commgrp@silver.ucs.indiana.edu (BACS Data Communications Group) (10/17/90)
jgk@osc.COM (Joe Keane) writes: >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... >I also noticed a 90 degree phase shift is one way to make a SSB AM >signal. They don't say how to do this, other than that it's difficult >to get exactly right... The phasing method of generating single sideband requires that the audio phase-shift network be accurate within a few degrees, else the unwanted sideband is insufficiently suppressed. The best explanation I've ever seen of the phasing SSB method is in _QST_ magazine, January 1988: "A New Breed of Receiver" by Gary Breed (editor of _RF Design_ magazine). The article contains plans for an all-pass 90-deg. audio network made of 8 op-amps with precision resistors and capacitors. 90^ audio phase-shift can also be done with a passive network of LOTS of resistors and capacitors. See old ARRL SSB Handbooks. -- Frank Reid W9MKV reid@ucs.indiana.edu
kchen@Apple.COM (Kok Chen) (10/18/90)
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 :-).
wayne@hpfcso.HP.COM (Wayne Covington) (10/19/90)
Do you need a single output shifted 90 degrees with respect to your input, or will you settle for two equal-amplitude outputs but 90 degrees apart from each other (bearing no simple phase relationship to your input)?
wayne@hpfcso.HP.COM (Wayne Covington) (10/19/90)
The original question seems to be pretty well answered, but if you need more accuracy on the phase tolerance or a wider frequency range than provided by the solutions mentioned so far, see the article by Allan Lloyd in Electronic Design 19, 13 September 1976. His approach is fairly generic and will let you explore design trade-offs.
davew@hp-ptp.HP.COM (Dave_Waller) (10/20/90)
Isn't it rather simple to modulate the signal via AM to some IF, use a steep notch to supress one of the sidebands (upper or lower), and then modulate the SSB signal to the transmit frequency desired? Dave Waller \ The opinions expressed are solely my own, and in no way Hewlett-Packard Co. \ represent those of my employer (but we all know dave@hpdstma.ptp.hp.com | hplabs!hpdstma!dave \ they should!)
ornitz@kodak.kodak.com (Barry Ornitz) (10/25/90)
In article <3899@osc.COM> 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. > >First, let me say what doesn't work. Joe then goes on with a discussion of several techniques he has investigated... The networks that are used to produce a 90 degree phase shift over a limited range of frequencies (which may be a number of decades) are nonminimum phase networks. The classic design is the Dome network used in many early phasing single sideband exciters. For references to this network, look in amateur radio publications of the mid-1950's through the early 1960's. Bill Orr's "Radio Handbook" has a discussion of this, and surprizingly Horowitz and Hill's "Art of Electronics" also discusses this network configuration. 73, Barry WA4VZQ ----------------- | ___ ________ | Dr. Barry L. Ornitz WA4VZQ | | / / | | Eastman Kodak Company | | / / | | Eastman Chemical Company Research Laboratories | |< < K O D A K| | Process Instrumentation Research Laboratory | | \ \ | | P. O. Box 1972, Building 167B | |__\ \________| | Kingsport, TN 37662 615/229-4904 | | INTERNET: ornitz@kodak.com -----------------