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How to Solve RF and Microwave System Phase Noise Problems

Posted on Friday, June 08, 2018

One obvious place to limit phase noise is in the choice of oscillator. This problem can be addressed by spending considerable time and money to design or procure a low noise oscillator. However, most oscillators do not generate enough output power, and indeed let us assume that for a particular application, the oscillator output of +5 dBm needs to be amplified to a level of +15 to +17 dBm, in order to drive the LO port of a mixer. The question then becomes, does the amplifier affect the phase noise of the LO signal?

In an ideal situation, the answer would be “no,” as the amplifier would simply raise desired LO signal and the skirts by the same level. However, in reality, microwave amplifiers add noise of their own to any signal, and herein lies the problem. All electronic devices exhibit a phenomenon called 1/f noise or “pink noise”, which is noise power that is added to an input signal spectrum but falls off proportionally to the inverse of the offset frequency. In Figure 1, we present the phase noise of the CMD167, a low noise amplifier covering the 10 to 17 GHz range, versus offset frequency away from the desired signal. The phase noise of the incoming signal has been canceled out, so this plot represents the noise generated by the amplifier.

FIGURE 1. CMD167 LNA Phase Noise

In Figure 1 (above), we note the phase noise falls off linearly on the logarithmic scale with increasing frequency offset, which is characteristic of 1/f noise. If this noise level is higher than the phase noise of the input signal, then the amplifier noise would dominate the output noise spectrum. In our example, this means the low phase noise of the oscillator would be replaced by the higher phase noise of the amplifier, thereby defeating the purpose of the low phase noise oscillator. A pictorial representation of this phenomenon is shown in Figure 2 (below).

FIGURE 2. Pictorial representation of the degradation of phase noise due to an amplifier. The skirts of the input signal on the left are increased after passing through the amplifier, with the output spectrum on the right.

One obvious question is, can anything be done to lower the phase noise of amplifiers? The answer lies in device physics. The 1/f noise is caused by random and thermal charge movement in the channel of an active device. The CMD167, for example, is manufactured on a Gallium Arsenide (GaAs) pHEMT process with a gate length of 0.13 um. The FET devices on this process typically have a high 1/f corner due to their high electron mobility. GaAs bipolar devices, on the other hand, tend to have lower electron mobilities, which means a much lower 1/f noise, so they are considerably better for phase noise than their FET counterparts. Therefore, one solution to lowering the additive phase noise is to use a GaAs HBT process.

At Custom MMIC we have used our extensive knowledge of amplifier design techniques to create a family of new Low Phase Noise Amplifiers (LPNAs) on a GaAs HBT process operating from 6 to 40 GHz. In Table 1 (below), we present the summary characteristics of these new amplifiers.

Product 

Frequency (GHz) 

Saturated Output Power (dBm) 

Phase Noise (dBc/Hz@ 10kHz offset) 

CMD245 

6 – 18 

20 – 22 

-165 

CMD246 

8 – 22 

18 – 20 

-165 

CMD247 

30 – 40 

15 

<-160 

Table 1: Summary of Custom MMIC’s new Low Phase Noise Amplifiers (LPNAs) die. SMT packaged versions are also available.

In Figure 3 (below), we present the phase noise versus offset frequency for the CMD245 amplifier as housed in a 4 mm QFN-style package, relative to the CMD167 HEMT LNA shown previously. We note the phase noise of the CMD245C4 is 15 to 20 dB lower than the CMD167 pHEMT LNA.

FIGURE 3. Phase noise of the CMD245C4 low phase noise amplifier vs CMD167 LNA.

Other components besides oscillators and amplifiers can contribute to phase noise, including frequency multipliers. Many microwave systems utilize a lower frequency oscillator that is then multiplied to produce a higher frequency. One common approach for multiplication is to use a harmonically terminated amplifier to generate the required output frequency. Unfortunately, such an approach will then add the amplifier’s phase noise to the multiplied signal, which will degrade the phase noise of the original oscillator.

A second approach is to use passive multiplication, which has the potential to add minimal additional phase noise to the multipliers signal (aka doublers). Custom MMIC, has also created a family of passive HBT style frequency multipliers which do not add to the phase noise of the input signal. In Table 2 (below), we present a summary of these multipliers.

Product 

Output Frequency (GHz) 

CMD225 

8 – 16 

CMD226 

14 – 22 

CMD227 

16 – 30 

Table 2: Summary of Custom MMIC’s passive multiplier die family. SMT packaged versions are also available.

 

Learn more about the remedies for phase noise by reading our blog, "What is RF Phase Noise and Why Should You Care About It?"

For the full story on phase noise, download out tech brief, "Addressing Phase Noise Challenges in Radar and Communication Systems."

Custom MMIC

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Chelmsford, MA 01824
Phone: 978-467-4290
Fax: 978-467-4294

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©2006-2017 Custom MMIC
All rights reserved.

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Custom MMIC

300 Apollo Drive
Chelmsford, MA 01824
Phone: 978-467-4290
Fax: 978-467-4294

Terms and Conditions

©2006-2018 Custom MMIC
All rights reserved.