Tektronix 1705A, Instruction Manual

Page: 46 / 223

Operating Instructions
1705A Spectrum Monitor
2- 7
Measurement Graticule
The 1705A is equipped with an internal graticule crt. The graticule has an 8 X
10 division scale that is lighted. Scale brilliance is controlled by the front-panel
SCALE control. Figure 2-4 shows the 1705A graticule. Refer to this figure and
subsequent figures when reading the following discussion of the graticule.
Tek
REF
--10
--20
--30
--40
--50
--60
--70
L
O
G
+ f
Figure 2-4: 1705A graticule scale
The vertical scale is eight divisions in height. The center vertical scale is broken
into five equal minor divisions per major division. Note that the 0 dB reference
is at the top of the graticule and that 80 dB (maximum division) is at the bottom
of the graticule. There are two gain selections: normal gain (10 dB/div) and
high gain (2 dB/div). In normal gain mode, major divisions are 10 dB, which
makes each minor division 2 dB. When the GAIN front-panel push button is
held, high gain is selected, and major divisions are approximately 2 dB, which
makes minor divisions about 0.4 dB each.
Since dB is a dimensionless ratio, and there are several scale variations, it is
essential that there be some discussion of the various scales. The unit of
measure described as dB (decibel) is 10 LOG P1/P2. If there is a specific scale
defined (m, k, w, p, etc.), there is a specific reference point established. The
1705A has input specifications in dBm, which means that they are referenced to
1 mW (milliwatt). Therefore, --30 dBm is 30 dB below 1 mW. Table 2--1 is a
handy reference table for dB and dBm. Table 2--2 provides a reference for
conversion from dBm to
v.
Vertical Scales