NIST Technical Note 1337: Characterization of Clocks and Oscillators

276 A. L. LANCE, W. D. SEAL, AND F. LABAARIt is convenient to consider the system equations and calibration techniquesin terms of frequency modulation of stable sources. If the source to bemeasured cannot be frequency modulated, it must be replaced, during thecalibration process, with a modulatable source. The calibration processwill be described using a modulatable source and a 20-kHz modulationfrequency. However, other modulation frequencies can be used. The calibrationfactor of this type discriminator has been found to be constant overthe usable Fourier frequency range, within the resolution of the measuringtechnique. The calibration factor of the discriminator is established after thesystem power levels have been set with the unit under test as the source.The discriminator calibration procedures are as follows.(1) Set attenuator No.4 (Fig. 20) to 50 dB.(2) Replace the oscillator under test with a signal generator or oscillatorthat can be frequency modulated. The power output and operating frequencyof rhe generaror must be set to the same precise frequency and amplitudevalues that the oscillator under test will present to the system during rhe measuremel1lprocess.(3) Select a modulation frequency of 20 kHz and increase the modulationuntil the carrier is reduced to the first Bessel null, as indicated on the spectrumanalyzer connected to coupler No. 1. This establishes a modulation index(m = 2.405).(4) Adjust the phase shifter for zero volts dc at the output of the mixer,as indicated on the oscilloscope connected as shown in Fig. 20. This establishesrhe quadrature condition for the two inputs to the mixer. This quadraturecondition is continuously monitored and is adjusted if necessary.(5) Tune the tracking spectrum analyzer to the modulation frequencyof 20 kHz. The power reading at this frequency is recorded in the programand is corrected for the 50-dB setting of attenuator No.4, which will be setto zero decibel indication during the automated measurements.P(dBm) = (-dBm power reading) +50 dB (88)This power level is converted to the equivalent rms voltage that the spectrumanalyzer would have read if the total signal had been applied:v'ms = jlO P11O /lOOO + R. (89)(6) The discriminator calibration factor can now be calculated becausethis power in dBm can be converted to the corresponding rms voltage usingthe following equation:v'ms = j(lOPilo/lOOO) x R, (90)where R = 50 n in this system.IN-227