HP 8568B manual Comb Generator Adjustments, Comb Teeth Display

22. Comb Generator Adjustments

Equipment Cable, SMA (m) to SMA (m) . . . . . . . . . . . . . . . HP 85680-20094

Procedure 1. Set instrument LINE switch to ON and press ~NSTR PRESET).

2.Connect CAL OUTPUT to SIGNAL INPUT 2.

3.Key in CCENTER FREQUENCY) 20 MHz, [FREQUENCY SPAN] 100 kHz, (ATTEN) 0 dB, LOG [ENTER dB/DIvj 2 dB.

4.Adjust front-panel AMPTD CAL for signal peak at top graticule line (-10 dBm).

5.Press QNSTR PRESET).

6.Key in [SHIFT) [RUN) “. This disables phase lock flags.

7.Position instrument on right side and remove bottom cover.

8.Disconnect cables from A23A5J2 (PILOT IF OUT) and A23A3Jl (1ST IF IN) and connect a short, low-loss coaxial cable with SMA male connectors ( do not use adapters) between A23A5J2 and A23A3Jl. Use coaxial cable, HP Part Number 85680-20094. If not available, remove A23FL2 FILTER and use between A23A5J2 and A23A3Jl to adjust comb generator.

9.Key in (START FREQ] 40 MHz. Wait for CRT annotation at lower left of CRT display to indicate START 40 MHz.

10.Key in (mFRE9) 1560 MHz. Wait for CRT annotation at lower right of CRT display to indicate STOP 1560 MHz.

11.Key in [REFERENCE LEVEL] -20 dBm, (ATTEN] 0 dB, LOG (ENTER dB/DIv) 2 dB, DISPLAY LINE (ENTER] -30 dBm.

12.Adjust Al6 COMB DRIVE A16R31 for peak amplitude of CRT trace until comb teeth begin to “wiggle.” Then adjust COMB DRIVE A16R31 slightly counterclockwise until the lowest comb tooth (near START frequency) just begins to fall. See Figure 3-84 for a typical comb tooth display. See Figure 3-83 for location of adjustments.

U P P E R L I M I T -22 dBm

D I S P L A Y L I N E

A T - 3 0 dBm

L O W E R L I M I T - 3 6 dBm

Figure 3-84. Comb Teeth Display

13.Adjust COMB BIAS A23A4A2R6 for peak amplitude of CRT trace until comb teeth begin to “wiggle. ” Then adjust COMB BIAS A23A4A2R6 slightly counterclockwise until the lowest comb tooth (near START) frequency) just begins to fall. See Figure 3-84 for

Adjustments 3-137