IN  ----+--\/\/\/\/\--+----\/\/\/\/\---+
           |             |                |
           |             |     |\         |
           |        C    +-----|-\        | 
           |                   |  >-------+---- OUT
                         |     |/
                         \  R3

  This circuit will shift phase from 180 degrees at DC to 0 degrees 
at very high frequencies.  Frequency response is flat, gain is unity.  
At the frequency where X(c) = R3, the phase shift is +90 degrees.  The 
phase shift is not uniform, but this can be managed as follows.

  Wideband 90 degree phase shift networks use several of these cascaded 
one after another, in two separate chains, with their 90 degree 
frequencies spaced according to some formula which I do not know.  The 
spacing allows the nonuniformity of the high frequency side of one 
network to compensate for the nonuniformity of the low frequency side 
of the next network in the chain.  With several networks in each chain, 
this results in a constantly changing phase shift over a wide frequency 
range, with constant gain.  It is the DIFFERERNCE between the outputs of 
the two chains which has a relative phase shift of 90 degrees.

  The 90 degree frequencies of the two chains are staggered.  Sweeping 
from DC up, the chain with the lowest "90 degree" frequency has 
progressively more shift until it reaches a little more than 90 degrees, 
whereupon the other chain is beginning to shift phase, and from then on 
until the highest "90 degree" network frequency, the phase difference 
between the two chains tracks at 90 degrees relative.  Above that 
frequency, the phase difference between the chains gradually falls off.  
I seem to recall that there were three or four networks in each chain for 
a 100 to 10,000 Hz, 90 degree PSN.

  There are a few other ways to accomplish a 90 degree wideband phase 
shift.  An older circuit by R. Dome covered 150 to 5000 Hz, if I 
recall, with several (5?) of the following cascaded networks in each of 
two chains, using vacuum tubes.  Again, the 90 degree phase shift exists 
between the outputs of the two chains. 

                 \ R1
                 |         | 
               -----      --- C 
   IN ---------- - -      --- 
               -----       |---------  OUT 
               '   |       \ 
                   |       / 
                   |       \ R3 
                   |       / 
                   |       \ 
                   |       | 
                   / R2

  And, there is a passive, multiple cross-linked network of numerous 
resistors and capacitors, which can produce a decent differential 90 
degree shift over a desired range - but I'm not going to try to create 
an ASCII rendition of that!  It's in recent issues of the ARRL handbook 
(ARRL, Newington, CT), although it turns out that this design is not as 
forgiving of component tolerances as the handbooks said.  Evidently, 
it wants matched caps and matched resistors, but their exact values are 
not ultra-critical.

 Finally, I've read about a wideband 90 degree PSN which uses a 
multiple-tapped delay line and several shift networks.  Unfortunately, 
I have never examined the design. 

  Bob Bruhns, WA3WDR,

Date: 8 Jul 1996 03:08:09 GMT

Original Subject: circuit sugestion need

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