VHF OCXO with Extra-low Noise Floor

Over the past 50 years crystal oscillators have become ubiquitous wherever a stable reference is needed in RF systems. Oven-controlled Crystal Oscillators (OCXO) in the 5 to 10 MHz range are typically used to provide very stable frequency references, or where ultra-low phase noise is required very close to carrier, at offsets up to about 10 Hz. Also, the high Q of the crystal (typically >10⁶ for 3rd-overtone SC-cut) exerts extremely tight control over the oscillator circuit, and crystals with very low phase noise are available.

Applications such as phase noise measurement systems, high performance radars and frequency synthesizers generally need low phase noise floors at offsets in the tens to hundreds of kHz region. It is not possible to achieve the required performance by direct multiplication or PLL synthesis from 10 MHz OCXOs because of the 20log(N) relationship which applies when a frequency is multiplied by N. For example, deriving a 1.6 GHz signal from 10 MHz will increase the phase noise by 44 dB. Even if the 10 MHz oscillator has a very low phase noise floor of -175 dBc/Hz, for example, the lowest possible floor at 1.6 GHz is -131 dBc/Hz, even before the noise added by the multiplier or PLL is taken into account.

VHF OCXOs can provide low close-to-carrier phase noise at the same time as offering an improvement of 20 dB or more in noise floor when used as a reference for multipliers or synthesizers. The standard Pascall OCXO offers best-in-class close-in phase noise combined with a very low noise floor. Figure 1 shows a phase noise plot of a 100 MHz level E OCXO, which has a guaranteed specification of -137 dBc/Hz at 100 Hz offset.

Figure 1 Standard Pascall 100 MHz OCXO phase noise.

However, enquiries from customers, together with a desire to enhance the performance of Pascall’s synthesizer products, suggested that there would be useful benefits for the most demanding applications if the OCXO’s noise floor could be reduced. This prompted the development of the new F-series OCXO, which has a lower noise floor while preserving the good close-to-carrier phase noise of the original oscillator.

Development Considerations

When developing the F-series OCXO it was important to understand and take into account the many mechanisms that can make an oscillator unexpectedly noisy and some of them are extremely subtle. For instance, put simply, a tuned oscillator is a feedback system in which the phase shift round the loop is a whole number of cycles at the operating frequency. The primary frequency control mechanism involves a tuned circuit or resonator. At low offsets this acts as a frequency discriminator, converting frequency variation into phase shift. The oscillator operates at a frequency at which the resonator’s phase shift compensates for that of the maintaining circuit.

Also, in addition to the phase shift criterion, the magnitude of the loop gain must be unity. This is generally achieved either by limiting within the oscillator circuit or by an ALC loop.

Because the combined phase shift of the resonator and the maintaining circuit is a whole number of cycles at the operating frequency, any phase perturbation originating within the maintaining circuit requires a compensatory phase change in the resonator. This results in a corresponding frequency shift. In this way, when an amplifier with flat phase noise is used in an oscillator, it will produce a signal with flat FM noise. As ΦM sideband amplitude is proportional to phase modulation and phase is the integral of frequency with respect to time, this leads to the characteristic 20 dB/decade slope seen round the carrier frequency.

Higher-Q resonators give more phase shift for a given change in frequency. In an oscillator this mechanism is used in reverse, so less frequency change is incurred for a given phase perturbation, leading to lower phase noise. This applies equally whether the phase disturbance originates from within the oscillator circuit or externally, from load variation or power supply noise. Hence, high Q is a good thing.

At lower offsets, various mechanisms cause the maintaining circuit to produce flicker phase modulation which the oscillator loop converts to flicker FM noise, giving a 30 dB/decade slope close to carrier.

The phase noise floor depends on both the signal level within the oscillator and the way the output is extracted from it. Compromises often have to be reached between load pulling, noise floor and close-in phase noise.

This description doesn’t cover AM noise, though. In a well-designed oscillator, however, it is generally much lower than the phase noise close to carrier, and typically falls to a similar floor at large offsets, so it isn’t normally a significant problem.

On the other hand crystal oscillators generally have much worse phase noise than would be predicted from the circuit noise and the resonator Q. This is because the crystals 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. Therefore, 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, 5th-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.

Design considerations

All of these factors are of importance, but practical oscillator design almost always involves compromises and trade-offs. The F-series OCXO aims to provide the lowest possible phase noise within a relatively compact 2 x 2 x 0.75 inch package.

The OCXO incorporates a low noise regulator followed by further filtering to maximize rejection of external supply noise and ripple. In particular, careful attention is paid to ground paths, in order to prevent oven current noise affecting the oscillator’s performance. The oscillator is based very closely on the company’s standard OCXO design, with the active devices operated linearly because nonlinear operation generally increases the transistors’ flicker noise and also increases modulation of the signal by power supply noise and ripple. The degree of degradation tends to vary with factors such as crystal motional resistance, temperature, etc. Linear operation also gives more predictable RF operating conditions, which helps when tuning the oscillator on either side of the crystal’s series resonance.

The extra-low noise floor of the F-series OCXO is achieved by maintaining high signal levels within the output amplifier. The signal is taken from the oscillator in a way that maximizes the power into the amplifier while using the resonator to reduce far-from-carrier noise.

Performance

The F-series has the same close-in phase noise as the standard Pascall OCXO, together with improved noise floor and higher output power. Table 1 summarizes the performance offered at 100 MHz. Note that the phase noise is guaranteed minimum performance, not typical figures.

The first application for the new OCXO design was at 120 MHz. Figure 2 shows a phase noise plot. The phase noise of crystals rises fairly rapidly with increasing frequency, so it is not possible to achieve the same close-in performance at 120 MHz as at 100 MHz. However, the plot clearly shows the improved phase noise floor.

Measurement of very low phase noise floor presents a serious challenge, and needs cross-correlation to push the test system’s added noise below that of the DUT. In this instance, the indicated performance in the 10 to 70 kHz range is probably limited by the test set.

Applications

With its exceptionally low phase noise floor, the F-series OCXO is ideally suited for driving ultra-low-noise frequency multipliers, phase detectors and mixers. Its high output power means that an additional drive amplifier will not normally be needed, thereby eliminating another source of noise.

For applications that do not require such a low noise floor, the standard OCXO may be a more appropriate choice as it has lower power consumption but still offers the same close-to-carrier performance. However, the extra-low far-from-carrier noise of the F-series can help designers to achieve real performance improvements in state-of-the-art systems such as high performance radars, ultra-low noise frequency synthesizers and phase noise test systems.

Conclusion

The new F-series OCXO combines the close-in phase noise performance of the company’s standard OCXO with an even lower noise floor and higher output power, offering designers a new tool to improve system performance in the most demanding applications. The oscillator has an ample electrical tuning range of ±≥6 x 10^-6, with mechanical tuning available as an alternative, and is in a standard 2 x 2 x 0.75 inch package size.

Pascall Electronics Ltd.,
Ryde, Isle of Wight, UK,
Tel: +44 (0) 1983 817300,
Fax: +44 (0) 1983 564708,
enquiries@pascall.co.uk,
www.pascall.co.uk.

RS No. 300