12. Time Base Adjustment (SN 2840A and Below, also 3217A05568 and Above)

Reading 11:

 

mHz

14.Subtract the shifted frequency reading in step 11 from the last recorded frequency in step 10. This gives the frequency correction factor needed to adjust the A27 10 MHz Frequency Standard.

Frequency Correction Factor:

 

mHz

15.On the Frequency Counter, select a 1 second gate time bv

pressing, ~~~ 1 @kiK]. The Frequency Counter should now display the difference between the frequency of the INPUT A signaland 10.0 MHz with a resolution of 0.01 Hz (10 mHz).

16.Remove the two adjustment cover screws from the A27 10 MHz Quartz Crystal Oscillator. See Figure 3-53 for the location of the A27 10 MHz Frequency Standard.

Note Do not use a metal adjustment tool to tune an oven-controlled crystal oscillator (OCXO). The metal will conduct heat away from the oscillator circuit, shifting the operating conditions.

17.Use a nonconductive adjustment tool to adjust the 18-turn FREQ ADJ capacitor on the A27Al 10 MHz Quartz Crystal Oscillator for a Frequency Counter indication of 0.00 Hz. See Figure 3-53 for the location of the A27Al 10 MHz Quartz Crystal Oscillator.

R F S e c t i o n ( b o t t o m view)

A27Al

Figure 3-53. Location of A27Al Adjustment

18.On the Frequency Counter, select a 10 second gate time by pressing, [GATE) 10 [GATE). The Frequency Counter should now display the difference between the frequency of the INPUT A signal and 10.0 MHz with a resolution of 0.001 Hz (1 mHz).

19.Wait at least 2 gate periods for the Frequency Counter to settle, and then adjust the 16-turn FINE adjustment on the A27 10 MHz Frequency Standard for a stable Frequency Counter indication of (0.000 + Frequency Correction Factor) fO.O1O Hz.

20.Replace the RF Section bottom cover and reconnect the short jumper cable between the FREQ REFERENCE INT and EXT connectors,

3-98 Adjustments