1. Home
  2. Electronics Design
  3. PLEASE HELP! cant get my generated sine wave down to almost zero RMS..

PLEASE HELP! cant get my generated sine wave down to almost zero RMS..

i want to make a metal detector.i would be uisng an LC tuned colpitts oscillator using the bjt of (lm389n) audio chip...i want the output variation from this wave to alert the presence of metal by using a chip that converts frequency to voltage(lm2917n). my problem is this:

::: i was able to generate a sine wave about 420khz but it has an RMS or average of about 4 V..this would be a problem for the F->V converter since it needs to detect zero crossings to properly perform.what i did was to put a DC block of 2pF.however its output is still with a DC offset similar to the signal before the blocking capacitor... is there another way to remove this DC offset?

thaks!

Reply to
e2
Loading thread data ...

Do you have a symmetric supply voltage like +9V *and* -9V? If yes, you could get it nulled with a DC-servo made from a precision Opamp. There might be also the possibility your opamps are oscillating at very high frequencies, becaquse of capacitive loading, poor layout or no decouple caps. Without a schema it's difficult to guess.

--
ciao Ban
Apricale, Italy
Reply to
Ban

..my chip only have +9 Vss. essentially,im using one of the 3 bjt transistors of the lm389n.i used fixed bias emiiter coupled setup. ..or maybe can you help me of another oscillator design as long as inductance dependent since i will be using this as a way to indicate presence of metal... ..

thanks

Reply to
electro

Connect a bias resistor between the tachometer input (pin 1) and ground: try something in the 10k - 100k range. You may need to slightly increase the value of the DC blocking capacitor.

Reply to
Andrew Holme

Connect a resistor from after the DC blocking cap, to the offset voltage you want. E.g. 1M ohm to 0V. That 2pF sounds a bit low, might try 22pF in which case reduce the resistor to 100k etc.

--

John Devereux
Reply to
John Devereux

e2 wrote in sci.electronics.design:

The RMS (root mean *square*) is not the average voltage of a wave form. It will be positive even for a perfectly symmetrical wave. In other words, you may be seeing a problem you don't have.

Anno

--
If you want to post a followup via groups.google.com, don\'t use
the broken "Reply" link at the bottom of the article.  Click on 
"show options" at the top of the article, then click on the 
"Reply" at the bottom of the article headers.
Reply to
Anno Siegel

First of all, post a schematic or an URL where you got the schema from, because your electronic and English skills are not sufficient to communicate your problem. If you want 0V on the output of your LM2917, you have to have 2 oscillators and a mixer(some multiplier I guess), so you get very low frequency output. With only one oscillator as long as it is working there will be always a high output voltage.

--
ciao Ban
Apricale, Italy
Reply to
Ban

got it work now! thanks

Reply to
electro

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.