Specifications

For example, suppose that the LO fundamental has a peak-to-peak deviation

of 10 Hz. The second harmonic then has a 20 Hz peak-to-peak deviation; the

third harmonic, 30 Hz; and so on. Since the phase noise indicates the signal

(noise in this case) producing the modulation, the level of the phase noise

must be higher to produce greater deviation. When the degree of modulation

is very small, as in the situation here, the amplitude of the modulation side

bands is directly proportional to the deviation of the carrier (LO). If the

deviation doubles, the level of the side bands must also double in voltage;

that is, increase by 6 dB or 20 log(2). As a result, the ability of our analyzer

to measure closely spaced signals that are unequal in amplitude decreases

as higher harmonics of the LO are used for mixing. Figure 7-10 shows the

difference in phase noise between fundamental mixing of a 5 GHz signal and

fourth-harmonic mixing of a 20 GHz signal.

Improved dynamic range

A preselector improves dynamic range if the signals in question have

sufficient frequency separation. The discussion of dynamic range in Chapter 6

assumed that both the large and small signals were always present at the

mixer and that their amplitudes did not change during the course of the

measurement. But as we have seen, if signals are far enough apart, a

preselector allows one to reach the mixer while rejecting the others. For

example, if we were to test a microwave oscillator for harmonics, a

preselector would reject the fundamental when we tuned the analyzer

to one of the harmonics.

Let’s look at the dynamic range of a second-harmonic test of a 3 GHz

oscillator. Using the example from Chapter 6, suppose that a –40 dBm signal

at the mixer produces a second harmonic product of –75 dBc. We also know,

from our discussion, that for every dB the level of the fundamental changes

at the mixer, measurement range also changes by 1 dB. The second-harmonic

distortion curve is shown in Figure 7-11. For this example, we shall assume

plenty of power from the oscillator and set the input attenuator so that when

we measure the oscillator fundamental, the level at the mixer is –10 dBm,

below the 1 dB compression point.

Figure 7-10. Phase noise levels for fundamental and 4th harmonic mixing