Non-Linear Transmission Line Comb Generators Part-1: The Phase Noise Problem and Comb Generation

Jan. 07, 2020

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Overview:

In this two-part series from MACOM, we will delve into Non-Linear Transmission Line (NLTL) Comb Generators, first understanding the phase noise problem, and understanding a potential solution to the problem. In the second part of the blog series, we will explore NLTL comb generation, compare it to its predecessor comb generation using Step Recovery Diodes and see how the NLTL comb generation approach can enable improved sensitivity and lower bit error rates in communication systems.

The Phase Noise Problem

Let’s start with the problem with circuits requiring low noise performance. Below we see a block diagram of the RF and IF portions of a typical superheterodyne receiver.  A weak signal is received at the antenna – 1) optionally amplified by a low noise amplifier, 2) filtered to reduce the effects of broadband noise and interferer signals whose frequencies may be close to that of the desired signal and then 3) downconverted to a lower, intermediate frequency for further processing. 

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Figure 1: Typical Superheterodyne Receiver

In the ideal case, the downconverter mixer mixes the received signal with a single-frequency local oscillator signal.  In the real case though, the local oscillator signal never comprises a single frequency, but is always accompanied by close-in noise sidebands which are generated in the local oscillator signal chain.  Also, the received signal may be accompanied by close-in interfering signals which cannot be completely removed by the band pass filter.     

What are the Noise Sidebands? – Phase and Amplitude Noise

This noise is composed of phase noise and amplitude noise.  Phase noise is random, instantaneous variations in the phase of a signal, in this case, the local oscillator signal.  Amplitude noise may be thought of as random, instantaneous variations in the amplitude of the signal.

The net effect of the noise sidebands is to self-jam very weak received signals whose frequencies are close to the local oscillator frequency.  Because the noise sidebands are so close to the local oscillator frequency that they cannot be removed by filtering, the internally-generated phase and amplitude noise must be minimized in any receiver intended to process weak signals or signals accompanied by interferer signals.

In addition to the self-jamming problem, in communications system which utilize phase modulation, phase noise can also cause high bit error rates.  Regardless of the type of modulation utilized, phase noise can cause spectral regrowth which can produce adjacent channel interference(1) and reciprocal mixing which can mask a low-amplitude received signal(2).

A Solution? Low Phase Noise Signal Generation

Low-phase-noise signal generation can be accomplished using several design approaches, such as crystal oscillators, ceramic resonator oscillators and SAW/BAW resonator oscillators.  One factor which is common to these various types of circuits is that they are only capable of producing signals at relatively low frequencies (typically in the HF up to the mid-UHF bands) due to the electromechanical modes of operation of their resonators. 

Comb Generation

Many systems require signals at frequencies which are not easily generated or in some cases impossible to generate directly.  The widely-employed mitigation strategy has been to apply a locally-generated, low-frequency signal known as the ‘fundamental frequency’ to a circuit containing nonlinear impedance, which translates energy from the fundamental signal frequency to its harmonics.  Such circuits are known as comb generators since these harmonics, when depicted in the frequency domain, resemble a comb.

Tune in for Part 2 in this series for more information on NLTL Comb Generators. Members of MACOM’s applications engineering team are ready to help you select the optimal diodes and circuit topologies for your application. For more information on MACOM’s solutions, visit: https://www.macom.com/diodes

References

(1). Khanzadi, M. R., “Phase Noise in Communications Systems”, Chalmers University of Technology, November 2015

(2). Henkes, D. D., “Analyzing the Role of Local Oscillator Phase Noise in Reciprocal Mixing”, Microwave Products Digest, October 2013