Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

Alignment ProcedureRATIO DETECTOR 389The ratio detector may be aligned by using either an unmodulatedsignal at the center frequency and a d-c vacuum-tube voltmeter, orby using an f-m signal generator and an oscilloscope.If an unmodulated signal generator isused, the procedure is to setthe signal generator to the center frequency. With the d-c vacuumtubevoltmeter connected to measure the rectified output voltage(thisis usually at the a-v-c take-off point) , the primary tuning is adjusted formaximum voltage.The procedure used for adjusting the secondary tuning dependsupon whether the center-tap of the stabilizing voltage is grounded orwhether one end is grounded. If the center-tap is grounded, the secondarytuning is adjusted so that the d-c voltage at the audio take-offpoint is equal to zero; the d-c vacuum-tube voltmeter is shifted to theaudio take-off point for this measurement. If the stabilizing voltageis grounded at one end (see Figure 25) rather than at the center, thesecondary tuning is adjusted so that the d-c voltage at the audio takeoffpoint is equal to half the rectified voltage.If sweep alignment isused, the primary can be accurately alignedby using a comparatively low deviation and adjusting itamplitude on the screen.for maximumThe secondary may be adjusted by using adeviation such that the total frequency swing (twice the deviation)is equal to the peak separation. This procedure makes it possible toadjust the secondary tuning so that a symmetrical detector characteristicis obtained.Peak SeparationThe separation between the peaks on the f-m output characteristicsof a ratio detector may be measured most simply by applying an f-msignal and increasing the deviation until the response is just observedto flatten at the peaks. When this is done, the peak separation is equalto twice the deviation.If an attempt is made to measure the peak separation by plottingthe output characteristic point by point, the peak separation obtainedwill usually be considerably less than is obtained under dynamic conditionswith the output voltage stabilized. The f-m detector characteristicmay be plotted point by point provided a battery of the propervoltage is connected across the stabilizing condenser. The voltage ofthis battery must be equal to the rectified voltage which exists at thecenter frequency.In practice it is convenient to use a 7.5-volt C-batteryand to adjust the signal input so that the condenser-stabilized voltageat the center frequency is equal to the battery voltage.