TM 11-6625-255-14/TO 33A1-13-170-1
Table 3-2. Typical DB Distortion Measurements
to be measured to the METER binding posts.
b. Set the function switch to METER.
3
1
2
4
Meter
DB setting of
Col 1 plus
c. Increase or decrease the setting of the meter
Distortion measured
reading
meter range
col 2
Col 3 + (-20 dB)
r a n g e switch R.M.S. VOLTS scale to obtain a
switch
r e a d a b l e meter indication.
-1.5
-21.5
-41.5
-20
-21.5
-1.5
-1.5
0
-1.5
-31.5
-51.5
-30
c e n t e r scale of the meter as designated by the
+8.5
-11.5
+10
-1.5
m e t e r range switch.
-1.5
-1.5
+18.5
+20
-19
-39
-20
+1
39. Use as Vtvm for Measuring Signals
+1
-29
-49
-30
-9
in DBm Values
+10
+11
+1
NOTE
3-8 a and b .
The maximum possible error in distor-
t i o n readings is - 1 1 p e r c e n t o f t h e
b. Set the meter range switch to DB; this posi-
r e a d i n g obtained. D i s t o r t i o n readings
tion gives an easily readable meter indication.
greater than -20 dB, such as -11.5 and
c. If the internal impedance of the signal source
-9 dB readings shown in column 4
being measured is 600 ohms, the dBm level is the
above, are not precise readings but only
meter indication algebraically added to the
indicate extreme distortion.
s e t t i n g of the meter range switch. Examples of
meter indications are given in table 3-4.
Table 3-3. DB Range of Meter Range Switch Positions
d. If the internal impedance of the signal
source being measured is not equal to 600 ohms, a
Range (dB)
DB setting
-30
-62 to
-48
c o r r e c t i o n factor must be applied to the meter
-52 to
-38
-20
reading obtained in column 3 of table 3-4.
-42 to
-28
-10
(1) Record the value obtained from the
0
-32 to
-18
meter.
+10
-22 to
-8
-12 to
+2
+20
(2) Refer to the impedance correction chart
+30
-2 to
+12
+40
+8 to
+22
the signal source under measurement and the
+50
+ 1 8 to
+32
diagonal line. The closest horizontal line to this
37. Unknown Input Signal Frequency
intersection is the dBm correction factor.
When the af signal frequency to be measured is
(3) This dBm correction factor must be
unknown, proceed as follows:
a l g e b r a i c a l l y added to the sum of the meter
reading and setting of the meter range switch.
( 4 ) Examples of dBm measurements and
t o DISTORTION.
applied correction factors are given in table 3-5.
b. Set the frequency RANGE switch to Xl.
Table 3-4. Examples of Meter Indications
c . Rotate the coarse FREQUENCY tuning
1
2
control from 20 through 200 on the FREQUENCY
DB setting of position
Meter indication
Measured signal
t u n i n g dial and, at the same time, watch the
of meter range switch
in dBm (sum of col
(DECIBELS scale)
1 and 2
meter pointer for a dip. If there is no dip, set the
0
-5
-5
frequency RANGE switch to X10. If there is still
-10
-15
-5
no needle dip, set the switch to X100.
-20
-25
-5
-30
-35
-5
+5
+10
-5
3-5 i through k .
+15
+20
-5
e . The approximate unknown frequency may
+31
+30
+1
now be read on the frequency dial multiplied by
Table 3-5. Examples of dBm Measurements and
the designated number of the frequency RANGE
Applied Correction Factors
Source
switch.
Algebraic
DBm correc-
Corrected dBm
impedance
sum of
tion factor
reading of
3 8 . Use as Vtvm for Measuring Signals
in ohms.
meter read-
obtained
actual sig-
ing and
in Rms Values
from fig.
nal being
setting of
5-4.
measured
Note that the AF-RF selector switch ( T S -
meter range
(algebraic
7 2 3 A / U ) , signal INPUT control, frequency
switch in
sum of COl
RANGE switch, and both FREQUENCY con-
d B ma r a n g e .
2 and 3).
trols have no effect when the spectrum analyzer is
60
+10
-8
+2
operated as a standard ac vtvm.
60,000
+4
-20
-16
a. Connect the test leads from the signal source
a
See table 3-4.
3-5
Change 1
