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SERVICE INFORMATION FROM HEWLETT-PACKARD
MAY-AUGUST 1977
FRACTIONAL N DIVIDE-BY-N LOOP
SIMPLIFIES In order to more readily understand
FREQUENCY how a fractional N loop works, one
must first understand the more tradi-
SYNTHESIS tional phase lock loop with a divide-
by-N element. The fractional N loop is
by Kenneth Jessen, really a modified divide-by-N loop.
HP Loveland Division
INTRODUCTION Figure 1 illustrates the basic concept
involved in a phase lock loop. The FR.Y..CY
in.,
Hewlett-Packardis introducing several phase detector compares the Voltage
new instruments that use a new fre- Controlled Oscillator (VCO) output to Figure 2. A Phase Lock Loop With a
the reference signal and produces a Divideby-N Element to Produce a Range
quency synthesis technique known as of Frequencies in Steps Equal to Integral
fractional N. This new technique over- tune voltage proportional to the phase Multiples of the Reference Frequency
difference of these two inputs. The tune
comes many of the limitations of the
voltage is "cleaned up" by passing it I I
C traditional divide-by-N loop. Any given
divide-by-N loop can only produce fre-
quencies which are integral multiples
through a low-pass filter to suppress
noise and high frequency components.
divide-by-N block can be stepped to
produce frequencies that are integral
The polarity of the tune voltage is such multiples of the reference. The VCO
of the reference frequency. The 33308 can now produce a range of discrete
Automatic Synthesizer, for example, that it will pull the VCO frequency in a
direction to phase-track the reference frequencies, all phase locked to the
uses four divide-by-N loops of different reference.
frequency rangesto producean overall frequency.
range fr6m 0.1 Hz to 13,000,999.9 Hz. FRACTIONAL N LOOP -
The first, second and third digits of the OPEN LOOP CONDITION
output frequency are controlled by the
first divide-by-N loop. The fourth and A fractional N loop is simply a modified
fifth digits are controlled by the second divide-by-N loop. The fractional N loop
divide-by-N loop; the sixth and seventh is capable of operating at frequencies
digits are controlled by the third loop which are not integral multiples of the
and the remaining two digits by the reference signal. In fact, the fractional
fourth loop. (Summation loops are also N loop can be made to operate at any
required to sum the outputs of the frequency greater than the reference
divide-by-N loops but they are ignored frequency up to the upper limit of the
to simplify this discussion.) FRq"e"C"
The new HP3335A Synthesizer/Level Figure 1. Basic Phase Locked Loop
Generator can produce frequencies Used to Produce Single Frequencies
from 200.00 Hz to 80,099,999,999 Hz
using only two loops. One loop is a
common divide-by-N loop producing
39 MHz to 79 MHz in 1 MHz steps. The In the previous example, the frequency
second loop, and topic of this article, is of the VCO must match that of the ref-
a fractional N loop able to generate erence. For single frequency applica-
c frequencies from 1 MHz to
1.999999999 MHz in 0.001 Hz steps.
The advantages of fractional N are ob-
vious; greater frequency resolution,
tions this is fine, but to extend the use-
fulness of the loop, a divide-by-N ele-
ment must be added as shown in Fig-
ure 2. The basic difference is that the
wider frequency range and fewer VCO frequency is N times the refer-
loops. ence frequency. The modulus of the
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FRACTIONAL N
L
VCO. As mentioned previously, the goes through 10.1 cycles. To put this integral multiple of the reference fre-
divide-by-N loop in the 3335A another way, the VCO advances one- quency (100 kHz in this example) and
Synthesizer/Level Generator produces tenth of a cycle relative to its integral the system can be phase locked. Be-
39 MHz to 79 MHz in 1 MHz steps part for every reference period. After tween removals of one cycle. however,
(using a 1 MHz reference), and the the passage of two reference periods, the instantaneous frequency is still
fractional N loop in the 3335A produces the VCO has gone 20.2 cycles and 1.01 MHz.
