494 BELL SYSTEM TECHNICAL JOURNAL 



the low-frequency filter, together with the reintroduced 125 kc. 

 carrier, is impressed on the input of balanced modulator No. 2. A 

 2,500 kc. carrier voltage, which is obtained from the 625 kc. crystal 

 oscillator by means of a harmonic generator, is also supplied to the 

 input of the second modulator. The intermediate frequency filter 

 which follows the second modulator passes the upper sideband gener- 

 ated in the second demodulator (from 2,625.1 to 2,630 kc.) and sup- 

 presses the other sideband and the carrier approximately 50 db. The 

 single sideband thus obtained is then amplified before it is impressed 

 on the input of the third modulator. The circuits up to and including 

 the intermediate amplifier are fixed and do not have to be adjusted 

 in order to change the final output frequency of the equipment. The 

 third modulator is of the unbalanced type and both the output of the 

 intermediate frequency amplifier and a third carrier are applied to its 

 input. The third carrier is obtained from a high-frequency crystal 

 oscillator through two harmonic generators in tandem. The fre- 

 quency of the carrier applied to the third modulator depends on the 

 output frequency desired and since either sideband may be selected 

 the carrier frequency must be 2,625 kc. greater or less than the desired 

 final output carrier frequency. In order to cover the range from 

 4,700 kc. to 21,000 kc, the carrier must range from 7,v325 to 18,375 kc. 

 No filter is required in the output of the third modulator since the 

 output tuned circuits are narrow enough to exclude the third carrier 

 and the unwanted sideband, which are respectively 2,625 and 5,250 

 kc. away from the desired sideband. The output circuit of the third 

 modulator is the first point in the equipment where the final frequency 

 to be transmitted is obtained. The output voltage of the third modu- 

 lator is applied to the input of a series of four amplifiers in tandem, 

 which serve to increase the amplitude of the single sideband and the 

 reduced carrier to a value which will excite to full capacity the power 

 amplifiers of a regular double-sideband transmitter. Receiving type 

 screen-grid tubes are used in all but the multi-vibrator, crystal oscil- 

 lator and the final amplifiers. Amplifiers 2 and 3 consist of one 75- 

 watt screen-grid tube each and amplifier 4 consists of two 1-kw. 

 screen-grid tubes in push-pull. 



Transmitting Monitor 

 It is extremely important in operating the single-sideband ecjuip- 

 ment to know that the distortion is within reasonable limits. With 

 the ordinary double-sideband type of transmission it is possible to 

 simulate the receiving equij)ment with a very simple rectifier, thus 

 allowing local distortion tests to be made on the transmitter. With 



