EE4435 Fall 2002Design Project 2An Analog Voice Scram blerObjectThe object of this experim ent is to design, implem e nt, and test an analog voice scram bler.Such a circuit might be used for secur e vo ice commun ication s, e.g. o ver telephone lines orvia sh ort wa ve transmission.BackgroundThe telephone band is defined as the band from 300 Hz to 3000 Hz. Voice limite d to thisrange is intelligible and the talk er can be r eco gnized by the listener. A meth od of scra mblinga voice signal that has been limited to the telephone band is to in vert the frequencies in thesignal spectrum . That is, 300 H z becomes 3000 Hz and vice v ersa. When speech is scrambledin this way, it is unintelligible unless a descrambler is used to restore the frequencies to theirorigina l value s.A possible speech scram bler circuit is diagrammed in Figure 1. The input signal is a voicesignal from the output of a microphone preamplifier. A band-pass filter limits the bandw idthofthesignal. Theoutputofthefilter is applied to a balanced modulator which multipliesthe signal b y the output signal from an oscillator. The output of the balanced modulatorconsists of t wo sidebands, a lower sideband and an upper sideband. A low-pass filter passesthe lower sideband to the output, which is the scram bled speech signal.Figure 1: Bloc k diagram of the voice scrambler.The Balanced ModulatorA balan ced mod ulator is a circuit which multiplies t wo signals. A four-qu ad rant multiplieris an example circuit which can be used for this. For this design project, a sim p ler circuitusing a JFET as a switch is to be used. A suggested circuit is sho wn in Figure 2. Theaudio input signal is applied to the vGinput. A square-wa ve is applied to the vCinput.The square wave causes the JFET to alternately switch bet ween a short circuit and an opencircuit. When the JFET is an open circuit, the gain is vO/vI= −RF/R5.Whenitisashortcircuit, the gain is vO/vI=(R4− R3RF) / (R3+ R4). The circuit should be designed so thatRF/R5=(R4− R3RF/R5) / (R3+ R4). For example, R3=100kΩ, R4=200kΩ, R5=2001kΩ,andRF=100kΩ gives the gains +1/2 and −1/2. Thus the polarit y of the audio signalis rever sed each time the square w ave switc hes states. This effectively m u ltiplies the audiosignal by a square w av e. Suggested values for R1and R2are R1=1MΩ and R2=10kΩ.Figure 2: Balanced modulator circuit.IftheoutputsignalvOis low-pass filtered with a filterhavingacutoff frequency less than thefrequency of the square wa ve, it can be shown that vOisthesameasifvGwere multipliedb y a sine wa ve having the same frequency as the square wav e. Th us the com bination of thesquare wa ve m u ltiplier and a low pass filter does the same thing as a four-quad rant multiplierdriven b y the audio signal and a sine w ave oscillator. The 2N54 57 is specified for the JFET.ThesquarewaveappliedtothevCinputmusthaveanamplitudethatisnothighenoughtobreak do wn the gate-to-c hannel diode of the JFET when the square wa ve goes negative. Arelaxation oscillator is described in the text whic h can be used to generate the square wa ve.Frequency ConsiderationsLet the audio input signal be filtered to the band f1<f<f2and let the square wa ve havethe frequency f0. When the audio signal is m ultiplied b y the square w ave, the frequencycomponents that lie below f0are in the band from f0− f2to f0− f1. The next band offrequenc ies are in the range f0+f1to f0+f2. The output of the scrambler is to be the lo wer ofthese two bands. Th us the bandw idth that lies bet ween them is (f0+ f1) − (f0− f1)=2f1.A target value for f1is 300 Hz. This gives a bandwidth between the bands of 600 Hz. Thelow-pass filterattheoutputmusthaveaverysharpcutoff in ord er to filter out the upperband with such a small bandwidth bet ween the bands. An elliptic low-pass filter is a goodc h oice for this.Specifications• The input band-pass filter is to hav e the target lower and upper −3 dB cutoff frequen-cies of 300 Hz and 3000 Hz.2• The square w av e oscillator is to ha ve the target frequency of 3.3 kHz.• The output lo w-pass filter is to have a response that is as flat as possible to 3000 Hzand roll off as fast as possible abov e that frequency. An odd-o rd er elliptical filter issuggested for this filter . The elliptical filter exhibits nulls in its response. The first nullin the elliptical filt er sh ould be in the b a nd from 3000 to 3600