Omni VI+ TCXO Information

From the Ten-Tec Reflector January 19, 2005

I suppose i could ask Ten Tec this but does anyone know when or if they put the TCXO in the OMNI VI+ (564)?

John N1JM

January 20, 2005

Approximately the last year of production of the Omni-VI Plus. There is no advantage over it vs. the master heater oven previously used - we made the change because the parts for the oven were no longer available to us.

Scott Robbins

January 20, 2005

Additionally, maybe the beginning serial number? I bought my VI Plus/564 from a ham in GA; not sure when he originally bought it.


73 de Jim - AD6CW

January 21, 2005

Ah - specific production information is often something that I don't have, particularly for a piece of equipment that has been discontinued. Our service department may be aware of the answer to that question.

If the radio had an oven that needed replacing, it's possible that it had a retrofitted TCXO installed. They were only used the last year or so of Omni-VI Plus production, but we've been installing them as repair parts as needed since the same time - 2001.

Scott Robbins

January 20, 2005

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:
    1. Accuracy/Year:
      • TCXO - 2 x 10exp-6
      • OCXO - 1 x 10exp-8
    2. Aging/Year:
      • TCXO - 5 x 10exp-7
      • OCXO - 5 x 10exp-9
    3. Stability:
      • TCXO - 1 x exp10-9
      • OCXO - 1 x exp10-12
    4. 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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