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.
Theory
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.
Measurements
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.
Baofeng BF-666S/777S/888S: not looking good.
The Waccom WUV-6R isn’t doing much better.
The Baofeng UV-B5, the surprise of 2012. Looking good.
The Wouxun KG-UVD1P, still the best of the pack (but only just)
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.
Not much phase noise, but lacking a decent front end.
I hope I didn’t bore you today. Have a great weekend!
Ham Radio Blog PD0AC 
Just curious, how about UV-3R+ or 3R mark II? The pirce of UV-3R+ is almost the same with UV-5R, It should have some improvements or difference from UV-3R except the well known harmonic suppression.
All UV-3R varieties have the exact same performance.
Very informative article as always! I was just curious if you did the same kind of tests with the Puxing 888K?
I read the part I review (not sure if you published a part 2, I don’t think I could find it).
I would also be very interested to know how the front end of the Puxing and receiver sensitivity compares to the Baofeng UV-B5 / Baofeng UV-B6.
Thanks again for your blog!
The Puxing review still needs finishing up… every time I tried to do that, some dual band mobile showed up which had to be returned quickly. I’ll see what I can do in the next few days.
How are you measuring this, is this just a handheld on TX or are you directly coupled into the DDS IC output ? Thanks, Jon
Hi Jon,
because phase noise is caused by instability of the local oscillator, it basically doesn’t matter. Both methods will give more or less the same result.
No they dont, that is why I asked !
Assuming the spectrum is taken from the handheld on TX all it suggests to me is that some of the handhelds are running one or more of the PA transistors at or near saturation.
You can prove it to your self with a simple setup. Take a pure source like an XTAL oscillator, run it through a simple amplifier with a variable gain. As you increase the gain the output signal increases, when you get towards saturation sidebands start to appear as a product of the amplifier not the source !
Without measuring directly from the IC you can say almost nothing about phase noise as its such a small part of the transmitter output relative to other effects.
Jon
Hi Jon,
interesting! I’ll take a few samples (most hand helds are still here) and do a comparison. Never to old to learn!
Hans
Sorry, saturation is the wrong term if its Bipolar it will always in be in saturation but you get the idea……
I may be proved wrong here – everytime I think I fully understand some aspect of RF something happens to prove I don’t. Simple logic though says if two radios are based on the DDS IC then they should have the same phase noise charactistics, if not something external to the IC is happening. Not that these radios have much external to the IC 🙂
Jon