I need to match the phase of two signals coming from a pair of transducers over the 10Hz to 4 KHz range, the difference between the two transducers is really close, however one of the transducer lags 7° in phase over the entire band with respect to the other transducer. I need something like an all-pass filter, but instead of it having linear phase shift which varies linearly over frequency, I need a constant phase shift, in my case I need it to be a constant 7° phase shift over the desired bandwidth, I know of no analog network that can perform this.

Any ideas?

  • \$\begingroup\$ Is the signal from the transducer broadband, or at any given moment is it approximately sinusoidal? \$\endgroup\$
    – The Photon
    Oct 3, 2018 at 22:56
  • \$\begingroup\$ It is approximately sinusoidal \$\endgroup\$
    – S.s.
    Oct 3, 2018 at 22:58
  • \$\begingroup\$ So you want something that delays by (1/f) * (7/360) seconds for f from 10 to 4000? \$\endgroup\$
    – crj11
    Oct 4, 2018 at 0:26
  • \$\begingroup\$ @TonyEErocketscientist I thought that maximally flat group delay meant linear phase, not a constant not linearly changing phase. \$\endgroup\$
    – S.s.
    Oct 4, 2018 at 2:25
  • \$\begingroup\$ @S.s. Brainfa/t. So Is there another way to correct this with PLL tracking>? \$\endgroup\$ Oct 4, 2018 at 2:39

1 Answer 1


Option 1: Generate the quadrature signal of your input. Sum it with the input in the correct proportions to get 7 degrees phase shift. How to generate a quadrature signal over more than 2 decades of frequency range is left as an exercise for the reader.

Option 2: Make a 1:1 phase locked loop and put an offset in the phase detector to produce 7 degrees phase shift. How to make a good VCO that spans more than 2 decades of frequency range is left as an exercise for the reader.

Option 3: Digitize everything and let software sort it out.

  • \$\begingroup\$ Option 1 is brilliant!, I cant really see how option 2 works, option 3 is precisely what I want to avoid. \$\endgroup\$
    – S.s.
    Oct 4, 2018 at 2:39
  • \$\begingroup\$ If the signal amplitude doesn’t matter a PLL IC can lock on with any phase error introduced by a type II mixer offset of Vcc/2 * 7/180 deg if Vcc/2 is 0 error but getting 3 decades of VCO is better using a log scale voltage-controlled Schmitt Trigger Relaxation Oscillator to change feedback ratio. \$\endgroup\$ Oct 4, 2018 at 5:51

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