# square to sine

Discussion in 'Electronic Basics' started by Sandeep, Nov 4, 2005.

1. ### SandeepGuest

Hi all,
I want to know how to convert a 1KHZ square wave into a stable sine
wave .Please can anybody help me out
Many Thanks
Sandeep

2. ### petrus bitbyterGuest

You just need a good 1kHz filter. Sine will be as stable as your square.

petrus bitbyter

4. ### Anthony FremontGuest

I could be wrong, but wouldn't a low-pass filter be easier to construct,
and more to the point?

5. ### Anthony FremontGuest

Never mind I suppose, I just notice a separate thread on SED covering
that very topic. It would appear that the bandpass method may be
preferred in some situations.

6. ### Ralph MoweryGuest

---
I would use a low pass filter also. It needs to start cutting off somewhat
above the fundamental frequency. As the square wave is all the odd
harmonics, if your cutoff is below 3 khz (using your 1 khz square wave) then
you will have a very good sine wave. after the filter.

7. ### skepticGuest

The problem with filters is that they work well only over a narrow
range of frequencies. Another method that will work over a wider range
of frequencies is to run the square wave through two integrators. The
first one will convert it to a triangle wave and the second one will
convert it to something very close to a sine wave. Of course the
amplitude of the resultant sine wave will vary inversely with the
frequency. Now you know why function generator ICs put out square
waves, triangle waves and sine waves.

8. ### Rich, Under the AffluenceGuest

Ever since I've heard of the 8038 and the phase-locked loop, I've had a
fantasy of locking in a sine wave to some square wave by PLL of some kind.

Set up an 8038 (or maybe these days it's the XR220whatever) as a 1 KHZ
oscillator, and run a phase detector output into the freq. control input.
Should be a piece of cake. ;-)

Cheers!
Rich

9. ### Roger DewhurstGuest

Two op amps in series with 100K on each (-) input and feedback from the
output to the (-) input through a .01mF capacitor. Both (+) inputs
grounded? Or should the values be changed?

R

10. ### Big Mouth Billy BassGuest

OP is only concerned with 1KHz, that's a pretty narrow range.

13. ### Bob MastaGuest

It would be a sine wave, if the input was a true square wave
with +/- swing, otherwise it would be a true sine wave with
some DC offset.

The advantage of the bandpass is simply higher Q, but
in theory a lowpass form should be superior since the
bandpass is wasting half of its slopes on the low side
where they do nothing useful. The trick is to use a
lowpass with high Q giving a peak at 1 kHz. The fact
that there is a plateau on the lower side is of no
consequence, since there isn't anything there anyway.

Whether lowpass or bandpass, at high Q there will be
the issue of stability. If the tuning drifts slightly, the
output amplitude may change substantially. This is
one argument for a more-typical flat passband lowpass,
since you can make it less sensitive to drift in components
or the input square wave frequency... if you are willing to
put more stages into the circuit.

The OP doesn't mention where the original square wave
comes from. If it is being generated in a circuit under your
control, you can instead generate at a much higher frequency
and divide it down to 1 kHz. This opens up two new
possibilitues:

1) Use a switched capacitor filter where the high frequency becomes
the filter clock, insuring solid stability even at high Q.

or 2), you can make a cute little D/A type of circuit where you
sum different amounts of the higher components such that
the harmonics cancel. I've seen several designs like this over
the years, but I don't know what they would be called to search
for them in Google. The big advantage is that they work over
a broad range of frequencies, and they virtually elimiante all
harmonics below 2^N - 1 where N is the number of divider
stages in the original (I think!). Basically, a handful of resistors
and a few divider stages is all this takes, and you can follow
it with a simple lowpass to remove the higher trash.

Best regards,

Bob Masta

D A Q A R T A
Data AcQuisition And Real-Time Analysis
www.daqarta.com
Home of DaqGen, the FREEWARE signal generator

14. ### Anthony FremontGuest

How so? I can see that there is a slight difference in the skirt slopes
of the two filters. But since no filter is perfect, can you really say
that the output of the low pass filter is not a sine wave, but the
output of the bandpass filter is? What is the distinguishing factor?

15. ### mikeGuest

Best help I can offer is to suggest that you disclose the secret
operation that you're trying to perform. You're question is incomplete.
The solution depends a LOT on unstated requirements.

The simplest thing I can think of is a high Q resonator.
I'd make a simple coaxial cavity resonator. But it'd have
to be somewhat bent to compensate for the curved surface of the earth.
mike

--
Wanted, Serial cable for Dell Axim X5 PDA.
Return address is VALID but some sites block emails
FS 500MHz Tek DSOscilloscope TDS540 Make Offer
Bunch of stuff For Sale and Wanted at the link below.
MAKE THE OBVIOUS CHANGES TO THE LINK
ht<removethis>tp://www.geocities.com/SiliconValley/Monitor/4710/

16. ### John FieldsGuest

---
My thinking was that with a simple single-pole lowpass with a
square-wave input:

SQIN>---[R]--+--->OUT
|
[C]
|
GND>---------+--->GND

You wind up with an integrator which merely charges and discharges
the cap as the square wave makes its excursions.

