EE4435 Fall 2001Design Project 3An Analog Telemetry SystemObjectThe object of this experiment is to design, implement, and test an analog telemetry systemthat will transmit an audio frequency representing a battery voltage level over a telepho neline or ove r a voice-freque n cy radio link. In this situation, the battery is the standby po wersupply at a remote comm unications site. It provides the power required to operate the siteequipm e nt for a minimum time if all external ac power is lost. Such equip m ent is oftenused at remote mountain-top locations of microwave repeater equipment. A power-line orsolar battery c harger k eeps the batt ery charged. T y p ically, a 48 V lead-acid battery is used.Becaus e the condition of the auxiliary power battery is importan t, some means of rem otelychecking its condition is necessary.Proposed CircuitA bloc k diagram of a possible circuit is give n in the Þgu re . The voltage is sampled by ahigh-impedance vo ltmeter having an input resistance greater than 1 MΩ. The signal fromthe voltm ete r controls a vo ltage-c ontrolled oscillator (V C O ) whic h functions as a voltag e-to-frequency converter. For 38 V ≤ V ≤ 58 V, the V CO has an output frequency giv enbyf (V ) = 1000 + 50 (V − 38)This formula gives f (58) = 2 kHz and f (38) = 1 kHz.The output of a ty pical op amp V CO is either a square wave or a triangle wave. Thesewav e fo rm s are unsuitable for transmission ov e r a telephone line and m ust be converted toa sine wav e . You ma y use a wa ve shaping circuit and a low-pass Þlter to generate the sinewavefromthesquarewaveortrianglewave.Becausetherangeoffrequenciesis1 to 2 kHzand the lo we st harmonic of a squar e wave or a triangle wave is the third harmo n ic, the Þltercutoff frequency m ust be between 2 kHz and 3 kHz. An elliptic Þlter is suggested to obtainthe sharpest cutoff.Telephone lines require balanced signals with a source impedance of 600 Ω. Therefore, yourcircuit should have a balanced output having an output resistance of 600 Ω. It should be1able to deliver a power of 1 mW into a 600 Ω load, which is the input resistance of thetelephone line.SpeciÞcation s• Input voltage range: 38 to 58 V.• Output frequency: f (38V)=1kHz ±10 Hz, f (58V)=2kHz ±20 Hz, f (48V)=1.5kHz ±15 Hz, linear response between thes e values.• Output po wer: Differen tial output, 1 mW into a 600 Ω balanced load from a 600 Ωbalanced source.• Output ßatness: Output po wer shall not deviate from 1 mW by more than ±0.5 dBover the 1 − 2 kHz frequency band.• Spectral purity of output: Total harmonic distortion (THD) shall not exceed 2%.• Operating volta ge: ±15 V.Simu lations• You shou ld v er ify the operation of y ou r V CO and wav e -sh ap in g circuits with SPICE .Sho w that the VC O co vers the desired frequency range for the speciÞed input voltages.Use realistic models for all op amps and comparators.• Determine the THD of your circuit as a function of frequency and v erify that it meetsthe speciÞcat i ons.• Verify that the output po wer is correct. Verify that the output ßatness meets speciÞ-cations.Tests of Fi nal Cir cuit• Make measurem ents that duplicate the required sim u latio ns. M e asure all performan cecriteria and sho w that your design meets the speciÞcations.• Present y our circuit to the lab instructor for Þnal