An Ultra-low Noise VHF OCXO
In order to better understand the OCXO’s development, consider the factors affecting phase noise, starting with the fact that if an oscillator is considered as a feedback amplifier combined with a resonator, Leeson’s model predicts that the close-to-carrier noise is determined by the signal level, the amplifier’s noise figure and the Q of the resonator. Assuming the signal is taken from the amplifier output, the single-sideband (SSB) noise density at a given offset is the amplifier’s noise floor plus the noise floor multiplied by a –20 dB/decade slope, with the two curves intersecting at the resonator’s half bandwidth. (At offsets lower than the amplifier’s flicker corner frequency, the slope is –30 dB/decade.)
In real oscillators, the steady-state loop gain is maintained at unity by either limiting or automatic gain control (AGC) action. This has the effect of suppressing AM noise, which typically shows a –10 dB/decade slope below the amplifier’s flicker corner frequency.
Crystal oscillators generally have much worse phase noise than the Leeson model predicts. This is because the resonators themselves exhibit 1/f FM noise, which translates to 1/f3 phase noise. In low noise oscillator designs, the close-to-carrier noise is generally dominated by the crystal’s noise rather than its loaded Q or the circuit noise. Crystal selection is essential if low phase noise is important, as there can be more than 20 dB variation even within a single batch of crystals.
High Q is still important, as it reduces the oscillator circuit’s contribution to close-in noise, and minimizes supply pushing and load pulling. At frequencies around 100 MHz, fifth overtone SC-cut crystals offer the best combination of low noise and high Q, together with a flat frequency/temperature characteristic at their turnover point, typically about 80°C.
At higher offsets, the noise is determined by the drive level and the circuit’s noise floor. SC-cut crystals generally allow higher drive levels to be used than AT-cut. Taking the oscillator output via the resonator utilizes the filtering action of the crystal, so that only the output buffer contributes to the noise floor.
It is important to operate the active devices linearly if the full potential of the lowest noise crystals is to be realized. Nonlinear operation generally increases the transistors’ flicker noise and also causes modulation of the signal by power supply noise and ripple. The degree of degradation tends to vary with factors such as crystal motional resistance and temperature.
Linear operation also helps when tuning the oscillator, as SC-cut crystals in particular need large series reactance to tune them away from series resonance. If the RF operating conditions are not well defined, the tuning range may be limited by wide variation in output power and/or unwanted oscillation modes, particularly with the drive levels needed for low noise floor.
Practical oscillator design almost always involves compromises and tradeoffs. The Pascall OCXO aims to provide the lowest possible phase noise within a relatively compact 2" x 2" x 0.75" package.
A low noise regulator is followed by further filtering and careful attention is paid to ground paths, in order to minimize the effect of supply noise and prevent oven current noise affecting the oscillator’s performance. Fairly high signal levels must be used in order to achieve a very low noise floor. In combination with the linear operation required for lowest possible close-in noise, this inevitably results in higher dissipation than in oscillators with lower performance.
To ensure good temperature control up to 70°C, the oven needs to be maintained at ~80°C. In the great majority of applications, the oscillator will not need to be operated continuously at its maximum temperature. In order to maximize reliability and reduce oven power requirements, only the crystal and certain critical components are held at 80°C. A temperature compensation circuit minimizes drift due to the oscillator circuit.
The Pascall standard part is designed to operate with base plate temperatures of –30° to +70°C. Alternative temperature ranges are available, down to –40°C and up to 85°C. The supply voltage is +12 V, while +15 V is available as an option.
The new OCXO has a relatively wide tuning range, to give ample allowance for aging or locking to external references. At 100 MHz the range is typically ±10 ppm for 0 to 10 V input, with virtually no change in phase noise across the range. The standard oscillator has electrical tuning. It is also available with internal mechanical tuning, or a reference voltage output can be provided to enable the oscillator to be tuned by an external potentiometer.
Figure 1 shows a phase noise plot of a pair of OCXOs at 100 MHz. This is an uncorrected plot, and therefore shows the combined noise of two oscillators. The noise floor measurement is limited by the test system noise, which is actually higher than the oscillator’s noise floor. (Accurate measurement of very low noise is always a difficult exercise.) Allowing for the test system noise and subtracting 3 dB for the two-oscillator measurement shows the OCXO’s phase noise floor to be ~–180 dBc/Hz per oscillator.
The best-selected crystals have been found to yield well under –140 dBc/Hz at 100 Hz offset from 100 MHz. However, it is important not to over specify when designing an OCXO into a system, as this will have a major impact on cost. As previously stated, a crystals’ phase noise varies widely even within a single batch, so careful screening is necessary to meet the most stringent specifications. Also, reliable measurement of low phase noise is very time consuming. The new unit’s output power is 13 dBm ±2 dB into 50 Ω and harmonics are ≤ –20 dBc.
The total frequency drift from –30° to +70°C is typically ~1 x 10–7 (see Figure 2). Warm-up power is typically 5 W and the steady-state power at 25°C is ~2.5 W. There is always a tradeoff between operating temperature range and power consumption. Specifying a higher maximum temperature will increase the consumption at all temperatures, as the oven must be held a few degrees above the highest working ambient temperature. Reducing the minimum operating temperature will increase the warm-up current, as the heater must be able to maintain a larger differential between oven and ambient temperature.
The new OCXO addresses the most critical applications where ultra-low noise is required and aims to provide the lowest possible phase noise within a compact package. It features a relatively wide tuning range and a temperature compensation circuit.
As for the future, Pascall is now well into development of a vibration-isolated variant. This unit utilizes low-g-sensitivity crystals and elastomer AV mounts to reduce the influence of shock and vibration on phase noise. This new unit will be suited to fast jet and helicopter vibration and environmental requirements.
Pascall Electronics Ltd.,
Ryde, Isle of Wight, UK
+44 (0) 1983 817412,
E-mail: email@example.com, www.pascall.co.uk.
RS No. 300