Simple to build isolation transformer that consumes less power than it gives out

[forest]

Thank You Jack I just wanted to help.  I think we need a tool which can do sweep at slow rate while displaying  frequency and able to pause when required still producing output. Such tool would allow to find resonant frequency easily. Sadly I must confess I haven't found such one yet.

[TinselKoala]

As Jack has described, GoldWave will do what you need in the audio range, it just needs a little programming.

But why not invest in a decent function generator with sweep capability? The FG, along with the oscilloscope, is the basis of any setup for looking at resonant circuits or just electronics tinkering in general.
Any signal generator will allow you to sweep manually, and you can just stop turning the knob when you see the resonant voltage rise on the scope, and read off the numbers from the FG's or scope's display. Most common, low-end FGs these days will cover a range up to 3 or 5 MHz and many also even include sweep function.
I have an old Interstate F34 Sweep Function Generator that I use for coil tuning, letting it sweep automatically, but this is a luxury. It's easy enough to do manually with the right tools.

[forest]

As Jack has described, GoldWave will do what you need in the audio range, it just needs a little programming.

But why not invest in a decent function generator with sweep capability? The FG, along with the oscilloscope, is the basis of any setup for looking at resonant circuits or just electronics tinkering in general.
Any signal generator will allow you to sweep manually, and you can just stop turning the knob when you see the resonant voltage rise on the scope, and read off the numbers from the FG's or scope's display. Most common, low-end FGs these days will cover a range up to 3 or 5 MHz and many also even include sweep function.
I have an old Interstate F34 Sweep Function Generator that I use for coil tuning, letting it sweep automatically, but this is a luxury. It's easy enough to do manually with the right tools.

Answer is simple. I'm short on money and function generators are costly. Do you know maybe about any cheap one , maybe extension slot for PC ?

[DreamThinkBuild]

Hi Forest,

If you need to generate more specific frequencies I suggest Octave(which is Open Source).

http://www.gnu.org/software/octave/

Using the chirp function is a snap to generate swept frequencies or any other kind of signal. You can keep changing the parameters until you narrow down the frequency range.

Quick example:

%Notes:
% - The duration should be set longer than this example for
%better low frequency accuracy
%
% - You can change the starting phase angle which is an
%optional parameter for chirp.

%Clean up workspace
clear;
clc;

%Inputs
StartFreq=1;    %Hz
EndFreq=100;    %Hz
Duration=2;    %Seconds
SampleRate=48000; %Samples per second
OutputFile='MySweep.wav'; %Output filename

%Set time frame
t=1/SampleRate;    %Time period
tp=0:t:Duration; %Time over duration

%Generate swept output signal with chirp (normalized)
OutSignal=chirp(tp,StartFreq,Duration,EndFreq);

%Save generated signal to wave file
wavwrite(OutSignal,SampleRate,32,OutputFile);

You can also play the signal out to the soundcard directly but it depends on if your computer will recognize the command(soundsc). soundsc(OutSignal,SampleRate);

[TinselKoala]

Answer is simple. I'm short on money and function generators are costly. Do you know maybe about any cheap one , maybe extension slot for PC ?

I advise not using PC plugin cards. Unless of course you can afford the quality and performance of NI products and software like LabView.

What is your budget range? Don't forget that your signal generator will become one of the basic and most-often-used piece of gear in your laboratory.

Can you spend a hundred dollars?

http://www.ebay.com/itm/5MHz-DDS-Digital-Signal-Generator-Module-Sweep-Function-CPLD-STM32-NEW-/320774544820?_trksid=p3284.m263&_trkparms=algo%3DSIC%26its%3DI%26itu%3DUCI%252BIA%252BUA%252BFICS%252BUFI%26otn%3D21%26pmod%3D271094933774%26ps%3D54

http://www.ebay.com/itm/PROTEK-B-801-SWEEP-FUNCTION-GENERATOR-/360454232368?_trksid=p3284.m263&_trkparms=algo%3DSIC%26its%3DI%26itu%3DUCI%252BIA%252BUA%252BFICS%252BUFI%26otn%3D21%26pmod%3D271094933774%26ps%3D54

http://www.ebay.com/itm/Interstate-20MHz-Sweep-Function-Generator-F74-/271036923351?_trksid=p3284.m263&_trkparms=algo%3DSIC%26its%3DI%26itu%3DUCI%252BIA%252BUA%252BFICS%252BUFI%26otn%3D21%26pmod%3D271094933774%26ps%3D54

Better hurry, I just might buy that Interstate 20 MHz unit.