from 1 MHz to 1.999999999 MHz in after ten reference periods, the VCO
0.001 Hz steps using a 100 kHz refer- has advanced 101 cycles (100 cycles The element required to achieve this
ence signal. plus one extra cycle which represents mode of operation is a pulse remover
the fractional component). Table 1 il- and is shown in Figure 5. The output of
The best place to begin this explana- lustrates this point quite well. the phase detector is a sawtooth riding
tion is to assume an open loop config- on a dc voltage. The output of the
uration for a normal divide-by-N loop Note that after ten reference periods, phase detector increases linearity as
(see Figure 3). For this example, as- the VCO has gone one full cycle be- the phase of the VCO divided-by-Nad-
sume that the reference frequency is yond what would be needed to close vances on the reference signal. Each
No. of Ref. No. of Completed Cycks of
Periods
(Fdz=lOOkHz) N x F M = l MHz N.P-1.01 MHz ?ham Advancement
1 10 10.1 0.1 cycle of phase
1 10 10.1 0.2 cycle of phase
3 30 30.3 0.3 cycle of phase
4 40 40.4 0.4 cycle of phase
Flgum 3 The Basic Block D & g m
. 9 90 90.9 0.9 cycle of phase
of and N Step Loop in an 10 100 101.0 1 full cycle of phase (3600)
open Loop condltlon
100 kHz and that the divide-by-N
number is 10. Also assume that the
VCO frequency is 1.01 MHz. This
means that the VCO is operating at a
fractional muMp/e (10.1) of the refer-
the loop and phase lock this system. If
one VCO cycle could somehow be re-
moved from the string of VCO cycles
after the passage of ten reference
I 1.0 I
ence signal. It would be impossible to
periods, the average phase advance-
close this loop if it were a divide-by-N
ment would be cancelled. That is, the I
loop because the phase detector com-
pares the reference signal to the output of the phase detector, insteadof
divide-by-N signal, and then generates increasing without bound as shown in
a dc level equivalent to the phase dif- Figure 3, would be reset to zero every RaMima
ference. That is, the VCO is not operat- ten reference periods as shown in Fig- P O kW
ing at an integral number times the ref- ure 4. By cancellingthe average phase
advancement, the average frequency Rgum 5. The Basic Block Diagram of a
erence signal, but rather as a fractional M o d W N Step Loop With a Pulse
component; this means the phase of from the divide-by-N block becomes an Remover A d d d to AHOW th.VCO to
the VCO is advancing. Sincethe phase op.ntl,a a FnOtionai Frequency
t
of the VCO signal (divided by 10) is
advancing relative to the phase of the time the VCO advances one full cycle,
reference signal, the phase detector the pulse remover is actuated and the
puts out an ever increasing dc level. In phase advancement is reset to its zero
the normal case (closed divide-by-N degree level. The dc voltage level
loop), the phase detector's output which the sawtooth is riding on can
would be used as the tune voltage back then be used as the tune voltage
to the VCO forcing its frequency to drop needed to phase lock the system.
from 1.01 MHz to 1.00MHz. In the frac-
tional N loop, the phase is allowed to The next step is to devise a method to
advance and the phase detector out-
puts a ramp. 1-
Rloh XN(M = 101" M
blnMC
zm
O M p * I
trigger the pulse remover by determin-
ing when the VCO frequency has ad-
vanced one full cycle. The fractional
'0
Reference periods are defined using portion of the VCO contains the infor-
Rgum 4. me Output of the Phase mation needed to accomplish this. The
Figure 3 as follows: While the refer- Detector Shown Here is a Sawtooth
ence signal goes through one period, Riding on a dc Voltaga. fractional portion is stored in a register
the VCO, operating 10.1 times as fast, and added to a second register each
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FRACTIONAL N 1
reference period. This second register,
called the phase register, then contains
a running total of the phase advance-
ment. The contents of the phase regis- Ken Jessen received his BSEE and
ter are represented in the right-hand MEA from the University of Utah
column of Table 1. When the phase before joining HP in 1965. Ken is in
register reaches or exceeds unity, the Customer Service at the Loveland,
Colorado, Division where he is
VCO has advanced one full cycle and Service Manager for the sources
the overflow of this register is used to and analyzer product line.
trigger the pulse remover.
