Phase noise and the Y-factor noise figure

-December 19, 2013

Y-factor method mechanics

One technique for measuring noise figure is the Y-factor method. The Y-factor itself is the ratio between the noise generated at the output of the DUT when a noise source at the input of the DUT is turned on, and the noise generated when the noise source is turned off:

Typically the noise source is a noise diode with a known ENR (excess noise ratio). After executing a series of calibrations and measurements, the noise figure is calculated as:

As the device's noise level drops, the value of Y approaches 1, forcing the equation to approach singularity. You can use a higher ENR noise diode to induce a higher measured value of Y, but doing so also spreads the measurements used in calculating Y across a larger amplitude range on the measurement equipment, increasing equipment nonlinearity. It is best to use the lowest ENR noise diode that delivers a stable, repeatable measurement.

Noise figure measurements across the mixer’s RF range are shown in Figure 3. The measurements were collected using a spectrum analyzer with a pre-amplifier and a calibration step to separate the DUT noise contributions from the instrumentation.

Figure 3: Noise figure across RF frequency.

The synthesizer and signal generator results overlay closely. The linear value of Y underlying these displayed measurements varies between 1.4 and 1.8. There are two perturbations in the curves, one at 1950 MHz and the other at 2350 MHz. With an IF fixed at 200 MHz, these are consistent with behavior expected from a signal radiated at 2150 MHz. These appear to result from an interfering signal.

Based on these results, the synthesizer’s phase noise dioesn't significantly affect the noise figure measurement. When making a double-sided measurement on a frequency conversion device such as a mixer, both upper sideband and lower sideband noise will fold into the measurement bandwidth. A low IF will minimize the effect from different frequency response of the noise source, the device, and the measurement equipment, by keeping the upper and lower sideband frequencies close together. You should examine the effect of IF frequency before selecting it for your design.

Figure 4 compares noise figure measurements across a range of IF frequencies. Notably, there are a number of small peaks in the LMX2581 result, most occurring at multiples of the phase frequency detector (PFD) frequency. These are the effect of pfd spurs. Since PFD spurs cannot be filtered in an integrated synthesizer device, the IF frequency should be offset from any expected spur frequency to avoid erroneous measurements. Above 300 MHz, the measured noise figure begins to increase due to rolloff in gain from DUT and the IF output balun. Since the LO is 1800 MHz in this measurement, the large spike at 175 MHz IF appears to result from an interfering signal at 2150 MHz.

Figure 4: Noise figure across IF Frequency

Introduce an intentional blocker
A blocker signal offset from the desired RF highlights the impact of phase noise on the noise figure measurement. At low blocker offset from the LO the in-band phase noise level mixes onto the noise figure measurement frequency. While at higher blocker offset the LO phase noise floor determines the impact on the noise figure measurement. The contribution to noise figure from a blocker is given by:

where LCN = conversion loss, T = temperature, To = 290K, L = LO noise, PBLOCKER = blocker power, and k = Boltzmann's constant.

The plots in Figure 5 show that the low-phase noise floor of the LMX2581 synthesizer drops the noise figure well below the signal generator measurements. At lower blocker offsets, higher LO phase noise at the lower offset frequency increases towards the in-band, also increasing the measured noise figure. Placing a filter on the LO signal drops the phase noise floor, and both the synthesizer and the signal generator curves drop towards similar levels at high blocker offsets.

Figure 5. Noise figure across blocker offset frequency.

The phase noise of the synthesizer is coupled into the measurement, which now has to be viewed as a synthesizer and mixer combined effective noise figure.

Also see
Impact of phase noise in signal generators
Accurately predict measured noise figures for transformer coupled differential amplifiers (Part 1 of 2)
Accurately predict measured noise figures for transformer coupled differential amplifiers (Part 2 of 2)
Noise Figure Minimization of RC Polyphase Filters
Noise Figure Measurement without a Noise Source on a Vector Network Analyzer
Fundamentals of RF and Microwave Noise Figure Measurements
Noise Figure Measurement in the 60 GHz Range

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