Processing system having distributed radiated emissions

A processing system (10) includes a backplane bus (13) that defines a plurality of locations (20-31) implemented as backplane slots each for connecting any option module (41) thereto. Each slot includes contact pins (201a-d) that carry binary logic levels forming a different digital number at each slot. Option modules include their own source of timing signals (220) comprising an oscillator (221) and an identical thermally-sensitive crystal (222) for driving the oscillator. Each option module includes receptacles (207a-d) for the contact pins. The receptacles are connected to the digital input port of a D/A converter (225) whose output port is connected to a heater (224) mounted in physical proximity to the crystal in the crystal's case (223). Depending on which slot an option module is mounted in, the D/A converter receives a different digital input and hence generates a different level of output. The heater generates heat in proportion to the converter's output and hence raises the temperature of the crystal to a different level at each slot, causing the crystal to generate somewhat different fundamental frequency at each slot. Hence option modules connected to different slots generate different fundamental frequencies and therefore emit different harmonic frequencies. Hence the amplitudes of the harmonics emitted by different option modules are non-additive, and radiated emission levels of the processing system equipped with all option modules are no higher than those of the system equipped with only one option module.

TECHNICAL FIELD 
This invention relates to processing systems in general, and in particular 
concerns control of electromagnetic emissions radiated by such systems. 
BACKGROUND OF THE INVENTION 
A processing system generally comprises a basic processor including a 
central processing module and a memory module, and a bus to which may be 
selectively connected one or more other functional modules that serve to 
enhance or expand the processor's capabilities and adapt the processor to 
its particular application. Such functional modules for example may 
include input and output (I/O) modules that interface the processor with 
equipment that allows it to communicate with the outside world. 
To properly time and synchronize operation of its internal circuits, each 
module commonly includes a source of timing, or clock, signals such as a 
crystal-driven oscillator. And to make the modules of a system compatible 
with each other, for example, such as to make them interchangeable, and to 
make the modules capable of efficiently carrying on communications with 
each other across the bus, commonly the oscillators--and hence the driving 
crystals--of all modules of a system are substantially identical and 
generate signals of the same frequency. 
Only the one principal, or fundamental, frequency that is generated by an 
oscillator is required for the operation of a module. However, because of 
inherent imperfections in crystals that are used to generate the 
frequency, and because of nonlinearities of circuit components that are 
driven by the oscillator output, other frequencies that are integral 
multiples, or harmonics, of the fundamental frequency are generated within 
a module as well. The harmonics are undesirable because they are emitted, 
radiated, by the equipment that has generated them and thus can interfere 
with operation of other equipment and with communications in the vicinity. 
The greater the strength, or amplitude, of the radiated emissions, the 
greater the possibility of their causing interference, and the greater the 
distance at which they can cause interference. Hence it is desirable to 
keep the level of the radiated emissions to a minimum, and governmental 
agencies such as the Federal Communications Commission set strict limits 
on the levels of emissions that are allowed for various kinds of 
equipment, including processing systems. 
Various schemes exist for limiting the radiated emissions of equipment, 
such as electromagnetic shielding of the emitting equipment. However, no 
effective scheme exists for eliminating the emissions completely. This 
presents a problem especially in systems having a plurality of identical 
frequency sources, such as the above-described processing systems, because 
the amplitudes of the plurality of emissions are additive at any one 
frequency. In spite of attempts to limit emissions by electromagnetic 
shielding, it may be difficult or impossible to meet limits imposed on 
emission levels with equipment having a plurality of like frequency 
sources. Hence the above-described processing systems may either be 
restricted in use to environments to which relatively high emission limits 
are applicable. Or such systems may be restricted in their processing 
power, versatility, and capabilities because they cannot be equipped with 
more than a limited number of functional modules in order to meet 
applicable emission level limits. 
SUMMARY OF THE INVENTION 
It is these and other disadvantages and problems of the prior art that the 
invention is directed to solving. Broadly according to the invention, 
apparatus that has a plurality of connecting arrangements, each for 
connecting to a frequency-generating apparatus, also has an arrangement 
for identifying the connecting arrangements, while the 
frequency-generating apparatus has an arrangement that is responsive to 
the connecting-arrangement-identifying arrangement and generates a 
frequency which is dependent upon the identity of the connecting 
arrangement to which the frequency-generating apparatus is connected. More 
specifically, a processing system that has a communication bus defining 
locations each for connecting to a circuit package, such as a functional 
module, also has an arrangement for identifying each location, while the 
circuit package includes a frequency-generating arrangement, such as a 
crystal-driven oscillator, that is responsive to the location-identifying 
arrangement and generates a frequency that is a function of the identity 
of the location to which the circuit package is connected. 
