gregw@sugar.hackercorp.com (Greg White) (03/24/91)
I'm looking for an inexpensive phone scrambler. I'm not looking to keep out the NSA, but just nosey neighbors listening in on cordless and cellular calls. I'm really interested in something I can build if possible. The dim reaches of my memory mumbles something about a ring modulator setup. Can anyone supply details? Any information will be appreciated. Thanks, Greg -- Greg White === POST NO SIGS ===
mcovingt@athena.cs.uga.edu (Michael A. Covington) (03/24/91)
Although I haven't tried it, you could make an audio scrambler out
of an NE602 mixer-oscillator chip. Run the oscillator at about 5 kHz
and mix it with the audio using the double balanced mixer. Out come the
sum and difference frequencies. Low-pass-filter it so that you get only
the difference frequencies; this gives you audio which is "scrambled"
because its spectrum is backward. (It will sound like an SSB radio
signal does when you're tuned for the wrong sideband.) Run the sound
through the same setup again to descramble it.
In sum:
300-to-3000-Hz bandpass filter on input
Oscillator 5000 Hz or so
(no, make that 3500 Hz)
Doubly balanced mixer
300-to-3000-Hz bandpass filter again on output
--
-------------------------------------------------------
Michael A. Covington | Artificial Intelligence Programs
The University of Georgia | Athens, GA 30602 U.S.A.
-------------------------------------------------------psfales@cbnewsc.att.com (Peter Fales) (03/24/91)
In article <1991Mar24.012754.9816@athena.cs.uga.edu>, mcovingt@athena.cs.uga.edu (Michael A. Covington) writes: > In sum: > > 300-to-3000-Hz bandpass filter on input > Oscillator 5000 Hz or so > (no, make that 3500 Hz) > Doubly balanced mixer > 300-to-3000-Hz bandpass filter again on output > > Sure, it's a standard Sophoomore communications lab experiment. It is not terribly secure though. Anyone one who wants to listen in will have little difficulty building the "descrambler." -- Peter Fales AT&T, Room 5B-420 N9IYJ 2000 N. Naperville Rd. UUCP: ...att!ihlpb!psfales Naperville, IL 60566 Domain: psfales@ihlpb.att.com work: (708) 979-8031
robf@mcs213j.cs.umr.edu (Rob Fugina) (03/25/91)
In article <1991Mar24.012754.9816@athena.cs.uga.edu> mcovingt@athena.cs.uga.edu (Michael A. Covington) writes: > 300-to-3000-Hz bandpass filter on input > Oscillator 5000 Hz or so > (no, make that 3500 Hz) > Doubly balanced mixer > 300-to-3000-Hz bandpass filter again on output What's a 'doubly balanced' mixer??? Rob robf@cs.umr.edu <--use this address!
mcovingt@athena.cs.uga.edu (Michael A. Covington) (03/25/91)
A doubly balanced mixer outputs *only* sum and difference frequencies; neither the input signal nor the oscillator signal appears at the output. I think "doubly" refers to the fact that both the oscillator and the input can swing to either side of 0 volts and the balancing still works. -- ------------------------------------------------------- Michael A. Covington | Artificial Intelligence Programs The University of Georgia | Athens, GA 30602 U.S.A. -------------------------------------------------------
mycal@everexn.com (Mike Johnson) (03/26/91)
In article <1991Mar24.012754.9816@athena.cs.uga.edu> mcovingt@athena.cs.uga.edu (Michael A. Covington) writes: >Although I haven't tried it, you could make an audio scrambler out >of an NE602 mixer-oscillator chip. Run the oscillator at about 5 kHz >and mix it with the audio using the double balanced mixer. Out come the >sum and difference frequencies. Low-pass-filter it so that you get only >the difference frequencies; this gives you audio which is "scrambled" >because its spectrum is backward. (It will sound like an SSB radio >signal does when you're tuned for the wrong sideband.) Run the sound >through the same setup again to descramble it. > > In sum: > > 300-to-3000-Hz bandpass filter on input > Oscillator 5000 Hz or so > (no, make that 3500 Hz) > Doubly balanced mixer > 300-to-3000-Hz bandpass filter again on output > > >-- >------------------------------------------------------- >Michael A. Covington | Artificial Intelligence Programs >The University of Georgia | Athens, GA 30602 U.S.A. >-------------------------------------------------------
jfw@ksr.com (John F. Woods) (04/02/91)
mcovingt@athena.cs.uga.edu (Michael A. Covington) writes: >A doubly balanced mixer outputs *only* sum and difference frequencies; >neither the input signal nor the oscillator signal appears at the output. >I think "doubly" refers to the fact that both the oscillator and the >input can swing to either side of 0 volts and the balancing still works. A doubly-balanced mixer is balanced with respect to the signal input and the local-oscillator input; a singly-balanced mixer is balanced with respect to only the signal input. The input signal and the oscillator signal are suppressed at the outputs (and the LO is also suppressed at the signal input port, resulting in less leakage of LO energy to (for example) your antenna). The ARRL Radio Amateur's Handbook or any other electronics text covering radio (i.e. not some compugeek text which starts with gates and ends with ICs) will have a nice explanation of just what this is all about.