Spare time activities include writing
Now assume that the VCO operates at both technical articles for trade
a frequency such as 10.3 MHz which is publications, as well as historical
not evenly divisible into 1.O. After one articles on Colorado rail transporta-
reference period, the VCO frequency tion for local newspapers. Ken is
has advanced 10.3 cycles; after two also a member of the Loveland
Public Library board of directors.
reference periods 20.6 cycles, and
after three periods 30.9 cycles. At this
point, the phase register contains 0.9.
In the fourth reference period, the VCO
has advanced 41.2 cycles which
causes the phase register to overflow, represents thar proper tune voltage to remove the stairs, invert it and sum it
in turn, triggering the pulse remover. be fed into the VCO. Anything other with the output of the phase detector.
Notethat when the phase registerover- than dc fed to the VCO will prevent This essentially eliminates the saw-
flowed, it contained a count of 1.2. To phase lock and cause frequency mod- tooth portion of the output of the phase
preserve the correct relationship be- ulation. detector and when filtered, produces a
tween pulse removal and the reference clean dc tune voltage for the VCO.
frequency, the next sequence begins Since the phase register is in-
with the excess count of 0.2 loaded in cremented each reference period, its
the phase register (insteadof zero if the contents represent an instantaneous
VCO frequency was 1.01 MHz). fractional sum which grows until one
cycle of phase advancement has oc-
Figure 6 shows the elements needed to curred. The contents of the phase reg-
properly remove a pulse automatically ister as viewed with respect to time, is
whenever the VCO has advanced one shown in Figure 7. The contents of the
full cycle relative to the reference fre- phase register can be representedby a
quency. The next step is to close the staircase ramp resetting each time a
loop. Note that the phase detector's pulse is removed. Note that the phase
output is a sawtooth riding on a dc register, when viewed graphically, has
level. The dc portion is the part which the same characteristics as the saw-
tooth output from the phase detector.
Figure 8. Highly Slmpflfkd Diagram of
afmctlonal N p ..Lock Loop
h.
This ends a simplified explanation of
the concepts involved in a fraction N
Flgure 7. A Pictorial view of the Contents loop. Actual implementation is more
of the Phase Reglater complex. For example, to convert the
staircase output of the phase register
into a smooth sawtooth requires cur-
Figure 8 shows the addition of a rent sources and an integrator. A sam-
digital-to-analog converter (DAC) to ple and hold circuit is used to produce a
Ftgure 6. Frrrctional N Loop showing a the output of the phase register. By transient free dc tune voltage. A tune
Phase Reglater Used to Trigger the converting the contents of the phase current is needed to allow the loop to
RHnovd of om vco cycle (orPulse). register into voltage through the DAC, relock after a large change in VCO fre-
the sawtoothoutput of the phase detec- quency. Despite the additional cir-
tor can be approximated. The next step cuitry, the basic principle given in this
is to smooth the output of the DAC to artiele will still hold true. 0
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COUNTRIES AND
Isreel . . . . . . . . ..22ov
b i y . . . . . . . . . . . 110/22ov
* Rhcdosla . . . . . .22OV
Romania . . . . . . . 110/220V
Rwanda . . . . . . .22OV
FAST L-C 3
J
THEIR VOLTAGES
Ivory Coast . . . .22ov
Jamaica . . . . . . .110/220V Saba . . . . . . . . . . 110/220v MEASUREMENTS!!!
Japan . . . . . . . . .110V St. Barthelamy 220V
Jordan . . . . . . . .22OV St. Euatatlus . . . 110/220V Here is a very old but convenient
Kenya . . . . . . . ..22OV St. KMs . . . . . . ,220V
Kuwait. . . . . . . . .22OV st. Lucla.. . . . . .22ov method for determining the value of
Laos . . . . . . . . . .110/22ov St. Mearten . . . . 110/220V unmarked capacitors or inductors
Lebanon . . . . . . . 110/22OV St. Wnwnt . . . . .22OV
Followingis a complete and up-to-date Lesotho . . . . . . .22ov Saudl Arabla . . . 110/22OV using instruments commonly found in
guide to foreign voltages. In general, all ubolia. . . . . . . . .110/22ov Scotland . . . . . . .22OV
S.negal ....... 11OV
most maintenance shops and labs.
