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  #1  
Old 03-08-2013, 11:51 PM
Reyo Reyo is offline
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Exclamation NE602 colpitts oscillator construction challenge ?!!

hey there experts , could you please help me on this ?

Im doing my engineering undergrad course and my final year project require to construct a a colpitts oscillator LC tank circuit in a bread board which will have a drop in frequency value as varying the inductor( the inductor am using now is fixed but will be replaced by a variable coil after making the circuit work) , one way of doing so is to use NE602 (same as SA602a) IC chip.

the chip has this pic configuration



The power supply was connected to a 150Ω resistor then to pin 8, also it was bypassed to ground with a 4n7F capacitor.



Pin 1 was grounded through a 10kΩ resistor while pin2 to through a 10nF capacitor and obviously pin3 to ground


and finally the oscillator configuration is shown below



measuring the output from pin 4 or/and 5 directly by a scope should give a frequency of 10MHz. However for some reason this didnt work, i tried couple of different values for capacitor and inductor in the LC tank according to colpitts oscillator equations but it seems its not connected to the circuit somehow.

any kind of help or tips by you engineers is very appreciated
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Old 03-09-2013, 04:43 AM
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Drop the one cap between pins 6 and 7 and see what happens.

If it doesn't start, put a 2.2k resistor between them and look again.
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Old 03-09-2013, 03:43 PM
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i tried both but nothing change .. i suppose that i shouldnt measure the output directly from pin 4 & 5 maybe im missing a resistor ?
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Old 03-09-2013, 05:16 PM
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This circuit oscillates in simulation (LTspice).
Note that you Have to unbalance the mixer by grounding either pin 1 or pin 2.
The duty cycle is asymmetrical, I think because the transistors in the upper half of the mixer are saturating in the model I used.
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Old 03-10-2013, 05:10 PM
Reyo Reyo is offline
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Quote:
Originally Posted by Ron H View Post
This circuit oscillates in simulation (LTspice).
Note that you Have to unbalance the mixer by grounding either pin 1 or pin 2.
The duty cycle is asymmetrical, I think because the transistors in the upper half of the mixer are saturating in the model I used.

Actually both pin 1 and 2 are grounded by 10kΩ and 10nF respectively

thanks for answering anyway
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Old 03-11-2013, 05:13 AM
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Ron H Ron H is offline
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Quote:
Originally Posted by Reyo View Post
Actually both pin 1 and 2 are grounded by 10kΩ and 10nF respectively

thanks for answering anyway
What do you mean, "thanks for answering anyway."?
You said you couldn't get it to oscillate. I showed you one way to do it. I had failed to notice that you had pin 1 grounded through 10k, but oscillation has nothing to do with pins 1 and 2.
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Old 03-11-2013, 03:23 PM
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I tested the NE602 that way with a 10 MHz xtal oscillator and worked on the first try. A small trimmer in series with the crystal allowed to vary the frequency. Had some 180 mV pk-pk at the output IIRC.
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Old 03-11-2013, 05:05 PM
Reyo Reyo is offline
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Quote:
Originally Posted by Ron H View Post
What do you mean, "thanks for answering anyway."?
You said you couldn't get it to oscillate. I showed you one way to do it. I had failed to notice that you had pin 1 grounded through 10k, but oscillation has nothing to do with pins 1 and 2.

i meant thanks for helping me to splve thew problem, sorry if it sounded a bit weird

i think my problem is in the values of components (c1,c2,c3 and c4) in the colpitts lc tank connection (pin 6,7)
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Old 03-11-2013, 05:12 PM
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Quote:
Originally Posted by atferrari View Post
I tested the NE602 that way with a 10 MHz xtal oscillator and worked on the first try. A small trimmer in series with the crystal allowed to vary the frequency. Had some 180 mV pk-pk at the output IIRC.
thats amazing, may you test it with a 10MHz colpitts tunable lc tank oscillator because ive been trying to do so for a while, also im kind of struggling with the value of capacitor used , c1 & c2 are obtaiend from C1=100/sqrt(F(MHz))
C2=1000/F(MHz)
but i field to get values for c3 and c4 (L is a tunable inductor )
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Old 03-11-2013, 05:21 PM
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I recall trying with a tank to also get 10 MHz. Kind of worked, but I am not THAT sure. The output was much much lower.

Other than a good scope I have nothing to deal with tanks and filters.

Will try again when I return from the Patagonia.
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