Bench measurements under 110dBc 3rd order intermodulation distortion

Emerging low power fully differential amplifiers (FDAs) are intended to support IF and ADC interface requirements with exceptional linearity. Offering intercepts exceeding 50dBm on very lower power, they can provide an attractive option to the more typical Class A RF amplifiers for applications below 500Mhz. An immediate practical issue is encountered in attempting to measure the IM3 when the spurious are >-110dBc below the carriers. Typical approaches of projecting from -1dB compression points do not apply to FDA type devices. Other projection techniques can certainly help, but at the end of the day generating a -120dBc clean input and measuring a -110dBc dynamic range are both useful capabilities in these types of measurements.

Extremely high 3rd order intercept amplifiers

Communications channels have always needed a mix of low noise figure, high intercept, and manageable quiescent power to deliver leading edge systems. The 3rd order intermodulation intercept is particularly important as it describes how low the spurious powers will be at the output of a stage receiving two closely spaced carriers that were not in the original input signal. These are particularly troublesome since they will fall “close-in” to the carries and cannot be filtered out. The classic definition of the 3rd order intercept is shown in figure 1. Also shown is the spacing around a center frequency where the resulting spurious will be. Essentially, for carriers spaced +/-Δf around f0, the 3rd order spurious will be +/-3Δf around f0 where f0 is the average (or center) frequency of the two carriers.


Click on image to enlarge.


Figure 1. 3rd order intercept definition.

For amplifiers that show an intercept characteristic, this simple approach gives an easy way to predict SFDR for different output carrier levels. From fig. 1, the intercept for equal carrier power (P0), is given by eq. 1.
                                     


From this single number, an estimate of the 3rd order SFDR may be made as eq. 2
                                     

The intercept is often constant over frequency for class A type RF amplifiers, but never so for more high open loop gain, voltage or current feedback based, fully differential amplifier (FDA) type devices. These lower power devices offer a frequency dependent loop gain and lower full power bandwidth (slew rate) that reduce the performance as the frequency increases.

The easy measurement is when the test power levels are at 0dBm. In the example drawing of fig. 1,which is drawn with a -60dBc to the 3rd order spurious at 0dBm output, so the intercept is 30dBm from equation 1. Then, at say 10dBm output level  (2Vpp on each tone for a sine wave test, 4Vpp output envelope), equation 2 would predict 40dB SFDR, which also can be seen in fig. 1. The name “intercept point” comes from the intersection of the 2 curves in fig. 1. That also equals 30dBm and is a projection of where the output spurious would equal the test powers. That 30dBm output power is of course not intended here and the model is only used to project the 3rd order spurious at output powers far below this “intercept” point.

Not all amplifiers show a strictly intercept performance, so it is also common to just see a 3rd order spurious level vs. frequency and/or output power level plot. This is particularly common when the loads are not intended to be 50ohm loads – such as driving ADC inputs. For example, a very low IM3 device like the ISL55210 (ref. 1) shows a data sheet plot such as fig. 2 (figure 9, ref. 2).



Click on image to enlarge.


Figure 2. Swept frequency, fixed gain, 200Ohm load IM2/IM3 SFDR plot for the ISL55210

This is showing the ΔdBc from equal test tone powers to the spurious levels for different fixed output 2-tone envelopes swept up in frequency using the 15dB gain test circuit of fig. 3. The output network of fig. 3 maps from a 200Ω differential load to a single ended 50ohm measurement path. The 2Vpp curve is 2, 1Vpp test tones at the output pins (Vo) spaced  +/-100khz around the x-axis frequency.



Above 150Mhz, it is starting to look like it might have an intercept characteristic, but the question here is how to generate and test these <-100dBc levels in a lab environment. While the IM2 is not nearly so low as the IM3 for the 115mW ISL55210, the intent was that a bandpass filter would filter those off. This is to follow the stage when it is the <100dBc 3rd order terms that are of interest in the application.

Developing the input test signal
Testing for OIP3 starts with summing two signal generators together and eventually ends with using a spectrum analyzer to measure very low spurious levels. To allow the spectrum analyzer some chance of making this measurement, it helps to use very low phase noise sources locked to the spectrum analyzer to zoom in on a narrow span knowing exactly where to look with no phase noise smearing of the measured power. Those synthesized sources are readily available (e.g. HP8662, HP8664, Gigatronics 6080A, R&S SMA100A, etc) and the fact they have very poor harmonic distortion (typically in the -50dBc to -60dBc range) is inconsequential to amplifier IM3 testing. Those straight harmonic distortion do matter to ADC testing and the test signal needs to be run through a bandpass filter in that case. No passive filtering is required in testing IM2/IM3 for amplifiers as none of individual source harmonics create terms at the intermodulation locations.