Читать книгу Handbook of Microwave Component Measurements - Joel P. Dunsmore - Страница 36

While the harmonic measurement provides a direct characterization of distortion, it suffers from the fact that the harmonic frequencies are far away from the fundamental, and in many circuits, the network response is such that the harmonic content is essentially filtered out. Thus, it is not possible to discern the non‐linear response of such a network by measuring only the output signal. Of course, if the gain is measured, compression of the amplifier will show that the value of S21 changes with the input drive level. But it is convenient to have a measure or figure of merit of the distortion of an amplifier that relies only on the output signal. In such a case, two signals of different frequencies can be applied at the amplifier input, at a level sufficiently large to cause a detectible non‐linear response of the amplifier. Figure 1.7 shows a measurement of a two‐tone signal applied to the input of an amplifier (lower trace) and measured on the output of the amplifier (upper trace).

Figure 1.7 Measurement of a two‐tone signal at the input and output of an amplifier.

It is clear that several other tones are present at the output and are the result of higher‐order products mixing in the amplifier due to its non‐linear response and creating other signals. The principal signals of interest are the higher and lower intermodulation (IM) products, PwrN_Hi and PwrN_Lo, where N is the order of intermodulation distortion (IMD). Normally, IM products refer to the power of the IM product relative to the carrier, in dBc, and these terms are called IMN_Hi and IMN_Lo. For example, the power in the lower third‐order tone is Pwr3_Lo; the level of the upper third‐order tone relative to the carrier is called IM3_Hi. The frequencies of the higher and lower tones are found at

(1.71)

And more generally

(1.72)

In Figure 1.7, the amplifier is driven such that the fifth‐order IM product is just visible above the noise floor in the upper trace.

IM products have the same attribute as harmonics with respect to drive power, and the power in the IM product (sometimes called the tone power, or PWRm for the mth‐order IM power) increases in direct proportion to the input power and the order of the IM product. Thus, if the tone power is plotted along with the output power against an x‐axis of input power, the plot will look like Figure 1.8, where the extension of the slope of the output power and IM tone‐powers at low drives will intersect. This point of intersection for the third‐order IM product is known as the third‐order intercept point, or IP3. Similarly, IP5 is the fifth‐order intercept point, etc.

Figure 1.8 Output power and IM tone‐power versus input power.

It is also interesting to note that in general at high powers, the IM tone‐powers may not increase but may decrease or have local minima. This is because of the effect of high‐order IM products re‐mixing and creating significant signals that lie on the lower‐order products and can increase or decrease their level, depending upon the phasing of the signals.

There is often some confusion about third‐order IM products (IM3) and third‐order intercept point (IP3), and both are sometimes referred to as third‐order intermod. For clarity, in this book, the intercept point will always be referred to as IP.

Finally, for amplifiers used as a low‐noise amplifier (LNA) at the input of a receiver chain, it is often desired to refer the IP level to the input power, which would produce an intercept point at the output. This is distinguished as the input intercept point (IIP), and in the case of ambiguity, the normal intercept point referencing to the output power should be most properly referred to as the output‐referred intercept point (OIP). The most common intercept points are the third‐order ones, OIP3 and IIP3. The input and output intercept points differ by the gain of the amplifier at drive level where the measurements are made.

The details of two‐tone IM measurements are discussed at length in Chapter 8.