Ubya . . . . . . . . . . 110/22OV
references to 11O apply to the range
V Urchtensteln . .22OV S.ychelles . . . . .22ov The method is based on the axiom that
si.m bone . . .22ov when an impedance is fed from a
100V to 16OV. References to 220V Luxembourg . . . t 10/220V
Macao . . . . . . . . . 110/220V . Singapore . . . . .110/220V
Somalia . . . . . . . 110/22ov constant-current source, the voltage
apply to the range from 200V to 260V. tladelra . . . . . . . .22OV
Majorca . . . . . . . . 11ov South Africa . . .22OV
Where 110/220V is indicated, voltage Malagasy Rep. .22OV South Korea .. .22OV
across the impedance is a function of
varies within country, dependingon lo- Malawl . . . . . . . . ,220V S p l n . . . . . . . . . .110/220V the magnitude of the impedance.
Malaysia . . . . . . .22OV Srl Lank8
cation. Mall . . . . . . . . . . .110/220v (Ceylon) . . . . ,220V See figure below. To calibrate the
Malta . . . . . . . . . .22OV Sudan ........ .22OV
Martlniqw . . . . . 110/220V Surlnam ....... 110/22OV setup connect a known value capacitor
Aden . . . . . . . . . .22OV Curacao . . . . . . . .1lOV Maurbnls . . . . .22OV Swdland . . . . .22OV or inductor to the TEST terminals. Set
Afghanlstan . . . .22OV cub. . . . . . . . . . . 11ov MeXlCO . . . . . . . .110/220v t Sw&n . . . . . . . . 110/22OV
Algerla . . . . . . . .110/220V * Cyprus . . . . . . . .22ov Monaco . . . . . . . . 110/22OV Switzerland . . . .110/220V the oscillator to the desired frequency
Angola . . . . . . . .22OV Cuchoslovakla .110/22OV Montwrrat . . . . .22OV Syria . . . . . . . . . . 110/22OV
Anguilla . . . . . . ,220V Dahomey . . . . . ,220V 'Morocco . . . . . . ,l10/220V Tahltl . . . . . . . . . . 110/22OV with maximum output. For example, I
Antlgua . . . . . . . . 110/220V Denmark, . . . . . .22OV Moumblqua . .22OV Talwan . . . . . . . . 110/220V used 10 Hertz to calibrate on a 1.O mic-
t Argentlna . . . . . .22OV Domlnlca . . . . . .22OV Nepal . . . . . . . . . ,220V Tanzanla . . . . . . .22OV
Aruba . . . . . . . . . 11O V Dominican Rep. 110/220V Netherlands.. . .110/220V Tobago . . . . . . . . 110/220V rofarad capacitor and 100kHz to cali-
t Australla. . . . ..22OV Ecuador . . . . . . .110/220V Neth. Antllles . . 1101220V Togo . . . . . . . . . .110/22OV brate on a 1OOpF capacitor. Set the
Austrla . . . . . . . .22OV Egypt, . . . . . . . . . 110/22OV Nevls . . . . . . . . . .22OV Tonga . . . . . . . . .22OV
Azom . . . . . . . .110/22ov El Salvador . . . . l l O V New CaWonla .22OV Trlnldad . . . . . . . 110/22OV VTVM to the 0.1 volt range and adjust
Bahamas . . . . . .110/22OV Ethlopla . . . . . . . 110/220V New Guinea.. . .220V Tunlala . . . . . . . . 110/220V the 100 ohm resistor for an indication
Bahrain . . . . . . . .22OV Fljl . . . . . . . . . . . .22OV New Hebrides . ,220V Turkey.. . . . . . . .110/220V
Bangladesh . . . .220V flnland . . . . . . . .22OV New Zealand . . .220V Turks I on the VTVM that agrees with the value
Barbados . . . . . . 110/220v France. . . . . . . . . 110/220V Nlcaragua . . . . .110/220V Calcosl . . . . . . l l O V
Belgium . . . . . . .110/22OV F n m h Gulana . llO/ZZOV Niger . . . . . . . . . .22OV Uganda . . . . . . . .22OV of the known capacitor or inductor.
