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oscillator design project

Thread starter #1
kkkkk.PNG
Am trying to design an oscillator that meets the following requirement
1. f_out = 500KHz (1st harmonic )
2. 2 volt peak to peak output (V_out) into a 50Ω load
3. both the second and third harmonic of the output must be -25dB relative to the magnitude of the fundamental frequency (1st harmonic)
4. determine and graph the full bode plot (magnitude and phase) of the loop gain of the design
need help on how to do this project
 
Last edited:

audioguru

Well-Known Member
Most Helpful Member
#4
The 741 opamp design is 49 years old and it performs poorly above only 10kHz. You want 500kHz so use a newer and better opamp.
You posted a Wein Bridge oscillator but the gain is much higher than the required 3 times so if you use an opamp that works then the harmonic levels will be very high.
I calculated its frequency to be about only 31.3kHz.
Please learn about a Wein Bridge oscillator.
 

audioguru

Well-Known Member
Most Helpful Member
#6
I do not know any opamp that works well at the 500kHz to 1.5MHz radio frequencies you want.
A Wien Bridge oscillator is usually used at audio frequencies. Maybe you should make an RF oscillator with a fast transistor.
 

JimB

Super Moderator
Most Helpful Member
#7
what op amp will you suggest I should use
You could try a TL071 which has a GBP (Gain Bandwith Product) of 3MHz, but even this could be a bit on the low side for good operation of this circuit at 500kHz.

If you type 'op-amp high GBP' into your favourite browser, it will give you a large selection to pick from.

A Wien Bridge oscillator is usually used at audio frequencies.
I have seen Wien Bridge oscillators used up to several megahertz, but they are not an oscillator of choice for use at RF.

When I first saw your post, my immediate reaction was that you should be using an LC oscillator of some kind, such as a Colpitts type.

JimB
 

jpanhalt

Well-Known Member
Most Helpful Member
#8
I can add the AD8035/36 or AD8055/56 as other candidates. Earlier this year I used the AD8055/56 as a SAB (single amplifier biquad) filter for 500 kHz. It oscillates quite well at that frequency with just a little too much positive feedback. ;)

John
 

ci139

Active Member
#9
i don't ask questions and tune/fail as it comes ("fuzzy" logic / intuitive)
as your course master likely does - you have to revisit this thing
Random osc xp-00-0100.png
the basic/trivial idea here is to pull your osc down (by it's dc offset) to limit the amplitude
there is likely transistor only option to this (would be easier with LC resonator/filter)
 

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