Run this through Linear's simulator to see what I mean:

Version 4
SHEET 1 880 680
WIRE -32 192 -32 160
WIRE -32 304 -32 272
WIRE -32 336 -32 304
WIRE 48 160 -32 160
WIRE 160 160 128 160
WIRE 160 192 160 160
WIRE 160 304 -32 304
WIRE 160 304 160 256
FLAG -32 336 0
SYMBOL voltage -32 176 R0
WINDOW 3 24 104 Invisible 0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V1
SYMATTR Value PULSE(-1 1 0 0 0 5e-4 1e-3)
SYMBOL res 144 144 R90
WINDOW 0 -38 58 VBottom 0
WINDOW 3 -36 62 VTop 0
SYMATTR InstName R1
SYMATTR Value 1000
SYMBOL cap 144 192 R0
WINDOW 0 41 34 Left 0
SYMATTR InstName C1
SYMATTR Value 159n
TEXT 0 320 Left 0 !.tran 0 .01 0

OTOH, a simple bandpass filter, like this:

SQIN>--[R]--[L]--[C]--+-->OUT
|
[R]
|
GND>------------------+-->GND

will give you a beatiful sine wave out:

Version 4
SHEET 1 880 680
WIRE -80 192 -80 160
WIRE -80 288 -80 272
WIRE -80 336 -80 288
WIRE 0 160 -80 160
WIRE 128 160 80 160
WIRE 272 160 208 160
WIRE 368 160 336 160
WIRE 368 192 368 160
WIRE 368 288 -80 288
WIRE 368 288 368 272
FLAG -80 336 0
SYMBOL voltage -80 176 R0
WINDOW 3 24 104 Invisible 0
WINDOW 123 -91 84 Left 0
WINDOW 39 -111 109 Left 0
WINDOW 0 -75 53 Left 0
SYMATTR Value PULSE(-1 1 0 0 0 5e-4 1e-3)
SYMATTR Value2 AC 1
SYMATTR InstName V1
SYMBOL cap 272 176 R270
WINDOW 0 32 32 VTop 0
WINDOW 3 0 32 VBottom 0
SYMATTR InstName C1
SYMATTR Value 159n
SYMBOL ind 112 176 R270
WINDOW 0 72 56 VTop 0
WINDOW 3 70 55 VBottom 0
SYMATTR InstName L1
SYMATTR Value 159e-3
SYMBOL res 352 176 R0
SYMATTR InstName R2
SYMATTR Value 100
SYMBOL res 96 144 R90
WINDOW 0 -36 59 VBottom 0
WINDOW 3 -31 59 VTop 0
SYMATTR InstName R1
SYMATTR Value 1000
TEXT 48 320 Left 0 !.tran 0 .02 0
TEXT -24 352 Left 0 !;ac oct 128 100 10000

18. ### Bob MastaGuest

To get the lowpass to work here, the corner frequency
needs to be just above the 1 kHz square wave frequency.
An RC circuit like the above is only a good integrator
approximation if its time constant is long, such that the
effective corner is well below the input frequency.

A simple RC is probably not going to be adequate
to recover a very good sine wave approximation, though.

Best regards,

Bob Masta

D A Q A R T A
Data AcQuisition And Real-Time Analysis
www.daqarta.com
Home of DaqGen, the FREEWARE signal generator

19. ### Bob EldredGuest

As has been mentioned, you can filter the higher harmonics out using a band
pass or preferably a low pass filter but that only works at a small range of
frequencies. Depending on the required purity of the sine wave, filtering
may prove to be difficult requiring a steep slope multi-pole filter to clean
out harmonics. If the square wave has any asymmetry, even a percent or two,
there will be a significant second harmonic present. This is a bitch to get
rid of because of its closeness to the fundamental. Even the third harmonic
can be troublesome.

Another idea is the synthesizer scheme: Use the squarewave to switch an
integrator to ramp positive then negative on each half cycle producing a
triangle wave from the square wave. Run the triangle wave into an automatic
gain stage to generate a constant amplitude. Use a diode-resistor shaping
network to round the triangle wave into a sine wave form. Run this into
another variable gain stage controlled by the amplitude of the original
square wave to re-constitute the original amplitude of the square wave for
the sine wave.

This scheme creates a sine and triangle wave out of the square wave and can
work over a range of frequencies. If the amplitude is not important, the
variable gain stages can be ommited.
Bob

20. ### Bob MonsenGuest

The OP can see this in action (or a similar scheme in action) by searching
for and viewing the ICL8038 datasheet. It uses this scheme by making a
triangle wave from a couple of current sources and a flipflop, and runs it
through an array of PNP transistors. There is a schematic for the chip in
the datasheet.

The ICL8038 is now quite pricy, since it appears to be out of production.
However, so you may as well go for the Cadillac:

http://www.maxim-ic.com/appnotes.cfm/appnote_number/650

or

http://www.futurlec.com/Maxim/MAX038CPP.shtml

For \$20, you can get a signal generator on a chip that goes from .1Hz to
20MHz. It also appears to feature a PLL input, which allows the chip to
lock the chip to an external reference, such as a square wave. I haven't
spent much time looking at it, but it might be just the thing the OP needs.

---
Regards,
Bob Monsen

When earlier, new functions were invented, the purpose was to apply them.
Today, on the contrary, one constructs functions to contradict the
conclusions of our predecessors and one will never be able to apply them for
any other purpose.
- Poincare