Advantageously, a functional module of a processing system according to 
this invention generates a somewhat different fundamental frequency at 
each location of the bus, and hence modules connected to different 
locations generate somewhat different fundamental frequencies from each 
other even though all of the connected modules may have identical 
frequency-generating arrangements. Since the fundamental frequencies of 
the connected modules are different, their harmonics are also different, 
and hence the levels of the harmonics are not additive. Therefore, the 
amplitude of radiated emissions of a processing system equipped with a 
single module is no greater than that of the system equipped with a 
plurality of modules. Yet this result is accomplished while the crystals 
of all modules of the processing system are allowed to remain identical, 
as the invention does not rely on differences among the modules and their 
crystals. Indeed, identical modules may be connected to different 
locations on the bus without affecting the level of the system's radiated 
emissions. And in processing systems wherein modules are interchangeable, 
in that any module may be connected to any location on the bus, the 
interchangeability of the modules is preserved, because the invention does 
not depend upon the positioning of modules. These and other advantages and 
features of the present invention will become apparent from the following 
description of an illustrative embodiment of the invention, taken together 
with the drawing.

DETAILED DESCRIPTION 
Shown in FIG. 1 is an example of a processing system 10 that includes an 
illustrative embodiment of the invention. The processing system 10 may be, 
for example, the AT&T 3B2 computer of AT&T Technologies, Inc. The 
processing system 10 includes a basic processor that comprises a central 
processing unit (CPU) 11 and a memory 12 connected to the central 
processing unit 11. Also connected to the CPU 11 is a backplane bus 13. 
The bus 13 defines a plurality--in this case twelve--locations numbered 20 
through 31, at which option modules may be connected to the bus 13. The 
connecting arrangement allows the processing system 10 to be equipped with 
up to twelve option modules. The system 10 may be equipped with fewer than 
twelve option modules, or no option modules. Three option modules 40 
through 42 are illustratively shown connected to the bus 13 at locations 
20, 28, and 30, respectively. 
The bus 13 may define other locations as well. For example, the CPU 11 and 
the memory 12 may occupy a pair of locations on the bus 13. The bus 13 
provides a communication path between the CPU 11 and the various option 
modules that may be connected to the bus 13. 
The option modules are circuit packages that provide various capabilities, 
such as I/O capabilities, to the processing system 10. For purposes of 
lower system cost, versatility, ease of manufacture, etc., the option 
modules are substantially identical, and any option module may be 
connected to the bus 13 at any location 20-31. 
FIG. 2 illustrates the physical structure of the bus 13 and the option 
modules. As its name implies, the backplane bus 13 is implemented in a 
backplane 200. The backplane 200 comprises a circuit board 210 through 
which extend a plurality of half-connectors such as contact pins 201. The 
pins 201 are arranged in a plurality of groups to define a plurality of 
backplane slots. Each slot represents one of the locations 20-31 for 
connecting an option module to the bus 13. The portion of the backplane 
200 shown in FIG. 2 represents locations 28-31. 
The option modules, of which the modules 41 and 42 are shown in FIG. 2, are 
circuit packages formed of printed circuit boards 205 that support various 
circuits, including frequency-generating arrangements, and printed 
conductors that interconnect the circuits. An edge of each circuit board 
205 is adapted to electrically contact and engage the contact pins 201 of 
any location 20-31, thereby to make electrical contact between the 
conductors of the board 205 and the pins 201 of the location and also to 
mount the board 205 on the board 210. Any option module may be connected 
to the bus 13 at any location 20-31. 
The bus 13 is formed by the pins 201 and by conductors 202, such as 
conductors printed on the backplane circuit board 210 or wires extending 
along the board 210, that connect to various of the pins 201. The 
conductors 202 implement various functional lines of the bus 13, such as 
signal and power lines. Included among the functional lines of the bus 13 
are a ground (GND) line and a positive voltage source (V+) line, shown in 
FIG. 3. Signal levels carried by these two lines represent the binary 
logic zero and one signal levels, respectively. The GND and V+ lines are 
connected to certain ones of the pins 201 in a manner that defines a 
unique identification code for each of the locations 20-31. In this 
manner, each of the locations 20-31 is uniquely identified. This 
location-identifying arrangement is illustrated in FIG. 3. 
