Making RF-to-baseband Noise Figure Measurements.

RF-to-baseband front ends consisting of a low-noise amplifier (LNA) cascaded with a mixer down-converting RF signals to baseband have become the epitome of RF devices tested in high-volume manufacturing (HVM) today. While the methodologies for measuring noise figure (NF) on these devices are the same as those for RF-to-RF devices, the implementations may appear to be somewhat different between bench and ATE and between RF-to-RF devices and RF-to-baseband devices.

There is a handful of methods for measuring RF-to-RF NF. (1) For mainstream RF-to-baseband devices, however, only two methods are popular: Y-Factor and Cold Noise, each with benefits for reducing both test time and cost of test.

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NF and Noise Factor

NF is used to determine how much noise is added to a system by a device. In RF-to-baseband receivers, it describes how much added noise comes from the downconversion and amplification processes. NF is related to the fundamental parameter, the signal-to-noise ratio (SNR), which is paramount in nearly all electronic applications from audio to the latest generation of personal communications devices.

Although the term noise factor is rarely used, it is the foundation of NF. Noise factor (F) is the linear format of signal-to-noise degradation imposed by a device

F = [[[S.sub.i]/[N.sub.i]]/[[S.sub.o]/[N.sub.o]]]|[.sub.T=[T.sub.0]=290[degrees]K] (1)

F is the ratio of input SNR to output SNR at a standardized reference temperature, T = [T.sub.0], designated by IEEE to be 290[degrees]K (~17[degrees]C). (1) Temperature comes into the definition because the dominant contribution of noise in electronics is thermal agitation of the electrons in conductive media of the devices.

Figure 1 depicts Equation 1, showing the impact of noise on a device. It shows the input power level of a DUT with amplification having a gain (G) and the increased noise at the output of the DUT resulting in a decreased SNR. Both the input signal and input noise are amplified by the DUT and higher at the output of the DUT. However, since the DUT adds noise, the total noise at the output is raised significantly.

The definition of NF is related to noise factor by the equation

NF|[.sub.dB] = 10[log.sub.10](F) (2)

NF Measurement Methods

Y-Factor

The Y-Factor method of measuring NF is perhaps the oldest one known used behind the scenes in most NF meters and analyzers. (2) It involves applying a noise source to the input of the DUT and making noise power measurements at the output of the DUT. By doing this, a ratio of noise power measurements, the Y-Factor, is determined, and NF is derived from that.

The Y-Factor method uses a noise source applied to the input of the DUT as shown in Figure 2. It is powered on and then off. Each time, a power measurement at the output of the DUT is performed. The Y-Factor is defined as the ratio of hot to cold measured noise power in watts

Y = [P.sub.hot]/[P.sub.cold] (3)

The term hot refers to the state of the noise source being powered on and adding noise to the device, much like a signal generator providing a voltage or power signal to the input of the device. Cold refers to the noise source being powered off but still connected to the input of the DUT Almost all noise sources in their off or cold state provide a 50-[OMEGA] termination to the input of the DUT.

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Every noise source has an associated parameter termed excess noise ratio (ENR). ENR is the power-level difference between hot and cold states compared to the thermal equilibrium noise power at [T.sub.0], again 290[degrees]K. Diode-based noise sources come calibrated with a statement of their ENR value.

Using the measured Y-Factor along with the ENR of the noise source, F is calculated as

F = ENR/(Y - 1) (4)

and NF in dB is

NF|[.sub.dB] = ENR|[.sub.dB] - 10[log.sub.10](Y-1) (5)

Y usually is much greater than 1 when testing the NF on RF-to-baseband devices so the -1 can be ignored, providing a simple equation

NF|[.sub.dB] = ENR|[.sub.dB] - ([P.sub.hot] - [P.sub.cold]) (6)

Equations 5 and 6 commonly are applied for measuring RF-to-baseband NF when using a noise diode built into the ATE, the RF arbitrary waveform generator (Arb) noise source, or noise diode on the load board.

Cold Noise

The Cold Noise or gain method is another technique considered to be very production test-friendly for RF-to-base-band devices. (1,3) It relies on measuring just the cold noise power of the DUT when a 50-[ohm] termination is applied to its input.

