Tektronix 2715, Руководство пользователя

Страница: 82 / 360

Dedicated Controls
4–10
2715 User Manual
Keypad Entry. As with frequency and span/div, you can directly enter the
reference level from the keypad. This feature is especially handy when you have
a good estimate ahead of time of what the signal amplitude is, or when you are
simply interested in how much below a given amplitude a particular signal is.
For instance, you might want to preset the reference level to +15 dBmV to
examine cable TV signals at a customer drop.
The following procedure shows you how to set the reference level to a predeter-
mined value:
3. Press [REF LEVEL] to place the reference level in the immediate entry
mode.
Notice that the abbreviation REFL has appeared at left center of the 2715
screen.
The REFL notation indicates that you can enter the reference level directly from
the keypad. This is done by keying in a permissible value followed by an
appropriate terminator key. You can enter any value from –21.2 to +68.8 dBmV
(or the equivalent in other units). You can enter up to 25 characters, but
regardless of how many you enter, the 2715 rounds and displays the reference
level to a tenth of a dB.
4. Press [2] [1] [ ] [3] [Y] to set the reference level to 21.3 dBmV.
The numbers you type appear on screen to the right of REFL. If you make a
mistake, you can correct it any time prior to pressing [Y] by repeatedly
pressing [BKSP] until the incorrect number disappears, and then typing the
correct value.
Pressing a terminator key (only [Y] or [Z] in this case) determines the units. The
[Y] key can represent units of kHz,
m
SEC,
m
V, or +dBx. Because the reference
level immediate entry mode requires decibels, and because the reference level
unit is dBmV, the 2715 correctly interprets the currently displayed value as
21.3 dBmV when you press [Y]. If a frequency unit had been required, the 2715
would have interpreted your entry as 21.3 kHz when you pressed [Y]. On the
other hand, had you wanted to enter a negative reference level, you would have
pressed [Z] for –dBx. The 2715 ignores the [W] or [X] terminator keys because
they do not represent acceptable reference level units.
REF LVL STEP.
The calibration signal peak is now less than one division below
the reference level.
5. Press [REF LVL STEP].
Nothing happened on screen, but the red LED next to the key is illuminated.
This indicates that the rate at which the reference level arrow keys change
the on-screen signal height and reference level is now 1 dB per press rather
than 10 dB.