Standard Communications mt930b, Руководство по обслуживанию, Страница: 42 / 155

Maintenance
Page 6-10 Service Manual MT930B
IF MEASUREMENTS
First IF Bandwidth at –3 dB
Measurement Conditions:
• Use a scalar network analyzer.
•
Input: J101, J102, J103, or J104, 1210 MHz at –35 dBm (75 Ω )
• Output: J105 (50
Ω at 479.5 MHz)
• AFC: OFF
• AGC: OFF
• MGC: 70 MHz IF TEST at –30 dBm (IF position)
Measurement Procedure:
While the center level is 1210 MHz, measure the upper and lower frequencies
where –3 dB results. The difference between the two frequencies is the 1st IF
bandwidth.
Measurement Specification:
−−−− Bandwidth at –3 dB: ≥≥≥≥
=
40 MHz
Second IF Image Rejection Measurement
Measurement Conditions:
•
Input: J101, J102, J103, or J104, 2150 MHz at –30 dBm (75 Ω )
• Output: 70 MHz IF TEST
• AFC: OFF
• AGC: OFF
• MGC: 70 MHz IF TEST at –30 dBm (IF position)
Measurement Procedure:
1.
Set the VIDEO OUT setting to Ku, NOR (or C, INV). (2nd LO: 549.5 MHz)
2.
Set the UUT and signal generator to the frequency to be measured.
3.
Set the 70 MHz IF level to –30 dBm, using the MGC. This level is the refer-
ence level.
4.
Set the signal generator to the image frequency, and read the 70 MHz IF
output level (the MGC is set the same as in step 3).
The difference in level between step 3 and 4 is the image rejection.
(2nd IF = 70 MHz)
Image Frequency = Receive Fre (½ x 2nd IF) = Receive
Fre 274.75 MHz
Receive Fre (2 x 2nd IF) = Receive Fre 1099 MHz
Measurement Specification:
−−−− 2nd IF Image Rejection: ≥≥≥≥ 30 dBm