In short: because there’s a strong relationship between the quality of receivers and phase noise. Phase noise or side band noise can destruct a receiver’s capability to distinguish wanted signals from unwanted signals.
In my reviews I often refer to ‘bad front ends’ if a receiver collapses when confronted with strong out-of-band signals. While this is technically correct, a bad front end is seldom the only cause of the problem. A large amount of phase noise is another. The less phase noise, the better. Unfortunately many modern day (cheap) designs aren’t doing very well in this area.
The amount of phase noise can be easily measured by connecting a transceiver to a spectrum analyzer. In an ideal situation a signal looks like a steep mountain without ‘shoulders’. The wider and higher these shoulders are, and/or the earlier they start to appear in the spectrum, the more problems you can expect.
No oscillator is perfectly stable. All are frequency or phase modulated by random noise to some extent. Any instability in the Local Oscillator (LO) is transferred to any mixing products resulting from the LO and input signals. So the LO phase-noise modulation sidebands appear around any spectral component on the display of a spectrum analyzer that is far enough above the broadband noise floor of the system. The amplitude difference between a displayed spectral component and the phase noise is a function of the stability of the LO. The more stable the LO, the farther down the phase noise.
Before we go on, reading this article on reciprocal mixing is a good idea. I can’t do better than they do.
I reviewed a lot of Chinese transceivers this year. Some of them had very good receivers, some of them didn’t. I wondered if my conclusions, which were mainly based on field testing, could be replicated by measuring phase noise. I switched on my Rigol DSA-815TG and checked some of them.
Take a look at four popular Chinese HTs. The first two are the bad boys; the last two are amongst the best.
As you can see it is possible to get an idea of potential problems by looking at phase noise levels. The system is not full-proof though. Low levels of phase noise without a decent front end will still translate into disappointing results. The opposite is also true: a reasonable front end can’t compensate for large amounts of phase noise.
An excellent example is the Baofeng UV-3R. This cheap HT doesn’t suffer from large amounts of phase noise, but lacks any kind of front end. This is the cause of the disappointing results we got.
I hope I didn’t bore you today. Have a great weekend!