As FIG. 3 shows, the pins 201 that define the identity of each location 
20-31 include four pins 201a-d to which are connected the GND and V+ 
lines. The arrangement in which the GND and V+ lines and the pins 201a-d 
are connected is different at each location 20-31. For example, at the 
location 20 the GND line is connected to all four pins 201a-d while the V+ 
line is not connected to any of the pins 201a-d. Because the GND line 
represents the logical zero level, the location 20 is marked by pins 
201a-d with the digital identification code "0000". At the location 21 the 
GND line is connected to the pins 201a-c while the V+ line is connected to 
the pin 201d. The location 21 is thus marked with the digital 
identification code "0001". At the location 22 the GND line is connected 
to the pins 201a, b, and d, and the V+ line is connected to the pin 201c, 
thereby identifying the location 22 as "0010". The pins 201a-d of the 
other locations 23-31 are similarly connected to the lines GND and V+ to 
mark the locations with the digital identification codes "0011" through 
"1011", respectively. Thus each location 20-31 is identified by a 
different digital identification code. 
Returning to a consideration of the option modules, while the circuitry of 
different modules may support or implement different functions, the option 
modules have certain features in common. The common features that are 
relevant to an appreciation of this invention are shown and described in 
FIG. 4 which shows the option module 41. The module 41 is representative 
of all option modules. As was mentioned before, any option module may be 
connected to the bus 13 at any location 20-31. For this purpose, the 
circuit board 205 of the option module 41 supports at one edge thereof an 
edge connector 206 which is adapted to engage and make electrical contact 
with the contact pins 201 of any location 20-31. The edge connector 206 
comprises a plurality of half-connectors, which in this example are 
contact receptacles 207, each of which is adapted to mate with a 
corresponding pin 201 of any location 20-31. The connector 206 includes 
receptacles 207a-d which are configured to mate with the pins 201a-d. 
Like all option modules, the module 41 includes a frequency-generating 
circuit 220, for generating timing signals for controlling the operation 
of the various other circuits of the option module 41. The 
frequency-generating circuit 220 includes a conventional oscillator 221 
which is driven in a conventional manner by a crystal 222. All option 
modules are designed to operate with timing signals having the same 
nominal frequency and to operate in a range of frequencies around the 
selected nominal frequency. Hence all option modules have substantially 
identical crystals 222 that output the same nominal frequency. For 
purposes of this illustrative example the nominal frequency output of the 
crystal 222 is about 8 MHz, but the various circuits of the option modules 
are designed to operate in a range of frequencies ranging from about 7.5 
MHz to about 10.5 MHz. 
The frequency-generating circuit 220 includes an arrangement for varying 
the frequency of oscillation of the oscillator 221 by varying the 
frequency generated by the crystal 222. The crystal 222 is housed in a 
protective case 223. Included in the case 223 in physical proximity to the 
crystal 222 is a heater 224, such as a diode. The physical proximity of 
the crystal 222 and the heater 224 and their encapsulation in the common 
case 223 ensure that the crystal 222 is thermally coupled with the heater 
224. The heater 224 is connected to the output port of a conventional 
digital-to-analog (D/A) converter 225. The digital input port of the D/A 
converter 225 is connected to the receptacles 207a-d. The output of the 
D/A converter 225 powers the heater 224. The output of the D/A converter 
225 is a function of the binary signal values that it receives at its 
digital input port from the pins 201a-d via the receptacles 207a-d. The 
D/A converter 225 converts the digital signal input into a proportional 
current level output. 
FIG. 5 illustrates in tabular form the operating characteristic of the D/A 
converter 225. FIG. 5 shows that the D/A converter generates a different 
level of output for every different digital input. For example, when the 
option module 41 is connected to the location 20, the D/A converter 225 
receives at its digital inputs the digital value "0000". The D/A converter 
255 responds to that input by generating no output. When the option module 
41 is connected to the location 21, the D/A converter 225 receives the 
digital value "0001" and responds thereto by generating a current output 
level having a value of X. At location 22 the D/A converter 225 receives 
the value "0010" and in response generates an output level of 2X. At 
location 23 the D/A converter 225 generates an output level of 3X, and so 
on, until at location 31 the D/A converter 225 generates an output level 
of 11X. Since the digital input of the D/A converter 225 is the digital 
identification code of the location 20-31 to which the module 41 is 
connected, the output of the D/A converter is a function of the identity 
of the connected location 20-31. 
