A wide range of temperature stabilities options are available in TCXOs, from 5ppm to 50ppb. TCXO oscillators vary in size from the smallest 2.0 x 1.6mm to larger sizes and even TH devices used in military applications.
The key factor to consider when selecting a TCXO is the temperature range over which it operates. -20°C to 70°C is common with -40°C to 85°C a premium especially at tight tolerances.
TCXO Oscillators use a compensation network that senses the change in frequency over temperature and creates a near inverse curve. This is used to pull the TCXO back to the nominal frequency. The remaining error we see is the stated stability, e.g. 1ppm over -20°C to 70°C meaning the error from nominal should be no larger over that temperature range.
One of the first types of TCXO compensation was &147direct compensation&148; these devices used a network of components to alter the capacitive load on the crystal to shift the frequency as it naturally changed with temperature. The other type is called the "indirect method", in early models this saw the use of thermistors again that created a temperature dependent voltage. This was used to drive a varactor whos changing capacitance changed the crystal frequency
Today we see the use of an IC to change the voltage to the varactor, and testing in small temperature steps to confirm the correction (due to differences in the crystal curve). These types are referred to as analog TCXOs.which are recommended for GPS/GNSS clocking applications; due to small shifts of frequency per °C change they help keep the GPS loop locked as temperature changes.
Digitally controlled TCXOs are also available and these tend to fall into the smaller packages and more cost-effective solutions. These oscillators use internal ADCs (analog to digital converters) monitoring the temperature but follow the temperature change in digital steps related to the ADC value.
These types are unsuitable for GNSS solutions due to the frequency jumps over temperature.
VCTCXO - Voltage Controlled Temperature Compensated Oscillator
VCTCXO Oscillators have the same features as the TCXO with the added functionality of a voltage controlled function to allow precision tuning of the output frequency. The VCTCXO differentiate themselves by having a VC pad (voltage control) in the oscillator. This input requires an analog voltage that works to control the varactor inside and is normally part of an external PLL that controls the VCTCXO frequency.