# HP AN174-6 Measuring the stability of a frequency source.pdf

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ELECTRONIC
CO LJ N] 1 — 2 HEWLETT hp; PACKARD
MEASURING THE STABILITY OF A
FREQUENCY SOURCE
Stability measurements on frequency sources are made possible by the high
resolution capability of the 5345 Electronic Counter. This application note
describes the use of the 5345A Electronic Counter in a calculator based HP
Interface Bus system which automatically measures and plots frequency devia-
tion as a function of time. The system is extremely useful in obtaining high
resolution plots of parameters such as oscillator long term drift and crystal
oscillator warm up characteristics. The program automatically sets the counter
gate time to give the necessary measurement resolution and uses the 5345
computer dump output mode to obtain a constant resolution of 2 parts in 10°
per second of gate time.
INTRODUCTION
In the 5345A counter, two scalers (registers) are used
to totalize events during the time that the gate is open.
One totalizes input events and the other totalizes
clock pulses. A processor in the 5345A operates upon
these two numbers and provides the desired output
display (frequency, period, ratio, etc.). As a result of
the requirement that the display resolution can not
exceed 2 parts in 10° per second of gate time, the
resolution of the displayed result is variable. Figure 1
shows resolution in parts in 10° per second of gate
time versus the most significant digits in the dis-
played number.
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MOST SIGNIFICANT DIGITS OF DISPLAY
Figure 1. Resolution of the 5345 counter is a function of the
most significant digits in the display. For a display of
499999999, the resolution would be 2 parts in 10? (for 1 sec
of gate time) whereas a display of 500000000 would be
resolved to 4 parts in 10? (for 1 sec of gate time).
The 5345A has two output modes: the talk mode and
the computer dump mode. The computer dump mode
of output bypasses the processor and is normally
used when it is desired to output 5345A readings at
a very high rate (up to 5000 readings/sec). It can also
be used whenever it is desired to analyze or process
the raw measurement data. In this application note,
a calculator is used to reduce the raw data. Since the
calculator is not subject to the constraints of a
display, a constant resolution of 2 parts in 10° per
second of gate time can be obtained regardless of the
input frequency. Figure 2 is a plot of measurement
accuracy (effects of time base error and trigger error
must be accounted for separately) versus gate time
when using the computer dump mode of output.
MEASUREMENT ACCURACY (t TIME
BASE ERROR + TRIGGER ERROR*)
2 x 10-12
2x 10710-
2x 108
2x 105
2x 10*-
”
e \_WORST CASE ACCURACY
ый USING 5345A DISPLAY
опт
1107
100ns | 10us | 1ms | 100ms | 10s |1000
lus 100us 10ms 1s 100s
GATE TIME
2x 1072-
*Refer to the 5345A training manual for a discussion of
time base error and trigger error.
Figure 2. Measurement accuracy as a function of gate time
for the computer dump mode of output (constant resolution
of 2 parts in 109).
Figure 3 is a plot of accuracy in terms of frequency
as a function of gate time and input frequency. Again,
the effects of time base error and trigger error must
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Figure 3. Frequency accuracy as a function of gate time
and input frequency for the case of constant resolution
(computer dump mode) of 2 parts in 109/sec gate time.
be accounted for separately. The dashed vertical lines
indicate where time base accuracies are of the same
magnitude as the measurement accuracy. At this
point, of course, extending the gate time cannot
improve the measurement accuracy since the time
base is limiting. Table I gives the stability character-
istics of various time bases used in the 5345A and
frequency standards which could be used to phase
lock the 5345A clock by connecting the standard to
the counter’s rear panel EXT FREQ. STD INPUT
connector.
