Tektronix 7D14, Instruction Manual

Page: 17 / 133

Operating Instructions— 7D 14
MEASUREMENT MODES
Introduction
The 7D14 has three primary measurement modes: fre­
quency, ratio, and totalize (number of events). The
following discussion gives general information about each
mode and instructions on making the measurements.
Frequency Measurements
The 7D14 provides direct measurement of the average
frequency of signals up to 525 MHz. The input to the
counting circuits is selected by the INPUT SENS/P-P
VOLTS control from either the signal connected to the CH
A INPUT connector or the internal trigger pickoff signal
from the Indicator Oscilloscope trigger circuits. (TRIG
SOURCE; see Counting a Displayed Signal).
The 7D14 counts the input pulses over an interval
selected by the MEASUREMENT INTERVAL 1 ms
through 10 s pushbuttons. The measurement interval
selected determines the measurement range and resolution.
These pushbuttons also automatically select the measure­
ment units and decimal point position in the display. Table
2-2 gives the frequency range and resolution for each time
interval and shows the display decimal point position and
measurement units.
Generally, use the shorter measurement intervals for
high-frequency, low-resolution measurements and the
longer intervals for measurements requiring high resolution.
Greater Accuracy. For greater accuracy and/or stability
when measuring frequency, an external 1 MHz crystal signal
can be substituted for the internal time-base oscillator. The
external signal must have greater accuracy than the internal
oscillator (see Specification section) and have a minimum
amplitude of 0.8 volt peak-to-peak.
TABLE 2-2
MEASUREMENT INTERVAL DATA
1 If the maximum frequency is exceeded, a "greater than" symbol
(» will precede the readout display.
MEASUREMENT MAXIMUM
FREQUENCY
INTERVAL UNITS RESOLUTION
1 ms MHz 1 kHz 525.000 MHz
10 ms MHz 100 Hz 525.0000 MHz
100 ms MHz 10 Hz
525.00000 MHz
1 s kHz 1 Hz
99999.999 kHz 1
10s kHz 0.1 Hz
9999.9999 kHz1
Connect the external signal to the REF FREQ/CH B
EXT IN connector and press the EXT IN pushbutton. The
function of the MEASUREMENT INTERVAL 1 ms - 10 s
pushbuttons is the same as when using the internal time­
base oscillator.
Ratio Measurements
Ratio measurements compare the frequencies of two
signals. Normally, the higher frequency signal is applied to
Channel A (via CH A INPUT connector or internal TRIG
SOURCE) and the lower frequency signal is applied to
Channel B (via EXT IN connector with the REF FREQ/CH
B EXT IN pushbutton pressed). The MEASUREMENT
INTERVAL 1 ms through 10s pushbuttons select the
counting time as derived from the signal applied to Channel
B. The 7D14 then counts the pulses derived from the
Channel A signal during a time interval established by the
Channel B signal.
The display ratio measurement is equal to:
Frequency Channel A 6 MEASUREMENT
Frequency Channel B INTERVAL
whose denominator is known to be 1. Thus, a ratio of
30.000:1 will be displayed as 30.000. The decimal point in
a ratio measurement display is correctly positioned only
when using the 1 ms, 10 ms, or 100 ms MEASUREMENT
INTERVAL pushbuttons. Readings obtained when the 1 s
or 10 s buttons are pressed must be multiplied by 10~3 to
correct the decimal position.
Ratio measurements are dimensionless (i.e., no measure­
ment units); however, the MHz or kHz legend will remain
displayed.
Totalize (Count Number of Events)
Introduction. The 7D14 will count the number of
pulses applied to Channel A (via the CH A INPUT connec­
tor or TRIG SOURCE) up to 99,999,999 pulses at a rate
not to exceed 525 megahertz. The pulses will be counted
during a time interval (gate time) determined either by the
MANUAL GATE ON/OFF pushbuttons or by a gating
signal connected to the EXT GATE input connector. Over­
ranging (count exceeding 99,999,999) is indicated by a
"greater than" symbol (>) preceding the digital readout.
Display Storage Modes. Two modes of displaying the
count can be selected when using MANUAL or EXT GATE
operation. These modes are selected by the Man Gate Stor­
age switch.
With the Man Gate Storage switch ON, the total count
is displayed at the end of the gate or count interval.
2-8