the case. No matter what tuning voltage I apply, the frequency is 41 KHz. Why is that so ? Can someone help me correct this circuit ?

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    \$\begingroup\$ As well as what Andy said - Varicaps need DC bias and are AC connected. \$\endgroup\$
    – Russell McMahon
    Jul 16, 2014 at 15:31
  • \$\begingroup\$ I tried connecting the varicap supply via different resistors,as @Andy said, from 100 K ohm to 1 Meg. But the results were miles away from ideal ( 41 KHz observed). I would really appreciate if someone can guide me on this. \$\endgroup\$ Jul 16, 2014 at 16:06
  • \$\begingroup\$ But you didn't note what I said. The varicaps ned DC bias. They are capacitively coupled so have no DC voltage across them. eg C14 blocks DC to D9. | Is the bottom of R2 conncted to ground as it should be? Or R1 & C6. Ground symbol looks to have "moved". \$\endgroup\$
    – Russell McMahon
    Jul 17, 2014 at 8:10
  • \$\begingroup\$ Yes, the ground is there in circuit 2 ( see the small triangle). Although I remember simulating both the circuits yesterday taking note of the suggestions, i'll try once again. It takes quite a long to simulate in multisim though. Just to be sure, I need to remove capacitor C14 and add a resistor 1 Meg or so with the battery ( circuit 1)? \$\endgroup\$ Jul 17, 2014 at 9:01
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    \$\begingroup\$ I do not know what simulator you are using. I do know that in some systems (probably many (possibly most)) that if you offset a symbol as that ground symbol is that it would not be connected. Similarly R2 may not be connected.| You still to be missing what I am saying about the varicaps. There MUST be a DC circuit with the varicap in it for an applied DC voltage to work. As well as the resistor applying the +ve voltage from +20V there needs to be a DC path to ground on the other side. ie -> ... \$\endgroup\$
    – Russell McMahon
    Jul 17, 2014 at 9:52

1 Answer 1

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Your first circuit won't work because you have no signal feedback to the base of Q1. Signal can't come via C8 because the other side is grounded and, signal can't come via the BB204 varactor because you have a solid 20V supply on the common pin of that device. Typically a 10k to 1M ohm resistor would be inserted in series with the 20V supply. Then it might work.

Signal feedback is needed for the circuit to oscillate and there is none!

On your 2nd circuit, C1 is not needed and can be shorted out - AC coupling will be provided via C3 and C4. This configuration will only ever have the output centred at the positive rail (+15V) because, if you analyzed it from a DC perspective, the collector is tied to +15V via the zero ohm resistance of inductor L1. If you require it to be at some other DC voltage then connect the right-hand side of C5 to that DC voltage via a 100kohm resistor and use the output from C5.

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  • \$\begingroup\$ I still did not understand why my first circuit doesnt work. Can you explain a little ? As for the second question, I removed the capacitor, the output is still centered at 15 V. \$\endgroup\$ Jul 16, 2014 at 12:48
  • \$\begingroup\$ I've added some text to my answer to explain - I'd missed the question about it being at 15volts. \$\endgroup\$
    – Andy aka
    Jul 16, 2014 at 13:28

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