Table 1
Identification Temperature Short Term Long Term Normalized Long Term
Option 001 _6 (950_950 —9 -7 =
VT RT XO 2 x 10-6 (25°-35°C) 2 x 10-9/sec 3 x 10-7/mth 1 x 10-8/day
Standard Crystal —s (N0_KKO lan 5 -—10/
10544 Oven XTAL J x 10-2 (0°-55°C) 1 x 10-11 /gec 5 x 10-!°/day 5 x 10-19/day
Rubidium 5065 4 x 10711 (0°-50°C) 5 x 10-12/8ec 1 x 10—!/mth 3 x 10-!#/day
Cesium 5061 A-Opt.004 5 x 10-12 (0°-50°C) 5 x 10-12/sec 3 x 10-!2/life of tube —
MEASUREMENT SET-UP
The measurement system consists of the 5345A Elec-
tronic Counter (Opt. 011), the 9820A Calculator
(Opt. 001 Extended Memory), the ASCII Interface
Card and PCII ROM (both included in 10593A),
11221A Math ROM, 11220A PCI ROM, and 9862A
Calculator Plotter (Opt. 20). The instruments are con-
nected to the calculator as shown in Figure 4.
Fa 9820 CALCULATOR* -— 4
|
HP INTERFACE PLOTTER |
B
US CABLE 1/0 CABLE;
2 |
5345 COUNTER 9862A PLOTTER
4 * Calculator should have Math,
FREQUENCY | PCI, and PCII ROM blocks
installed in ROM slots 1, 2,
OE and 3 (left to right) respectively.
Figure 4
Since the 9820 Calculator remotely controls all front
panel controls of the counter, there is no need to set
these controls to any particular positions. The 5345A
Electronic Counter is interfaced to the 9820 Calcula-
tor in the following manner: insert the ASCII Bus
Interface card into any of the four slots on the
rear panel of the 9820 Calculator; connect an ASCII
Interface Cable (10631A, B, or C) from the inter-
face card of the calculator to the rear panel plug
of the 5345A Electronic Counter. To interface the
plotter to the calculator, plug the 9862 Plotter 1/0
Card into any of the three remaining slots on the
rear panel of the 9820 Calculator.
Set the Talk/ Listen address on the frequency counter
as specified in the following table:
Table 2
Talk/Listen Mode
5345A K/* addressable | 0 | 1 | O | 1
Counter | (K=computer
dump mode)
These switches are located on the rear panel of the
instrument and must be set so as to agree with the
OPERATION
Key into the calculator the provided program. The
program will request the following parameters:
1) “NO. OF POINTS” — enter the number of meas-
urements to be made and plotted.
2) “CENTER FREQ” enter the nominal fre-
quency of the frequency source in Hz.
3) “VERTICAL DEV EXP” — enter the vertical
deviation exponent. For parts in 10% resolution,
enter 6. For parts in 108, enter 8.
The program automatically sets the 5345A gate time
so that the counter resolution is 50 times greater than
the major divisions on the plotter output. After let-
tering the plot, the program halts to allow changing
of the plotter pen. Push the RUN PROGRAM key to
resume execution. The calculator then reads the two
scalers in the counter, computes the frequency, and
outputs the data to the plotter. Figure 5 shows the
frequency stability of the 8601A Generator/Sweeper
over a time period of approximately 1/2 minute.
FREQUENCY STABILITY
CENTER FREQ — 1.000063000E 07
100MS GATE TIME
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FREQ DEVIATION—PARTS IN 108
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Figure 5. Frequency stability of 8601A Generator/Sweeper.
100 measurements were made over a time period of approx-
imately one half minute.
MEASUREMENT CONSIDERATIONS
b) Misleading results will be obtained if the re-
a) Resolution is a constant 2 parts in 10° High quested resolution exceeds the stability of the
resolution plots will result in long gate times and crystal in the counter. For very high resolutions,
hence, large time intervals between measure- phase lock the counter clock to a “house
ments. standard”.
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For more information, call your local HP Sales Office or East (201) 265-5000 » Midwest (312) 677-0400 + South (404) 436-6181 + West (213) 877-1282. Or, write: Hewlett-Packard, 1501 Page
Mill Road, Palo Alto, California 94304. In Europe, Post Office Box 85, CH-1217 Meyrin 2, Geneva, Switzerland. In Japan, Yokogawa-Hewlett-Packard, 1-59-1, Yoyogi, Shibuya-Ku, Tokyo, 151.
025952-0883 PRINTED IN U.S.A.
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