[TenTec] TCXO vs. OCXO

["Dave Bowker" <dbowker@mail.sjv.net>]

The recent discussions on the reflector regarding Temperature Compensated Crystal Oscillator (TCXO) 
versus Oven Controlled Crystal Oscillator (OCXO) in the OMNI-VI (retrofitted) and VI+ (retrofitted 
or production) seem to be void of an understanding of the merits of each.  First is a general 
comparison of TCXO and OCXO oscillators based upon frequency stability versus temperature:

1.  A TCXO contains circuitry in which the output from an ambient temperature sensor is used to 
develop a correction voltage which is applied to a variable reactance in the oscillator circuit to 
compensate for the crystal's frequency variation due to temperature.  The temperature compensating 
components are generally exposed to the local ambient and are chosen to maintain a relatively stable 
frequency versus temperature curve relative to ambient temperature changes.  Typically, analog 
TCXO's can provide 20-30 times better frequency stability versus temperature than that of a basic 
crystal oscillator (XO).  A TCXO will also exhibit a temporary frequency drift if the temperature 
sensing element (usually a thermistor) does not have the same thermal time constant as the crystal 
and a TCXO's phase noise [can] be inferior to standard XO and OCXO circuit designs.  TCXO's 
generally are used in low power applications, when fast warm-up is desirable, and when costs are a 
significant factor.

2.  An OCXO, on the other hand, includes all of the temperature sensitive components (including the 
crystal) within a temperature-stable oven whose temperature is set to maintain a frequency versus 
temperature curve slope of zero, or nearly so, for the particular crystal cut (SC, AT, etc.) used in 
the oscillator.  The oven temperature is maintained nearly constant at the required crystal design 
temperature, regardless of changes in the ambient temperature.  Typically OCXO's exhibit 1000 or 
more times better frequency stability than the general XO and 50 or more times better frequency 
stability compared to the TCXO.  Even a low cost OCXO provides better temperature stability than 
TCXO's, but at the expense of much greater power consumption, typically several watts.  The lower 
cost OCXO's typically use AT-cut crystals which can exhibit a significant thermal transient effect 
and require considerable time to stabilize at their design operating temperature (sometimes ranging 
into the hours) while an SC-cut crystal stabilizes as soon as it reaches (or nearly so) it's design 
operating temperature.  Cost is a significant factor!

3.  The following is a direct comparison of some of the more important parameters of TCXO's and 
OCXO's:

a.  Accuracy/Year:
    TCXO - 2 x 10exp-6
    OCXO - 1 x 10exp-8

b.  Aging/Year:
    TCXO - 5 x 10exp-7
    OCXO - 5 x 10exp-9

c.  Stability:
    TCXO - 1 x exp10-9
    OCXO - 1 x exp10-12

d.  Warm-up time in minutes:
    TCXO - typically < 0.05 minutes
    OCXO - typically < 4 minutes

In summary, the OCXO is preferred over the TCXO when long term accuracy, aging, and stability are 
concerned.  If rapid warm-up time is the primary consideration, the TCXO is preferable.

Cost is probably the major factor to be considered in [manufacturing] when the other factors are 
relatively insignificant;  there is a substantial difference between the cost of TCXO's and OCXO's. 
Typically a TCXO design implementation can range in price from < $10 to around $100 while an OCXO 
design can start at well over $100 and range up into the thousands of $$!

73, Dave, K1FK 


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