8.rmuda . . . . . . 110/22ov Gabon . . . . . . . . .22OV * Nlgarla . . . . . . . .220V Upper VOlta . . . ,220v
Bhutan . . . . . . . .22OV Gambia . . . . . . . ,220V Northern Inland 22OV Uruguay . . . . . . .22OV Example: 100pf = O.lv, 50pf = 0.5v,
etc., or the reciprocal for a known in-
a
Bolivia . . . . . . . . .110/220v tGermany ..... .110/22OV Norway . . . . . . . .22OV United Arab
Bonaln . . . . . . . .110/220V Ghana . . . . . . . . .22OV Okinawa . . . . . . . 110V Emlmtes . . . . .22OV
Botswana.. . . . ,220V Glbraitar ...... .22OV Oman . . . . . . . . .22OV USA . . . . . . . . . . . 110V ductor. Connect the unknown to TEST
* Gnat Brltsln . ..220V USSR. . . . . . . . . . 110/22OV
t Brezll . . . . . . . . . . 110/220V
Brit. Honduras .110/220V t0 - . . . . . . . . 110/22OV
Pakistan . . . . . . .22OV
Panama . . . . . . . 11O V U.S. Vlrgln I. . . . 110V
and read directly capacitance or recip-
Brit. Vlrgln 1. . . . 110/22OV Gnenland . . . . ,220V t Paraguay . . . . . . 2 m v Venezuela . . . . .110/220V rocal of inductance. NOTE: For best
Bulgarfa . . . . ..110/220v Grenada . . . . . . .220V Peru . . . . . . . . . . .22ov Vlstnam . . . . . . . 110/22OV
Burma . . . . . . . ..22OV Gnnadlnes . . . .22OV Phlllpplnea . . . . 110/220V Wales . . . . . . . . ,220V
accuracy the ratio of X to R should be
Burundl . . . . . . .22OV Gusd.loupe.. . . 110/220V Poland . . . . . . . . 1101220v Yemen.. . . . . . . .22OV 1OO:l or greater.
Cambodla . . . . . 11w220v Guatemala . . . . . 110/220V Portugal . . . . . . . 110/220v Yugoslavls.. .. .22ov
Cameroon . . . . .110/22OV Gulnea . . . . . . . .220V P~orioRIco . . . l l O V
. Zaln . . . . . . . . .,220v EOUIVALENT C K T X R
Canada . . . . . . ..110/220V Guyana . . . . . . . .110/22OV Oatar . . . . . . . . .22OV Zambia . . . . . . . ,220V
AUDIO
Canal Zone . . . .110/220V HaHl . . . . . . . . . ..110/220V OSCILLATOR ACVTVM
Canary I . . . . . . . . 1101220V Honduras . . . . . .110/220V
Cayman 1. . . . . . .110V * Hong Kong . . . .22OV
Can. Amcan Hungary . . . . . . .22OV
Rep. . . . . . . . . .22OV Iceland . . . . . . .22ov
Chad . . . . . . . . ..220V tlndla . . . . . . . . ..22OV
"Denotescountries in which plugs with 3 square
Channel 1. (Brit) 22OV Indonesla . . . . . . 110/22OV pins are used (in whole or part).
t Chile . . . . . . . . .22OV Iran . . . . . . . . . ..22OV tCountries using DC in certain areas.
Chlna.. . . . . . . . ,220V Iraq . . . . . . . . . .22ov
Colombia . . . . . . l l O V Inland.. . . . . . ..220v
Costa RIw . . . . . 110/220V isle of Man. . . . .22OV
Digital Troubleshooting 0 Most recent logic symbology (ANSI There is ample use of reinforcement in
Y32.14AEEE 91-1973 the presentationand in the self-scoring
quizzes at the end of most of the mod-
AN HP VIDEOTAPE 0 Useful troubleshooting tips ules.
