Service Manuals, User Guides, Schematic Diagrams or docs for : HP Publikacje HP-Bench-Briefs-1973-07-08
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VOLUME 13 NUMBER 3 JULY-AUGUST 1973
QUARTZ CRYSTAL at a constant temperature usually
around 65OC regardless of the am-
OSCILLATORS bient temperature outside the oven.
Ovens are of two general types,
on-off and proportional.
by Marv Willrodt
The on-off oven uses a snap action
thermostat as a temperature sens-
The accuracy of most Hewlett- ing element. The cost is low but
performance is only moderate
Packard counters and many other
since the thermostat by nature has
instruments is determined to a
a temperature differential between
great extent by the accuracy of
the "on" condition and the "off'
the mechanical vibrating fre- condition, thus the crystal temper-
quency of a thin crystalline quartz ature is constantly being cycled
plate used in the time base refer- by some small amount which
ence oscillator. To make a quality quartz oscillator, a shows up as output frequency
Since the quartz plate is vibrating source of pure quartz must be located. cycling. Wear and sticking of the
mechanically, its frequency will be with ambient temperature change thermostat points can cause a
influenced among other things by is much greater than for good oven gradual shift of oven temperature
physical size, which can change as crystals. or even erratic operation. Also
the quartz plate expands or con- arcing of the points can introduce
tracts with temperature changes. Temperature effect can be reduced electrical noise into the oscillator
by combining temperature com- output. All of these things are
Several things are done to reduce pensating elements--temperature
the magnitude of this frequency undesirable since they cause fre-
sensitive capacitors for instance-- quency error.
change on high quality quartz with the crystal. Such units are
oscillators: known as temperature compen- Proportional ovens are throttling
1. Controlling carefully the angle sated crystal oscillators or TCXO's devices in that the rate of heat
at which the crystal is cut from for short. A good TCXO can be input is constantly adjusted to
the raw quartz. 10 or 100 times better than a room match the rate of heat loss to the
2. Adding temperature compen- tern peratu re crystal ; however, the oven surroundings. A thermistor
sating elements in the crystal nature of temperature-frequency in a bridge circuit senses tempera-
circuit. characteristics of crystals varies ture and controls an amplifier
3. Mounting the crystal assembly enough so each one must be care-
in an oven to maintain con- fully measured and then individ-
stant temperature. ually compensated with its own
Other factors such as poor mount- specific network. This process is
ing, contamination, etc., also de- time consuming and consequently
grade performance of poorly made expensive. Bear in mind that im-
crystals. proved output stability is achieved
Non-oven crystals have the advan- by keeping the oscillator output
tage over oven crystals in that they frequency within predetermined
do not require warm-up after acceptable Iimits by cornpensating
power has been removed from the the circuit, not by changing the
counter for some time and are thus quartz plate itself.
desirable for many of the portable
applications for which counters are TYPES OF OVENS
used. They have the disadvantage, A crystal oven, as the name implies,
of course, that frequency change heats the crystal and maintains it
Www. H PARCHIVE.COM
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QUARTZ OSCILLATORS
I
which supplies heat at the required removed, overnight--or even for a The three most important specifi-
rate--the farther the oven is off short time, the crystal goes through cations define oscillator perform-
temperature, the faster heat is a warm-up cycle before it again ance with respect to: time, am-
supplied. stabilizes. This can take from a bient temperature, and line voltage
Operation might be compared to fraction of an hour to several days variations.
keeping the speed of an automobile depending on the stability specifi-
constant by changing the throttle cations and can be a disadvan- AGING RATE:
setting to provide continuous, step- tage, particularly for a portable Aging rate specifies maximum fre-
less adjustment. By contrast a instrument. quency change with time. Any
thermostatically controlled oven oscillator can be much better than
might be compared to trying to specified but will never be worse
maintain a constant speed by turn- than the indicated rate unless it is
ing the ignition switch of an auto- malfunctioning. You may have
mobile on and off. This would noticed that HP has two kinds of
supply energy at a maximum rate specifications--some oscillators are
or not at all. specified as having a daily aging
The most stable quartz oscillators rate, such as <3 x lO-Vday, while
use a double oven--that is, one pro- others are specified as having a
portional oven mounted within a monthly aging rate, such as < 3 x
10-7/month. (3 x can also be
second proportional oven. Since
the inner oven now never sees a written as
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