🚀 go-pugleaf

jgk@osc.COM (Joe Keane) writes:

>I have what seems to be a fairly simple problem.  I want to buld a circuit to
>phase shift an audio signal by 90 degrees.  I also want this to work over a
>range of frequencies and have fairly flat amplitude response.


Take a look at the Phase-Sequence filter that is described in both
the 2nd edition of Horowitz and Hill ("Art of Electronics," McGraw-Hill,
I believe) and the 1990 Radio Amateur Handbook that is published by
the American Radio Relay League.  It also appeared in some IEEE 
Transactions a while back. The ARRL Handbook didn't actually call 
it a Phase sequence filter, so you won't find that in the index. 
Just leaf through the SSB transmitter section of the Handbook and 
you can't miss it.

The P-S filter starts with the original signal and the 180 degree 
phase shift (i.e. inverted signal) of it.  These are passed through 
4 intercoupled R-C networks in what appears to be a succesive approximation 
of the desired result.  The output of the P-S filter are 4 signals that 
are at 4 successive 90 degree phase shifts from one another.

The P-S filter looks far superior to the old time phase-shift networks 
that comes in octal tube sockets (remember those? I think Miller made 
one of them).  And the R-C values are *far* less critical.

Be aware that although the output phases of these filters are in 
quadrature, they do *not* have a nice flat phase response w.r.t. the 
original input signal.  If that is what you want (i.e., absolute 90 degree 
shifter rather than one that provides a pair of in-phase and quadrature 
outputs) you would have to find an approximation to the Hilbert Transform.
The Pade' approximation would be one way -- e.g., take the Euler expansion 
of exp(-i.T) and approximate as many terms as you can afford with
RC filters, and summing their outputs.

In answer to your comment about SSB generation - the method that
is more in favour nowadays seems to be the one that generates a DSB
suppressed carrier signal and pass that through a really sharp cutoff 
crystal filter.  Good crystal filters in the 5 - 10 MHz range are 
relatively inexpensive nowadays and are far less "tweeky" than SSB 
exciters that use phase shift networks.  You need phase accuracies in 
the sub-degrees to get sideband suppresion that approaches that of even 
a pretty sloppy filter (just try subtracting sin(x) from sin(x+delta)
to see how small delta has to be so that the resultant amplitude is,
say, 40 dB down -- I would offhand guess about 1/100 of a radian).


Kok Chen, AA6TY			kchen@apple.com
Apple Computer, Inc.

Disclaimer: the last time I even attempted the phasing method of SSB
         excitation was for an undergrad lab over two decades ago :-).