TRAINING Individual tapes are:
PROGRAM 1. Introduction To Digital
This course was designed, developed, Electronics (12 minutes)
"Digital Troubleshooting" . . . a vid- and made for technicians. It is an ap- 2. Binary Nature of Digital Circuits
eotape series developed to train HP's propriate bridgefrom transistors to digi- (18 minutes)
own technicians is a course especially tal electronics, or a digital refresher 3. Basics of Transistors and IC`s
useful in showing how to approach real course. Equivalent in coverage to a col- (18 minutes)
problems in real equipment. These lege term of 13 weeks, Digital Troub- 4. Logic Gates and Symbols
videocassettes provide: leshooting is presented in color on 14 (25 minutes)
videocassettes having a total running 5. Introduction To Digital IC
time of 5 hours and 31 minutes. The lab Families (29 minutes)
0 Practical demonstrations 6. Modern Digital IC Families
demonstrations shown in video are
0 Flexibility of use for classroom or in- from the workbook included with the (27 minutes)
dividual study series. Also included is a 180 page text 7. Simple Troubleshooting
0 Latest in digital troubleshootingtools and a study guide. Techniques (18 minutes)
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--r
SAFETY SERVICE NOTES
8. Troubleshooting Digital IC's 14. Memories 1700 Series OscilloscoDes
(27 minutes) (25 minutes)
9. Flip-Flops FOCUS PROBLEMS?
(31 minutes) The videocassettes are available in the
10. Counters and Shift Registers NTSC Standard only. Formats other Are you having problems adjusting the
(30 minutes) than 3/4' videocassette can be quoted beam focus on your HP1700 series os-
11. Combinational Logic Circuits on request. The part number is 90420D cilloscope? Service Notes for the
(30 minutes) which includes all tapes and study ma- 17108, 1712A, 1720A, and 1722A
12. Display Technologies terial, plus midterm and final exams, scopes describe the problem as being
(30minutes) exam solutions and certificates of related to resistors AlR11 (6.5
13. IC Manufacturing completion. See your local HP field en- megohms) and A15R13 (13 megohms)
(11 minutes) gineer for details. changing value. Improved resistors
from a different vendor are now being
used (same part number). The new re-
sistors have a reddish-brown body.
The original resistors had reddish-
brown, gray, or white bodies and are
substantially smaller than the current
resistors in use. The Figure below
shows the relative size of the new and
I old resistors.
A 1 5 R l l . 6.5mn--------HP P a r t NuRlber 0698-6441
A15R13, 13Ml---------HP P a r t Number 0690-6442
Figure 1. N w Focus Rdetor
.
SAFETY-RELATED 745AC CALIBRATOR OUTPUT terminals. Use the following
i procedure to test your instrument for
SERVICE NOTES this condition.
Service Notes from HP relating to
personal safety and possible equip- 1. Turn the power switch off, discon-
ment damage are of vital importance. nect all power cords and signal
I
To make you more aware of these cables. Disconnect the ground
important notes, HP has recently strap between LO OUTPUT and
modified the Safety Service Note chassis ground.
format. The note is now printed on 2. Set an ohmmeter to the 1 kilohm
paper with a red border, and a "-S" range and connect one end to the
suffix has been added to the note's LO SENSE terminal.
number. In order to make you
immediately aware of any potential
I I 1 . 1 I
4 3. Connect the other ohmmeter lead
to the outer shell of the COUNTER
safety problems, we are highlighting OUTPUT jack on the rear panel.
safety-related Service Notes here with HP 745A Calibrators with serials
a brief description of each problem. 00741-00101 thru 1319A01250 have 4. The ohmmeter should indicate
Also, in order to draw your attention to the COUNTER OUTPUT BNC connec- infinity. If not, order the following
safety-related Service Notes on the tor shell connected to the output LO parts and Service Note to modify
Service Note order form at the rear of SENSE terminal. When the 745A the 745A to conform to current
Bench Briefs, each appropriate num- output is floated above ground, this safety standards.
ber is highlighted by being printed in BNC connector will have the same Service Note 745A-12A-S
color. potential as the LO SENSE and LO Modification Kit 00745-89503
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