Shrinking Oscillators

Historically, the highest performance oscillators were packaged in
cold-rolled steel cans approaching the size of a lunch box. Many of these
designs were mechanical marvels filled with custom-made hardware surrounded by a
large silvered Dewar flasks for insulation. Modern-day versions of these
oscillators measure about 3 x 3 x 5" and offer excellent long-term, short-term,
and temperature stability. Insulation is often provided by a glass Dewar flask
and some designs are fragile. A typical stability specification is 0.1 ppb for
24 hours for all causes, including temperature variations and aging.

Smaller, higher volume oven oscillators insulate the oven components with a
foam clam-shell or hollowed-out cavity. The larger packages, typically 2 x 2 x
4" or 2 x 2 x 3", have sufficient room for thick foam walls and large oven
structures. These ovens can easily accommodate the larger precision crystals,
high quality mechanical tuning components, and older leaded components. The long
can also leaves room for custom circuitry for special outputs, frequency
multipliers, dividers, etc. Reduced-size versions of these oscillators have been
available for several years but the power consumption is higher, perhaps
requiring several watts at room temperature, and the short-term and temperature
stability is degraded.

Wenzel's "Small Fry" oscillator introduced in the early 1980s is essentially
a miniaturized version of the high performance Dewar oscillators. The Small Fry
measures 1.5 x 1.5 x 3" and contains a tiny vacuum flask insulator. Power
consumption is about one watt at room temperature despite the large oven
structure, 20-turn frequency adjustor and large third-overtone SC-cut crystal.
An even smaller Dewar insulated oscillator dubbed the "WAMO" for "Wenzel
Associates Miniature Oscillator" measures only 1.3" on all three sides.
Surface-mount circuitry allows for such miniaturization with ordinary
manufacturing techniques.

Surface-mount components have also allowed for the miniaturization of
foam-insulated oven oscillators. An example is the new L.O. series from Wenzel
Associates. A typical 100 MHz unit measures only 1.5 x 1.5 x 0.5" and consumes
onder 2 watts at room temperature. Surface-mount miniaturization allows this
unit to have a high performance voltage regulator and oven controller, 20-turn
tuning control, over 13 dBm output, and "ultra-low" phase noise.

As the oscillator packages shrink the materials used are changing. In the
past the typical oscillator package was a deep-drawn cold-rolled steel can with
nickel plating or paint. Newer oscillators are packaged in corrosion-resistant
alloys to avoid the costly plating or painting steps. Oven oscillator
performance is in much higher demand than in years past and the packaging is
reflecting the need for higher volume production at lower cost.

Connectors Virtually every style of RF connector that is manufactured has
found its way onto crystal oscillators. The same style of connector is often
used throughout a system and, since many are microwave systems, it is common to
encounter SMA, SMB, SMC, MCX and various miniature proprietary connectors. BNC
and TNC connectors are also used on the larger oscillators. Many oscillators use
ordinary pins for the RF output but harmonics might cause problems inside the
system or cause problems meeting emissions testing standards. Power and other
control or signal pins may be individual feed-through filters or arranged
together on a filtered header or connector. Many low-cost oven oscillators are
PCB mounted along with other RF or high-speed circuitry in a shielded
compartment. In these applications the RF signal is simply brought out of the
oscillator on another pin.

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