Mastering Precision: A Guide to Setup Your Laboratory for IPn Measurement

Introduction 

In the previous blog, you learned how IPn is obtained graphically based on the polynomial representing a general non-linear device. More specifically IP3 and IP2 are the most looked at parameters for many RF components. In this last blog of this series, I will give more practical details on how the intercept points are measured with lab test setup, examples with real components, and also introduce the 1dB compression point; a very similar spec to IPn.

Lab Setup for IP3 Measurement 

Suppose now that you want to measure the IP3 performance of a given LNA, a device under test (DUT). First, you will need two independent frequency sources: generators GEN-A and GEN-B (Figure 14). The two signals will have the same amplitudes and very close frequencies, for example, ωa = 2.00GHz and ωb = 2.01GHz (thus spaced with 10MHz). You can also take 1MHz and 1.001MHz, etc. The frequency selection depends on the actual device to be tested, i.e., around 433MHz for a European ISM band or 900Mhz for a GSM band. 

Figure 14. Block diagram for IP3 measurement. 

These two frequencies are first applied to a combiner (a sort of “adder”) and then injected into the DUT. Some filters can be found between generators and the combiner and from the combiner to the DUT. (Note: make sure that a filter is applied only to the two selected sources to the DUT). 

On a spectrum analyzer, you can observe the two sources at fundamental frequencies and all the harmonics and the intermodulation products (IMs).  

Figure 15. Schematic view of data generated by a spectrum analyzer of IP3 measurements. 

In Figure 15, POUT and ΔP are measured directly on the screen; further, OIP3 = POUT + ΔP/2. 

Figure 16 shows a typical view of a spectrum analyzer screen of an IP3 measurement: 

Figure 16. Spectrum analyzer screen view during IP3 test. 

In Figure 16, M1 and M2 are the traces of the two fundamentals; both were measured around -11dBm (= POUT). M3 and M4 are the IM3 signals; they were both measured both approximately -45dBm. Thus: 

 ΔP = -11dBm - (-45dBm) = +34dBm                     (Eq. 28) 

Therefore:  

 OIP3 = -11dBm + 34/2dBm                                   (Eq. 29) 

 Or: 

 OIP3 = -11dBm + 17dBm = +6dBm                       (Eq. 30) 

In this device, the gain is +7dB. Therefore:  

 IIP3 = OIP3 - G = 6 - 7 = -1dBm                            (Eq. 31) 

The results from Equation 31 show that this DUT is a standard, good LNA. 

Some functions like the first stage of a receiver RF front-end require higher IP3 devices. This is where the MAX2062 can help. 

The MAX2062 dual, 50MHz to 1000MHz high linearity RF/IF VGA can be configured for many purposes such as a PA pre-driver, a diversity IF amplifier (thanks to its dual construction), and any VGA for multipath and transmitter applications. The linearity performance of this device is outstanding with an OIP3 of +41dBm and an OIP2 of +56dBm.  

In the MAX2062 data sheet, the OIP3 has been characterized with two RF tones of 0dBm each and separated by 1MHz. The tests were made at seven different frequencies: 50MHz, 100MHz, 200MHz, 350MHz, 450MHz, 750MHz, and 900MHz. 

The -1dB Compression Point (CP1 or CP1dB) 

The IPn are excellent parameters to quantify the device’s linearity on some specific intermodulation products; especially IM3 and IM2. The -1dB compression point (CP) is also a figure of merit for measuring nonlinearity. Graphically (Figure 17), it is the point where the actual input-output response curve deviates (i.e., drops) by 1dB from the linear asymptote.  

Figure 17. Graphical view of a1-dB compression point.

The -1dB compression point can also be seen as the point where the actual curve crosses the linear line dropped by 1 dB. As for the IP parameter, the compression point can be expressed as input (iCP1) or output (oCP1). It can also be observed that CP1 is strongly linked with the IP3 values, even though there is no strict relationship.  

 OCP1 = OIP3 - 8 to -12dB                     (Eq. 35) 

For example, the versatile MAX2645 is configured as a PA pre-driver with a gain of 15.2dB. Here the input 1dB compression point (CP1) is -1.8dBm, while its IP3 under the same setup is +11.8dBm. Its IIP3 and ICP1 differ by 13.6 dB. 

Conclusion 

I hope that you now have a clear understanding of how IIPn or OIPn originate and can reconstruct their relationship with input/output powers and gain. Normally one does not need the above analysis to make an IP3 measurement with a spectrum analyzer. Occasionally, however, engineers require deeper, detailed explanations when faced with an unexpected phenomenon or, perhaps worse, systematic absurd results.