Need help in oscillator design

Harros · Jan 23, 2008

Hi guys,

I would like to make a 1MHz oscillator using op-amp, however, the design cannot give desired wave with the power of 100mW... Any suggestions of this circuit?

on1aag · Jan 23, 2008

Hi Harros,

Please check the datasheet page 12.

https://www.alldatasheet.com/datasheet-pdf/pdf/28777/TI/TL082.html

on1aag.

shaam · Jan 23, 2008

I hope u know the condition of oscillations f=1/(2(pi)sqrt(R1R2C1C2)).Once u check this in ur circuit.Then see.

audioguru · Jan 24, 2008

The frequency calculation is correct 1/(2 pi RC) but the circuit uses the wrong opamp:
1) At 1MHz the TL081 has an open-loop voltage gain of only about 3.
2) At 1MHz its max output swing is only 3Vp-p without a load.
3) It is not powerful enough to drive two 160 ohms resistors in series. Its minimum load is about 2k ohms.

The rectifier diodes are designed for 60Hz, not 1MHz and are way too slow.

How can it provide a power output of 100mW when it doesn't have a load?

Harros · Jan 24, 2008

I have redesigned this oscillator using LM6181, it seems working, but there is distortion on the wave form, and the output power is only about 23mW. Is there any correction can be done on the circuit to perform better? Or any other op-amp can be used for this case? Thank you.

audioguru · Jan 24, 2008

Now you have a very fast current-feedback opamp with a high output current. But the level stabilizing diodes are clipping the top and bottom of the waveform.

If you reduce the opamp's gain to be only slightly higher than 3.0 then the distortion will be reduced but the oscillator will take a long time to reach its final output level and one day might not work.

Harros · Jan 24, 2008

audioguru said:
Now you have a very fast current-feedback opamp with a high output current. But the level stabilizing diodes are clipping the top and bottom of the waveform.

If you reduce the opamp's gain to be only slightly higher than 3.0 then the distortion will be reduced but the oscillator will take a long time to reach its final output level and one day might not work.

So, should I remove the clipping diodes? The gain now is around 3.03 (1k/330). Is it hard to get the desired wave form in practical? How about crystal oscillator?

audioguru · Jan 24, 2008

If you remove the diodes then the output will increase until the output of the opamp clips.

There are many high frequency oscillators that use tuned LC circuits to make a sine-wave. I think some oscillators with a quartz crystal have a sine-wave.

I made a low distortion sine-wave oscillator as a phase-shift type.
A resistor in series then a capacitor to ground creates a lowpass filter with a 60 degrees phase shift. It is buffered with an opamp then feeds another and another to make a total of 180 degrees phase shift. The last one is buffered and is the low distortion output which is inverted by a 4th opamp that clips and feeds the input.
The 3 lowpass stages reduce the clipping distortion to nearly nothing.
The lowpass filters have a little amount of loss that can be made up if the opamps have a little gain.

Harros · Jan 24, 2008

audioguru said:
If you remove the diodes then the output will increase until the output of the opamp clips.

There are many high frequency oscillators that use tuned LC circuits to make a sine-wave. I think some oscillators with a quartz crystal have a sine-wave.

I made a low distortion sine-wave oscillator as a phase-shift type.
A resistor in series then a capacitor to ground creates a lowpass filter with a 60 degrees phase shift. It is buffered with an opamp then feeds another and another to make a total of 180 degrees phase shift. The last one is buffered and is the low distortion output which is inverted by a 4th opamp that clips and feeds the input.
The 3 lowpass stages reduce the clipping distortion to nearly nothing.
The lowpass filters have a little amount of loss that can be made up if the opamps have a little gain.

What op-amp did you use to make this oscillator? Is it a power-consuming system? This is because I am thinking of making this oscillator to be portable. (Drived with a 9V battery, if possible) Can I have the schematic for reference?

audioguru · Jan 24, 2008

My portable oscillator was for audio frequencies and used ordinary but pretty fast MC33077 dual opamps. The schematic is stuck on the broken hard drive of my old 486 computer.

Texas Instruments have a phase-shift oscillator for 2.8kHz that has a low output level because its buffer opamps don't have any gain.

Harros · Jan 24, 2008

audioguru said:
My portable oscillator was for audio frequencies and used ordinary but pretty fast MC33077 dual opamps. The schematic is stuck on the broken hard drive of my old 486 computer.

Texas Instruments have a phase-shift oscillator for 2.8kHz that has a low output level because its buffer opamps don't have any gain.

Is it possible for me to transmit the sine wave through the ferrite core loop antenna with this oscillator? I am thinking of transmitting it for 100 meter range... 100mW should be enough right?

audioguru · Jan 24, 2008

You are making an AM transmitter?
I listen to an AM station in my car sometimes to hear the news. The sound quality is horrible.

I made an AM crystal radio when I was 13 and made an AM super-het radio when I was 15. I never made an AM transmitter. Ordinary sparks make an AM transmitter. The RF cops caught me "transmitting" interference with a badly sparking fan.

Isn't the ferrite loop tuned with a parallel capacitor so it becomes a high-Q bandpass filter to reduce harmonics?

Harros · Jan 24, 2008

Well, its just a simple sine-wave transmitter without any modulation. This is the transmitter part of my project of Near Field Ranging System by the way. So, any suggestions on the type of the antenna that i can use for the transmitter?

audioguru · Jan 24, 2008

There are some AM radio station transmitters near my home. Their antennas are huge! They can't use a little ferrite bar antenna but you can.

Harros · Jan 24, 2008

Any idea of where to get the ferrite bar antenna? I can hardly get the old radio in my place....

audioguru · Jan 24, 2008

Some people donate old radios to charity stores.
I have a bag full of old radios and radio control transmitters and receivers I got from a guy I worked with.
My son met a guy who worked at a audio-video store and came home with tonnes of "unrepairable" electronic equipment. I fixed some and use the rest for parts.

Harros · Jan 24, 2008

I cant have this circuit working, when I am trying to simulate the circuit... Can you tell me what's going wrong with this circuit? Thank you.

audioguru · Jan 25, 2008

1) The MC33077 cannot drive a load resistance as low as 100 ohms. Try a minimum of 1k ohms.
2) Its max full output frequency is 200kHz but your RC has a phase shift of 60 degrees at about 1.5MHz. Try much higher value capacitors for a lower frequency then increase the frequency after you get the oscillator working.
3) Each 60 degrees of phase-shift reduces the level by about half. So the gain of the inverting opamp needs to be a little more than 8.

Harros · Jan 25, 2008

audioguru said:
1) The MC33077 cannot drive a load resistance as low as 100 ohms. Try a minimum of 1k ohms.
2) Its max full output frequency is 200kHz but your RC has a phase shift of 60 degrees at about 1.5MHz. Try much higher value capacitors for a lower frequency then increase the frequency after you get the oscillator working.
3) Each 60 degrees of phase-shift reduces the level by about half. So the gain of the inverting opamp needs to be a little more than 8.

Thank you for your info. I think this op amp cannot operate at 1MHz as i cant do any simulation of that circuit at the frequency...

k7elp60 · Jan 25, 2008

You may want to check with Ramsey Electronics, and North Country Radio.
Both of these firms are in the USA and have AM broadcast transmitters in kit form near the 100MW level.

Need help in oscillator design

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