User Manual

Performance Ver
ification
Common Mode Rejection Ratio
If veri fication of analog bandwidth was not performed, or the calibration of the network analyzer has been altered, perform the
calibration and normalization steps in the Preparation section of the Analog Bandwidth verification.
In this test, you first plot the differential mode gain, and then the common mode gain. Next, using the math function on the
network analyzer, you create a plot that represents the reciprocal of the CMRR.
Verification
42 V Range (÷ 50).
1. Set the refer
ence value of the network analyzer to –34 dB and position the reference near the top of the screen.
2. Verify that t
he test port power is set to +10 dB m.
3. Connect the p
robe input to the D M pins on the BNC-to-probe tip adapter. Polarity is unimportant.
4. Set the probe
to the 42 V range (÷ 50), full bandwidth, and DC reject off.
5. The plot that
is displayed represents the differential m ode gain of the probe. Save this plot to the instrument memory.
6. Disconnect t
he probe input from the DM pins and connect it to the CM pins on the BNC-to-probe tip adapter. Make
sure to connect the probe ground socket to the long ground pin on the fixture.
7. The plot that is displayed represents the common mode gain of the probe. You may need to adjust the reference
level and scale to view the plot.
8. Use the math function of the network analyzer to divide this plot by the differential plot that you saved in step 5. The
resulting pl
ot is the reciprocal of the common mode rejection ratio. The CMRR can be read by inverting the sign
of the magnitude.
9. Measure the CMRR at 30 kHz, 1 MHz, and 250 MHz. Analyzers with marker capability can do this directly by setting
the marker intercepts at 30 kHz, 1 MHz, and 250 MHz. If necessary, turn on the network analyzer Average mode
with 16 aver
ages to stabilize the reading.
10. Verify that
the CMRR is greater than the values listed in the following, and record the r esults in the test record.
Table 6: CMRR limits
Frequency 42 V Range (÷
50) CMRR
30 kHz ≥55 dB
1MHz ≥50 dB
250 MHz ≥18 dB
11. Disconnect the equipment.
P6250 & P6251 Technical Reference 27