Help with 5.8 GHz oscillator using BJT
I would like to design oscillator for 5.8GHz. It has to be with current consumption <5mA with 5V or 3,3V it doesn't matter. Phase noise should be as low as possible. Output power has to be over 3dBm. And of course price as low as possible.
I would like to use coaxial dielectric resonator which has 5.8GHz resonance.
I have some types of BJT - for example - BFU725, BFU660 or BFU630.
Is it possible to make it from these points?
How should I start because I am rookie at oscillator designing. I read some articles but the most important is to try it practicaly (meet paper stuff with practical board). I can use MWO for that.
Thank you
While ago designing a 5.8 GHz VCO I did experiment few SiGe transistors. The best results I had using BFU760, followed closely by BFP405. The last one is obsolete but you may find some available to online stores. Other SiGe transistors from NXP or Infineon should work, but have to allocate time for evaluation.
BFU760 has low VCEO voltage (collector-emitter with open base, max. 2.8V), so have to be careful how you design the DC bias network.
Next step would be to find the varicaps that fits your VCO design. Depending by the VCO frequency range, you have to look for varicaps with maximum capacitance no bigger than about 7pF (check Skyworks or NXP).
I would like to use biasing with 4 resistors (1 for C, 2 for B, and 1 for E,see atttached file).
Attachment 143420
Well, I will focuse on BFU760 as you said vfone. Do you think that is possible to make this output power from this low current consumption - this point is critical for me.
How can I calculate oscillation conditions? Here is URL for that resonator. I don't know how to calculate with him.
https://ferrite.ru/uploads/pdf/produ..._CRS%20(1).PDF
Should I use CC, or CB or CE?
The link to the attachment that you post is not working, so I cannot say what output power you want.
The output power that I got using BFU760 was about -3dBm for about 6mA DC current consumption. I used CC.
I didn't use coaxial resonators, but should work fine using them. Due to their high Q factor, the output power might be (a bit) higher..
Well, ok.
And do you know if there is some theory about Ic and Pout(max)?
Please see attached files of resonator and sorry for not working URL.
RF efficiency of an oscillator (Pout vs Ic) depends by many factors, which are a bit different than those that affect the efficiency of an amplifier.
The output power of an oscillator is somehow inverse proportional to the open loop gain of the active device. Placing the active stage in linear Class-A (which is recommended for most of the oscillators) you get the highest stage gain but the output power is lower.
If you move the oscillator bias from the linear class (to AB, or B, or even class-C), the gain will decrease but the output power will increase. In this situation also the harmonics will increase, due to distortions that appear in the waveform. A LPF at the output will fix this issue.
The oscillator feedback also affect the output power. Various places around the oscillator stage could be used to pick-up the output signal, but each of them have pros and cons, in terms of output power, harmonics, impedance, influence on the oscillator parameters as frequency stability and phase noise..
An optimized simulation will give you the best oscillator performances.
Thank you for reply. This is very helpful experience. Could you recommend me some book where is oscillator designed by step by step?
Thank you vfone and wish you merry christmas and happy new year.
The best books for learning designing oscillators are:
1.Discrete Oscillator Design - Rhea
2.RF and Microwave Transistor Oscillator Design - Grebennikov
3.The Design of Modern Microwave Oscillators for Wireless Applications - Rohde, Poddar, Bock
All the best wishes to you too...Merry Christmas!