HP 654A - 5-35. Distortion Check

Model 654A
Section V
k. Adjust the 654A to 10MHz and reset the
AMPLITUDE control for OdBm indication on the
654A meter.
I. Repeat steps d through j of this paragraph to
check tracking at 10MHz.
m. These checks verify meter tracking accuracy of
+/-0.05dB; perform the adjustments of Paragraphs
5-58 through 5-60 if the limits are not met.
5-22 ATTENUATOR ACCURACY CHECKS.
5-23. MEASUREMENT PROBLEMS.
5-24. The test setup ( shown in Figure 5-6) for these checks
is critical. Ground loops, noise, interference from other
instruments, etc., can give measurement errors of up to
greater than ldB. Placement of the test instruments relative
to each other may affect measurement accuracy; it is not
possible to give an exact position for each instrument as
this may also vary with the location of the test setup. To
reduce ground loops between instruments a large coil can
be placed in series with the interconnecting cables between
two of the instruments. The coil can be easily constructed
as follows:
1) Wrap about twenty turns of 50 ohm coaxial cable
around any large diameter, ferrite, circular core ( a
3in. o.d. X 1-3/4in. i.d. core, - hp- Part No.
9170-0995 is suitable) and tape the turns in place.
2) Cut the leads from the coil to a convenient length
and attach BNC connectors ( male) to both ends of
the coax.
The effect of this coil is to present a high impedance to
60Hz signal between grounds without appreciably
attenuating the required signal on the center conductor of
the coax. The exact location of the coil in the test setup
will depend on which instruments have a ground loop
problem; this can be determined by trial and error.
5-25. If the 654A attenuators do not appear to be within
the limits given in the following checks, carefully check the
test setup before troubleshooting the attenuators; the
placement of the shield around the 654A attenuators is
extremely critical and, if disturbed, can adversely affect the
high frequency response of the attenuators ( see Paragraph
5-97).
5-26. 10dB-STEP ATTENUATOR CHECK.
NOTE
See Paragraphs 5-24 and 5-25 before
making this check.
a. Connect the equipment shown in Figure 5-6.
(Figure 5-7 shows the parts required to build the
75 ohm to 50 ohm impedance converter). Use the
10dB/step external attenuator ( with known
accuracy at 300kHz and 10MHz), do not connect
the 1dB/step attenuator at this time.
b. On the rear panels of the instruments connect the
312A Wave Analyzer RECORDER OUTPUT to
the 313A Tracking Oscillator RECORDER
INPUT. Set the tracking oscillator METER MODE
switch to EXPAND 312A.
c. Set the 654A controls as follows:
FREQUENCY dial 3
FREQUENCY RANGE X1 00K
OUTPUT LEVEL dBm + 10,0
IMPEDANCE 150 BAL
AMPLITUDE Adjust for OdBm
on 654A meter.
d. Set the external attenuator to - 90dB position and
the external amplifier gain to + 40dB.
TEST OSCILLATOR
hp 6544
750 FEED-THRU
TERMINATION
hp 110944
EQUAL LENGTHS
OF CABLE
TRACKING OSCILLATOR
lip 3134
I DB/STEP WAVE ANALYZER
ATTENUATOR hp3I2A
hp 355C
0
IODB/STEP
ATTENUATOR
hp 355D
0
750 TO 500
IMPEDANCE CONVERTER —
SEE FIGURE 5-7
AMPLIFIER
hp 4614
II _ . Il
11
? 9 •
o
©G§C Cc)
M7 0 u 00
65.-B-1697
Figure 5-6. Attenuator Accuracy Checks
5-7