[SOLVED] simple VCO circuit with BJT

[peanutnjelly]

Hello everyone!

I am inexperienced in RF circuits, and I currently testing with some different circuits in "deadbug mode" to change that .

I am having a problem with this circuit:

All elements are the same except varactor diode is bb182.
Circuit is from Complete wireless design book.

Circuit starts to oscillate but it stops after about 4us and than starts again and again.


On left side of the board is almost the same circuit but without varactor(fixed frequency) and it works just fine at 50MHz.
On right side is mentioned circuit. For the time that it starts to oscillate frequency is 50MHz.


Any idea where is the problem?

 

[nilses]

Hi,
Check out this link: https://www.qsl.net/v/va3iul//Homebrew_RF_Circuit_Design_Ideas/

Here you will find a lot of working and tested oscillator circuits. Good luck

 

[BradtheRad]

I constructed a simulation. It duplicates the regular dead spots between periods of oscillation.



For the varactor, I increased the value of C4. Since the diode carries less current than C4, I let it remain as a diode.

By creating a number of scope traces, it becomes obvious that there is a lead-up time to oscillations. Current must go through C8-L1-L3 for a while. When C8 has charged sufficiently, oscillations begin.

Resonant action builds around a loop consisting of C3-L1-C4-C2. Oscillating action is strong and consistent.

However in the meantime, charge is drawn off C8, while charge accumulates on C3. At some point oscillations die.

The process begins all over again.

How to cure the dropouts?

I believe you need to prevent the extreme charge level from developing on C3.

There may be unneeded components in your schematic.

L3 for instance. It is a high value for the frequency you are operating at. It provides a path to ground, allowing DC current to continue to go through C8 while oscillations continue. This may lead to the dropouts.

Further experimentation could discover whether L3's value should be changed, or perhaps whether it should be omitted.

 

[Terminator3]

I am learning from book RF Circuit Design (Ludwig, Bogdanov), and tried only FET oscillators.
Try to alter base (gate) voltage, change it bias. Add more RF bypass at lower frequencies. Maybe your circuit have lower frequency of oscillation, and it knock out 50MHz each time low frequency sine puts transistor in unwanted region of operation. Experienced members correct me if i am wrong =)

- - - Updated - - -

BradtheRad, did you use https://www.falstad.com/circuit/ for simulation? I am very impressed that this problem can be simulated. Nice simulator..

 

[BradtheRad]

I am learning from book RF Circuit Design (Ludwig, Bogdanov), and tried only FET oscillators.
Try to alter base (gate) voltage, change it bias. Add more RF bypass at lower frequencies. Maybe your circuit have lower frequency of oscillation, and it knock out 50MHz each time low frequency sine puts transistor in unwanted region of operation. Experienced members correct me if i am wrong =)

Yes, that could be the case. A second slower resonant action which interferes.

BradtheRad, did you use https://www.falstad.com/circuit/ for simulation? I am very impressed that this problem can be simulated. Nice simulator..

Bingo!
Below is the link that will open the falstad.com website, load my schematic into his simulator, and run it on your computer. Same animation I see. Same interactive capability.

https://tinyurl.com/kqef89s

 

[nilses]

Hello everyone!

I am inexperienced in RF circuits, and I currently testing with some different circuits in "deadbug mode" to change that .

I am having a problem with this circuit:
View attachment 96997
All elements are the same except varactor diode is bb182.
Circuit is from Complete wireless design book.

Circuit starts to oscillate but it stops after about 4us and than starts again and again.
View attachment 96998

On left side of the board is almost the same circuit but without varactor(fixed frequency) and it works just fine at 50MHz.
On right side is mentioned circuit. For the time that it starts to oscillate frequency is 50MHz.
View attachment 96999

Any idea where is the problem?

Hi,

Why don't you go 2 steps back and find a working oscillator circuit and try to simulate ?
Your circuit looks a bit strange for my old eyes.

I have already sent you a link to about 30-40 tested and working oscillator circuits.

Find a Colpits oscillator at about 50 MHz or thereabout and try it out.

Your best simulator is a soldering iron and a board.

 

[keith1200rs]

If you want to understand the problem with the oscillations starting and stopping, look up squegging or squegging oscillators - it is a well known phenomenon.

Keith

 

[Bob60]

Probably schematics is worst. I recommend instead L3 resistor some kilohm, R3 C1 may be
you don`t need at all. And you can try series 10ohm resistor with BJT emitter.

 

[E-design]

What are the design goals? Is it supposed to oscillate at 50MHz with the varactor diode in place?

I agree with others that your problem is caused by the 270uH inductor.
This can be seen when we do a open-loop analysis on the circuit. There are two frequencies (one low and one high frequency) that the circuit wants to oscillate at.

Replace the inductor with 10k-100k resistor and reduce the value of the resistor that ties the varactor to the control voltage to say 1k.

The 4.7nF coupling capacitors are also way too big. The open-loop gain should be around 10-12dB max to achieve good stability and low phase noise.

- - - Updated - - -

Here I have replaced the 270uH with 100k and reduced the coupling capacitors to 33pF, and the circuit seems to have no problem oscillating at the correct frequency.

- - - Updated - - -

If we put the 270uH and 4.7nF caps back we get the original symptom.

 

[peanutnjelly]

Thanks for the response!
I simulate circuits with LT Spice and i have tried many elements values and options. Now circuit finally works in simulation and on soldered board.
https://obrazki.elektroda.pl/8087628200_1381172578.png
It oscillate from 42 to 77MHz with tuning voltage from 0 to 12V.

Goal of this is just to learn something .
Now I will try to make PLL.
Thanks again.

 

[E-design]

Is that your final circuit?
R3 and C1 lower the overall gain but increase the bandwidth. It also helps with impedance matching and suppresses spurious oscillations. It is a good idea to keep it in the circuit.
The coupling capacitors only need to be large enough to overcome the insertion loss of the resonator with some gain margin to spare for reliable starting. The smaller you make the coupling caps, the higher your insertion loss will become but at the same time will raise the loaded Q for better frequency stability and phase noise performance.