The output of the D/A converter 225 provides energy for powering the heater 
224. The more current the converter 225 provides to the heater 224, the 
more heat the heater 224 generates and hence the more it raises the 
temperature of the crystal 222. Therefore, because the digital input to 
the D/A converter 225 is different at each location 20-31, the temperature 
of the crystal 222 is different at each location 20-31 and is dependent 
upon the location 20-31 to which the option module 41 is connected. The 
D/A converter 225 is selected such that the difference X in its output 
level per unit change in its digital input causes the heater 224 to 
produce a temperature change .DELTA.t in the crystal 222. Hence the 
temperature of the crystal 222 is a function of the identity of the 
connected location 20-31. 
Turning to FIG. 6, there is shown in graph form the thermal characteristic 
of the crystal 222. The crystal 222 has a high thermal coefficient. The 
crystal 222 is therefore thermally responsive, in that its frequency 
output f changes significantly for a given change .DELTA.t in temperature. 
The thermal characteristic of the crystal 222 is rather linear, in that 
equal-size changes .DELTA.t in temperature of the crystal 222 produce 
approximately equal-size changes .DELTA.f in the frequency output of the 
crystal 222. The crystal 222 is selected to produce a nominal frequency 
f.sub.1 at ambient operating temperature T.sub.1. The ambient operating 
temperature T.sub.1 is the temperature in the case 223 with the heater 224 
not powered. The nominal frequency f.sub.1 in this illustrative example is 
about 8 MHz. This is the fundamental frequency generated by the crystal 
222 when the option module 41 is connected to the location 20. The 
harmonics generated by the option module 41 in the location 20 are 
therefore integral multiples of f.sub.1. At location 21, the D/A converter 
225 generates an output level of X, causing the heater 224 to raise the 
temperature of the crystal 222 by .DELTA.t over the ambient temperature 
T.sub.1. The crystal 222 responds to the temperature increase of .DELTA.t 
by increasing the fundamental frequency that it generates by .DELTA.f over 
the nominal frequency of f.sub.1. In location 21 the module 41 therefore 
generates harmonics that are integral multiples of (f.sub.1 +.DELTA.f). In 
this illustrative example, .DELTA.f may be, for example, 200 KHz. 
At location 22, the temperature of the crystal 222 is (T.sub.1 +2.DELTA.t), 
the fundamental frequency generated by the crystal 222 is (f.sub.1 
+2.DELTA.f), and hence the harmonics generated by the option module 41 are 
integral multiples of (f.sub.1 +2.DELTA.f). The temperature and 
frequencies increase correspondingly at the other locations 23-31 of the 
bus 13. Hence the fundamental and harmonic frequencies generated by the 
crystal 222 are a function of the identity of the connected location 
20-31. 
Ambient temperature changes will also cause shifts in the frequency output 
of the crystals. However, changes in the ambient temperature will 
generally be common to all modules and hence will affect the crystals of 
all boards substantially equally. Hence the frequency outputs of all 
modules' crystals will tend to drift in unison and maintain their relative 
frequency differences. 
FIG. 7 shows the net result of this invention as applied in the 
above-described illustrative embodiment of the processing system 10. As a 
result of each option module with which the processing system 10 is 
equipped operating at a slightly different fundamental frequency, the 
harmonics emitted by these modules likewise fall at slightly different 
frequencies. And because harmonics of different modules are not of the 
same frequencies, the amplitudes of the harmonics are not additive. 
Therefore the harmonics of different modules do not reinforce each other, 
and hence the peak harmonic signals produced by the processing system 10 
equipped with all option modules are substantially no higher than those 
produced by the processing system 10 equipped with only one option module. 
Yet the various option modules still retain their common characteristics, 
such as being driven by substantially identical crystals, and the system 
still retains its versatility of accepting connection of any option module 
at any of the locations 20-31. 
Of course, various changes and modifications to the illustrative embodiment 
described above will be apparent to those skilled in the art. For example, 
each option module may have an assigned location on the backplane bus, 
depending upon the modules' function. The system may be capable of 
accepting fewer than or more than twelve option modules. The modules may 
operate at a different nominal frequency or with different frequency 
changes among the bus locations than the ones disclosed. The disclosed 
linear D/A converter and diode may be replaced by a D/A converter that 
generates a current level output proportional to the square root of the 
digital signal input and by a resistor, respectively. Or the D/A converter 
and heater may be replaced by a resistive network that may be powered 
directly from the V+ signal line. Such changes and modifications can be 
made without departing from the spirit and the scope of the invention and 
without diminishing its attendant advantages. It is therefore intended 
that such changes and modifications be covered by the following claims.