[SOLVED] quadrature oscillator design problem

[johnson933]

Hi all,

i have read the post about reply quadrature oscillator design(early posted by 17-05-08 , do a search about quadrature oscillator design at right top corner can get it).
and i am now able to generate out both sine and cosine wave already.
but, i think there is a formular which state f = 1/2pi RC.
may i know that if i wants a frequency 1MHz output,
and i assume that C = 0.047uF, i found that R = 3.3kΩ.(closest standard value)
is the calculation correct?
(btw i want to get both sine and cosine wave same amplitue.)

hence i connect the circuit like state in


however i only can get a output frequency like 13+MHz,
and the Vp-p is about 100mV.

how can i do to make the output Frequency 1MHz and Vp-p like 3-4.5V?

apreciate if anyone can help.

Best Regards,
Johnson933

 

[KJ6EAD]

No, bad math. Those component values get you 1.026 kHz. You can change the capacitor to 47pF and try again. Use a quad op amp and add an amplifier stage to each output to get the amplitude you want.

 

[johnson933]

Thank you KJ6EAD.
umm..i tried with different Capacitors already, which is mentioned by you as 47pF.
but i am not able to get the two waveforms,even i changed back to the original what i did test yesterday,
i could not have the two wave forms already..then i change IC chip,still not help..0.0may i know the possible problem?
btw,if i do not have a quad op amp in hand, can i do it with dual op amp?

 

[keith1200rs]

What opamp are you using?

Keith

 

[LvW]

Hi Johnson933,
Question 1: Is R1=R (equal resistors) ?
Question 2: What is the GBW of the opamps used? (Same as KEITH's question).

 

[johnson933]

thanks for your reply~
Hi keith1200rs, i am using a LM358(dual op amp.)

Hi LvW, sorry for late reply, i was afk, what i did is, R1 = R, but at the same time,
i did connect a 1Mohms in parallel with R1, cause from the website i searched told me that R1 is needed to be slightly smaller than R.
and the op amp i am using i have the pdf, u all can have a check either it is suitable or not.

i am about to leave work place maybe will reply u all later when i reach home like 1-2 hrs later.

thanks again for your concern. = )

 

[LvW]

i did connect a 1Mohms in parallel with R1, cause from the website i searched told me that R1 is needed to be slightly smaller than R.

Don't do that. It's not a good recommendation. Both time constants should be as equal as possible. Note: Not each recommendation that can be found in the internet is good, helpful and serious.
(Hint: The name of the positive integrator is BTC-integrator. BTC= balanced time constants).

Regarding the opamp used: LM358 is not suited for your purpose and tthe operating frequency (GBW not sufficient).

 

[KJ6EAD]

Some application notes that may help: Sine wave generating techniques **broken link removed**,
op amp circuit collection **broken link removed**.

 

[FvM]

LM358 will possibly work at a few kHz. To build a MHz oscillator, you'll need a high speed OP in the multiple ten MHz BW range. It will require a good circuit layout and suitable supply bypassing. I won't suggest it without respective experience in high speed circuit design.