This method also requires the gain of the device to be measured. It is common practice to place this test after the gain test in the production test program. In this way, effectively only the noise power measurement has to be made. Having these two values, gain and noise power, F is calculated as

F = [P.sub.cold]/kTBG (7)

or in dB

NF|[.sub.dB]=[P.sub.cold]-(-174 dBm/Hz)-10[log.sub.10](B)-G|[.sub.dB] (8)

B is the bandwidth over which the cold noise power measurement ([P.sub.cold]) is made. The value -174 dBm/Hz is the thermal noise power associated with the temperature 290[degrees]K. It is the product kT (1.38 x [10.sup.-23]J/K)290[degrees]K converted to logarithmic format in dBm.

Comparing the Methods

The Y-Factor method uses the ratio of two power measurements to calculate NF. Since it is a ratio, the measurements are relative, and the absolute power accuracy of the measurement equipment is of less concern.

Unfortunately, it often utilizes a diode-based, fixed-ENR noise source, which can be problematic when measuring either very high or very low NF values. (3) The reason for this can be seen from Equation 5 where, if the NF is too large relative to the ENR of the noise source, the measured hot noise power causes Y to approach unity and can yield a different-than-expected NF.

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When a diode-based noise source is used, it has a fixed ENR. This ENR may be suitable for some devices but not others, specifically with larger NF. In some cases, an Arb noise source has been used. (4,5) It provides an adjustable ENR to combat this situation.

The Cold Noise method only requires one power measurement to be made, consequently taking less test time. Overall, the measurement setup and implementation are very simple.

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Both methods perform a cold noise power measurement with the input of the DUT terminated in 50[OMEGA]. The difference in the Y-Factor method is the hot noise power measurement. This measurement provides a means to calculate the gain of the DUT in addition to NF. This is how an NF meter or spectrum analyzer is able to display both gain and NF over frequency.

Choosing a Measurement Method

The key factor differentiating RF-to-baseband devices is the large number of gain states available. This is a result of the combined gain control available in the LNA as well as the mixer.

Figure 3 shows the Cold Noise and Y-Factor methods in a matrix composed of four combinations of states of gain and NF found in RF-to-baseband devices. It depicts which method is best suited for the given gain and NF combinations expected to be measured on the DUT.

Device settings having high gain with low or high NF are the easiest to measure, with either method working well. Typically, the higher the sum of gain and NF in dB, the easier the NF measurement can be made.

One caveat with the Y-Factor method: For those devices that have both high gain and high NF, you must use a noise source having a higher ENR. This assumes traditional noise sources for RF-to-RF testing having ENR values of 12 dB to 22 dB.

Both methods become a little weak in the case of low gain, low NF devices because the tester's own noise becomes significant relative to the noise of the DUT. This primarily affects the cold noise measurement in both methods.

For this special class of conditions, neither method is very easy to implement in production and would likely require a preamplifier to reduce the effective NF of the tester. (2) Fortunately for RF-to-baseband devices, this combination of low gain and low NF is not a common set of conditions.

In the case of low gain, high NF devices, the Y-factor method with a fixed-ENR noise source can become inaccurate if its ENR is not large enough because the noise output from the DUT is significantly greater than the noise of the noise source and Y approaches unity (Equation 5).

Comparison of Methods

A study was conducted to analyze the differences between the following methods of measuring RF-to-baseband NF:

* Y-Factor, using a noise diode

* Y-Factor, using an Arb noise source (ENR = 12.8 dB)

* Y-Factor, using an Arb noise source (ENR = 36.8 dB)

* Cold Noise

This work was done on ATE in a dual-site load board configuration. The baseband digitizer used to make the power measurements had a 16-bit resolution. The device being tested was an 802.11b/g device operating at 2.4 GHz. All measurements were performed at the same frequency, but the gain settings of the DUT were varied. All noise power was measured in a 2-MHz bandwidth.

The noise diode had an ENR of 12.8 dB. To be consistent, the Arb noise source was configured to generate a noise output also having a 12.8-dB ENR. To address the wide variation of gain setups of the DUT, a higher ENR noise source also was required. This was achieved only with the Arb noise source where the noise output was able to be increased to have an ENR of 36.8 dB.