Simple phase-shifter?

Assuming that the freq is fixed, a dual 555 timer. first unit, a time on delay for the shift. Second unit, a one shot equal to the fixed square wave.

for variable frequency, something like a shift register would work. You can simply vary the clock rate to adjust the offset how ever, there would be an error in frequency reproduction due to the bit res and clock rate..

Other option, use a uC (microchip) like PIC's, AVR's etc and use a hardware timer event that inputs the signal from your existing square wave and shifts it through a pack of bytes using the bits of each byte as the value as circular buffer that gets output to an IO as the phase offset. the rate can be adjusted via one of the ADC inputs which would simply adjust the interrupt and offset.

Many solutions.

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A simple RC lowpass filter, followed by a comparator, will work. Vary the R to vary phase shift. You can reasonably get up to, say, 45 to maybe 90 degrees of lag (more if the comparator has hysteresis), and you can cascade stages to get more. Phase shift will vary with frequency.

PLLs can do a lot more, lead and lag, but are more work.

You could also pipe it into a clocked shift register, and vary the clock frequency to directly add delay. But that would introduce a

1-clock jitter.

John

Strictly speaking, phase shift applies only to sine waves, and a square wave is made up of many different sine wave components. What you are needing is not a phase shift, but a variable time delay, that is independent of frequency (equal time shift for many frequencies). But the time delay for a 10 degree phase shift for the fundamental frequency in the square wave is a 30 degree phase shift for the third harmonic, and a 50 degree phase shift for he fifth harmonic, etc. So an analog circuit that does what you want must produce a phase shift that is proportional to frequency, with a variable proportionality constant. A tall order.

Variable time delays are usually performed with a sampled representation of he waveform passed through a shift register with variable speed clocking. The signal representation may be analog (voltage samples stored in a bucket brigade analog delay line) or digital (binary numbers made by an analog to digital converter that are shifted through a multi bit shift register, the output converted back to voltage samples by a digital to analog converter).

DSP (Digital Signal Processor) chips have all the stuff needed for the second case, except, possibly for memory to hold the shift register.

The analog delay lines are getting really hard to find because DSP chips have gotten so good and so cheap.

You might find some on eBay:

or from surplus places electronics sellers.

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________________ __________ SQIN___| |________________|

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The RC approach that others have described is probably as close as you're going to get to "super simple". But just to toss another idea into the pot, you can integrate the square wave into a triangle, then use comparators to set the trigger points. The simplest way to do that is probably to run your original square wave at twice the frequency you really want, with a single comparator that triggers at the desired level (which is now linearly proportional to phase). Its output will be a pulse of variable width, namely only as wide as the portion of the triangle wave that is above the threshold. But the output of the flip-flop will give you a clean square wave, at the desired frequency.

Note that this approach is best with a fixed frequency. If you vary the frequency, the triangle out of the integrator will change its amplitude.

Best regards,

Bob Masta DAQARTA v3.50 Data AcQuisition And Real-Time Analysis

Scope, Spectrum, Spectrogram, FREE Signal Generator Science with your sound card!

I'm intrigued by the low-pass filter idea. I'm using a comparator to square the wave already.

Now, do I have to be close to the cut-off to get the phase lag, or do I get the lag even if I'm well below it? In other words, can I make the filter with a cut-off far above my working frequency and still get an appreciable phase lag?

Thanks for all the ideas so far.

Don

I'm intrigued by the low-pass filter idea. I'm using a comparator to square the wave already.

Now, do I have to be close to the cut-off to get the phase lag, or do I get the lag even if I'm well below it? In other words, can I make the filter with a cut-off far above my working frequency and still get an appreciable phase lag?

Thanks for all the ideas so far.

Don

Alas, the easy ways are either very limited or require sinewaves. If you had a quadrature oscillator, outputs SIN(wt) and COS(wt) you could feed those to a pair of balanced mixers that accept low-frequency local oscillator input, sum the outputs, and get

cos(phi)sin(wt) + sin(phi)cos(wt) =3D sin(wt + phi)

where some trickery with custom potentiometers makes the constants cos(phi) and sin(phi)...

For square waves, you could generate with a PLL a signal at 20*f, then use a CD4017 to generate ten staggered output pulses. A toggle flip/flop on any of those pulses will have a characteristic phase (you might want to reset all flip/flops with (Q #0 .and. pulse# 0) to ensure repeatable phase). Between the Q and /Q outputs of ten flip-flops, you'd have twenty phases of the same square wave to choose from.

If you tell us more about how the whole system is supposed to work, perhaps we can come up with an optimized approach. For example, are you generating the initial square wave yourself? Does the phase shifter have to work across varying input frequencies automatically, or can you readjust when the frequency changes?

Best regards,

Bob Masta DAQARTA v3.50 Data AcQuisition And Real-Time Analysis

Scope, Spectrum, Spectrogram, FREE Signal Generator Science with your sound card!