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Topic Title: Producing a sine wave from a square wave circuit design
Topic Summary: Producing a sine wave with frequency bandwidth of 150kHz to 30MHz
Created On: 12 October 2012 12:53 AM
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 12 October 2012 12:53 AM
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devilmusic

Posts: 13
Joined: 07 February 2010

I am currently trying to design a circuit to produce a variable frequency sine wave in the range of 150kHz to 30MHz. Is it recommended to first generate a square wave using a 555 timer in astable mode then put the square wave through an amplifier and finally into a bandpass filter to obtain the sine wave in that bandwidth?

Kindly advise,
Jon
 15 October 2012 11:48 AM
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amillar

Posts: 1918
Joined: 28 May 2002

First point: I wouldn't recommend a 555 in any case, I've seen the maximum frequency given as between 500kHz and 2MHz (depending on the particular type) but getting it to work reliably at those frequencies is not going to be easy and to get a 50:50 duty cyle square wave would be particulalry challenging.

On the bigger question, the problem with doing it this way is that you have to adjust the filter to exactly the same frequency as the oscillator, so it usually easier (although not easy) to make an adjustable sine wave oscillator in the first place - given that a sine wave oscillator is just a filter with a lot of feedback! However if you get hold of circuit diagrams of analogue oscillators which do this you will see that they are still pretty complex, in particular keeping the level constant over the frequency range and temperature is very difficult. So, what would normally be done now is to digitally generate the sine wave. Whilst I have done this in the past I'm by no means an expert, but a web search on Direct Digital Synthesis (DDS) will find a lot of information.

Best place to look for examples of simple circuits are amateur radio websites, search for Variable Frequency Oscillator (VFO).

-------------------------
Andy Millar CEng MIET CMgr MCMI

http://www.linkedin.com/in/millarandy

"The aim of argument, or of discussion, should not be victory, but progress." Joseph Joubert
 15 October 2012 12:27 PM
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MAWilson

Posts: 48
Joined: 22 February 2006

I haven't designed circuits in a long while myself, but you may want to consider using a PIC micro-controller to generate the variable sine wave. From my recollection there are controllers now with opto-oscillator clocks which should be able to get to the 30 MHz frequency and a bit of code manipulation would vary the frequency.

There would probably be some set backs as you'd get a 0 - V square wave but from memory you should be able to float the wave as desired using an opamp. Don't know for sure if this would work for your design but it may be applicable.
 15 October 2012 12:35 PM
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devilmusic

Posts: 13
Joined: 07 February 2010

I found this circuit http://www.dxzone.com/cgi-bin/dir/jump2.cgi?ID=24600 but I do not quite understand the entire circuit. I have questions like

1) What are the FETs for and are they all of the same model?

2) Does the capacitors (2.2pf and 4.4pf) affect the frequency?

3) I still don't understand how the frequencies can be varied in the ranges that you mentioned (3-10 MHz and 10-30MHz). Can you explain in detail to me?

4) What are the diodes for?

5) What is the transformer for?

The circuit that i need to produce must be battery powered and it will then be concealed within a box or so. Therefore, i do not think i can digitally generate the sine wave.

Is this in your opinion the correct way my circuit should look like:

Battery - Inverter - amplifier - VFO - output
 16 October 2012 08:55 AM
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amillar

Posts: 1918
Joined: 28 May 2002

Unfortunately your link has been removed (presumably contravened IET guidelines). If it pointed to something openly available on the web you may need to describe where it is.

It would help greatly if you could describe what your circuit is for?

-------------------------
Andy Millar CEng MIET CMgr MCMI

http://www.linkedin.com/in/millarandy

"The aim of argument, or of discussion, should not be victory, but progress." Joseph Joubert
 16 October 2012 09:48 AM
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devilmusic

Posts: 13
Joined: 07 February 2010

the link is www<dot>dxzone<dot>com/cgi-bin/dir/jump2<dot>cgi?ID=24600

I am planning to use this circuit to generate a signal of frequency band from 150 kHz to 30 MHz to excite an antenna. I will then perform near field scanning on the antenna. The reason for needing to build a circuit is because the antenna and this circuit will be enclosed in a box. I need the circuit to be small so it can be placed into the box.
 16 October 2012 03:02 PM
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amillar

Posts: 1918
Joined: 28 May 2002

Ahh...you're trying to make a mini EMC test chamber (I suppose I should have guessed that from the frequency span). I hate to discourage anyone but - you almost certainly don't want to do that. You won't know how much power you're actually putting into the antenna or (unless you've got some good test equipment around) whether you're putting out the right signal at all, let alone what your antenna is actually coupling into whatever you're testing.

The question is, are you trying to find a fault with something, or are you trying to prove it will work? If you're trying to find a fault then it may be worth knocking up a simple circuit to wave near it, if you're trying to prove it will always work (e.g. to an EMC spec) there really is no easy way around this - you need to use real kit and know how to use it.

Or is this a college project to build a "conceptual" EMC test system?

-------------------------
Andy Millar CEng MIET CMgr MCMI

http://www.linkedin.com/in/millarandy

"The aim of argument, or of discussion, should not be victory, but progress." Joseph Joubert
 16 October 2012 05:21 PM
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devilmusic

Posts: 13
Joined: 07 February 2010

I am experimenting on materials that can be used as an EMI shield. After putting some thoughts on this, I believe it is better to produce a square wave instead since it have harmonics. This should give a better evidence that the material can be a good EMI shield. What do you think? Would it be easier to build a square wave generator with that frequency band? My problem is that I do not know how much power that I need to put into the antenna.

Out of curiosity, would you happen to know the nominal value of current that is used in electric vehicles?
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