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Colpitts oscillator in ADS

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mirkata0

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Hello everyone,
I am really struggling with designing a Colpitts oscillator for 2.4 Ghz. I know the principles and read a lot about it but my simulation in ADS does not work. I am spending hours on this without any progress and can't find where is my mistake. Would really appreciate if someone can give me advice on what I should do? Here is the circuit which I have designed. (and yeah i know that my bypass capacitor is not connected).
1636327832357.png
 

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This variant of Colpitts Oscillator is not very appropriate for 2.45GHz.You should consider Modified Clapp Oscillator for this band.
 

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I see the circuit oscillating at about 700 MHz in LTspice. A kick-start pulse is required as in any oscillator simulation, I presume it's also the case in ADS.

I agree with BigBoss, that colpitts is probably not the best oscillator topology for 2.4 GHz, in addition the LC values are inappropriate, 0.1 pF is lower than parasitic circuit capacitances, respectively you get much lower resonance. I assume an useful inductance value of a few nH.

1636361085560.png
 

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    mirkata0

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There are few design mistakes on your schematic.
Use the schematic attached and it will work.
Try using a real RF transistor from ADS library.
 

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    mirkata0

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Since you're using ADS, why don't you use Oscillator Design Guide ?
Let me give you a sample design here to start-up.
Also, I recommend you to read Ulrich Rohde's textbooks and papers about oscillators at

Synergy Microwave Corporation - Published Technical Articles

synergymwave.com synergymwave.com
He is the best known/living Oscillator/Signal Generation Guru. Follow him..

1636385968931.png

1636385985820.png

Synergy Microwave Corporation - RF & microwave signal sources and signal processing products.

RF and Microwave Voltage Controlled Oscillator (VCO), Dielectric Resonator Oscillator (DRO), Phase Locked Loop (PLL) frequency synthesizer, Phase Locked Tranlator, Crystal Oscillator (OXO), Splitter/Combiner, Hybrid, Mixer, Transformer, coupler, filter and more.
synergymwave.com synergymwave.com
 
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    mirkata0

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Hello,
Thank you all for the help and suggestions. I want to use a real RF transistor BFR540. Is anyone familiar with downloading libraries from NXP for their models as I am only finding High Power Design Kits and not the one for my application? I cannot find the BFR540.
Thanks.
 

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mirkata0 said:
Hello,
Thank you all for the help and suggestions. I want to use a real RF transistor BFR540. Is anyone familiar with downloading libraries from NXP for their models as I am only finding High Power Design Kits and not the one for my application? I cannot find the BFR540.
Thanks.
Click to expand...
BFR540 is an obsolete BJT, not recommended for new design.
I have designed an 2.4GHz VCO yesterday using with BFP193W ( Infineon) that is still available.
For your reference..
1636647998152.png

1636648130803.png

--- Updated ---

High Resolution Output
 

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  • 2.4GHz Clapp.pdf
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1- try using a short stub transmission line for your tank circuit inductor, in oscillator designs using lumped inductors create problems in pcb.
2-since tank circuitry is a ratio of inductor/capacitor; first see how much inductor value you can get for a short stub inductor, then choose the capacitor value accordingly. you can use FR4 as your substrate.
3- try making your design more realistic by using transmission lines after first couple simulations. In oscillator design, stray capacitance from transmission lines are very critical in determining your Q.
 

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    mirkata0

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Hello. I followed your suggestions and designed this circuit:
1637493707076.png

However, I am not getting any oscillation from Vout.
1637493772517.png

Do you have any suggestions what could be wrong with the circuit?
Thank you all!
 

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Not sure if the RF path fulfills the oscillation condition, but the bias point is surely inappropriate. Vbe of 450 mV doesn't even cause minimal collector current with a Si transistor.
 

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FvM said:
Not sure if the RF path fulfills the oscillation condition, but the bias point is surely inappropriate. Vbe of 450 mV doesn't even cause minimal collector current with a Si transistor.
Click to expand...
I got an oscillation but its not at the correct resonant frequency. I know that there are mismatches due to the distortion from microstrip lines but do you have any sugggestion how to increase the frequency. I am getting 1.6 Ghz while aiming for 2.4 GHz.
Thanks!
 

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Make the value of C21 between 2pF and 3pF..
If is not oscillating, increase the value of R2.
 

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Another example and verification of the previous circuit. Inspire from my designs.
Transmission Lines don't create distortion. You have to work much on Microwave Design Principles buddy..
 

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  • 2.4GHz Oscillator.pdf
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  • 2.4GHz Oscillator HB Output Voltage.pdf
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  • 2.4GHz Oscillator Output Spectrum.pdf
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vfone said:
Make the value of C21 between 2pF and 3pF..
If is not oscillating, increase the value of R2.
Click to expand...
Thanks!
I did that and managed to increase the frequnency up to 1.9GHz but no more. Do you have any other suggestions for changing components values to increase frequency?
 

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Using a BFU630 transistor, and assuming there is not much parasitic inductrance of the seires microstrip transmission lines between L1 and base of M1, making C2 about 2pF you have to get around 2.4GHz oscillation.
If the parasitic inductance of the lines is high, you may have to decrease L1 value down to 0nH (short circuit).
 

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