Local Oscillator - Sideband Noise Measurement

Oscillator sideband noise measurement overview

Accurate inspection of oscillator sideband noise is one of the more "esoteric" measurements needed during the design and construction of a high dynamic range frontend. General purpose test equipment is usually not sufficient. The SNA62 spectrum analyzer I am using, with its 100dB dynamic range, is not capable of inspecting the noise sidebands of low noise oscillators. Using a spectrum analyzer to directly measure the noise sidebands of an oscillator has the following limitations:

  • Because of the presence of the strong carrier the spectrum analyzer is easily overloaded and therefore its dynamic range is the limitation.
  • Also dynamic range related is that reciprocal mixing effects may occur between the carrier and the analyzers LO. This may cause close-in measurements to be further limited by the noise sidebands of the analyzer.
  • In case overloading does not occur, the sensitivity is still limited by the high noise floor of the spectrum analyzer, even with the smallest RBW available.

In practice, even today's high-end general purpose spectrum analyzers are not good enough to evaluate the noise sidebands of ultra low noise oscillators. Therefore specialized test equipment is needed such as a Signal Source Analyzer, but SSE's are usually not within the budget of most radio amateur homebrew enthusiasts. Fortunately, a couple of measurement methods exist that are well suited for homebrew reproduction with good results. The following methods have been implemented, during the evaluation of the AD9910 DDS chip:

  • Reciprocal Mixing Method
  • Crystal Notch Filter Method
  • Quadrature Mixing Method

The following paragraphs give a brief overview of those methods. Follow the links at the bottom of the page to read in more detail about each method.

Reciprocal Mixing

This is the method used by most equipment reviewers for amateur radio magazines. It measures the single sideband noise of the LO signal as part of the receiver. So the real performance of the receiver is measured with regard to SSB noise of the LO. This makes the reciprocal mixing method the only ultimate test with regard to the receivers Phase Noise Dynamic Range (PNDR), as all possible side effects acting on the LO signal inside the receiver are taken into account.

The method is based on the principle that a strong signal injected at the desired offset from the Rx frequency will mix with the noise sidebands of the receivers LO producing noise directly in the Rx channel. This noise will be perceived as an elevated receiver noise floor. By measuring the signal level that is needed to raise the receivers noise floor, actually the PNDR of the receiver is measured. From the PNDR, the dBc/Hz value of the LO noise sidebands can be easily computed. This makes the reciprocal mixing method an indirect but valuable sideband noise measurement method.

Crystal Notch Filter

The oscillators single sideband noise is measured directly using a spectrum analyzer with this method. The dynamic range and sensitivity of most spectrum analyzers however is not enough to inspect the noise sidebands of low noise oscillators directly. A very selective notch filter is used to suppress the carrier from the signal that is measured, but leaving its noise sidebands untouched. Now that the strong carrier is gone, so is the issue of overloading the spectrum analyzer! In order to overcome the rather high noise figure of a general purpose spectrum analyzer a low noise amplifier is used to amplify the noise sidebands to become visible above the analyzers noise floor.

Quadrature Mixing

This is also a direct measurement method, which is used in most commercial phase noise measurement systems. With this method the carrier of the signal is removed in order to overcome the limitations of the spectrum analyzer. That is quite like the crystal notch method, however the mechanism involved is entirely different. With the quadrature mixing method the signal to be measured is mixed down to the base band by means of a clean reference signal with exactly the same frequency. The clean reference signal is locked to the signal that is measured by means of a PLL. The phase detector used in the PLL is such that a precise 90° phase offset is maintained between the two signals. Because of this 90° phase shift, the carrier is suppressed almost completely at the IF port of the mixer, theoretically to 0Hz without any DC component. Now the baseband signal can be easily amplified and inspected with a low frequency spectrum analyzer without much dynamic range problems.

Reciprocal Mixing

Crystal Notch Filter

Quadrature Mixing

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