[TenTec] RX and TX audio cleanliness

[Duane - N9DG]

A radio performance testing item that I think has been ignored for far too long is its “in-band” or recovered audio cleanliness. Nobody is or has been consistently testing this that aspect of radios that I'm aware of.

I have long believed that the cleanliness of the RX audio is a major component of what people are actually describing when they say one radio is less fatiguing than another.

And I'm quite convinced that the cleanliness of the LO, or phase noise in general inside of 1 kHz distance from the carrier is a major component of that listener “fatigue factor”. Because that phase noise that close to the carrier has to be mixing with the desired audio and degrading it. Take notice of some of the radios which have good phase noise traits outside of 1 kHz, but look closely at the slope of their phase noise plots as it approaches the carrier, they rise very steeply. Often much more steeply than some radios criticized for having poorer wider spaced phase noise.

And also AGC behaviors certainly play a big role too. Not exactly sure how that kind of recovered audio quality testing could be achieved with consistency over the years and over different generations of radio technology. And the different signal path architectures.

So how would or could the audio quality test process or protocol deal with the radios that have a lot of AGC parameter adjustments? And then layered on top of that, the interactions of RF or IF gain settings relative to the AGC setting. Remember that not all radio's analog IF or RF gain is directly tied into the AGC system, the Corsair's come to mind, as do most PC-based SDRs. So then there is the aspect of testing for the radios that only invoke the analog AGC when the AGC in the DSP reaches a predefined limit. So the DSP AGC interactions with analog AGC aspects would also needed to be looked at. The testing matrix for all of those adjustment variables would be huge. And as such it would be easy for the tester person completely and inadvertently get results that may not be completely valid due to not fully understanding the details of the radio's overall AGC implementation.

And then there's the issue of IMD contributions by passive components in the signal chain. It is interesting to note that some radio's close-spaced IMD DR performance is now limited by the IMD contributions of the roofing filter itself.

In addition to the cleanliness of the RX audio, these same basic factors and components where they overlap in the TX signal chain would also apply to the in-band cleanliness of the TX audio as well.

Then addition to this audio cleanliness aspects there are the AGC interactions with short duration impulse noise that Rob Sherwood has been calling attention to. That short impulse response testing is complimentary aspect to a radio's RX audio cleanliness in general, even when there is no impulse type noises in the mix.

Duane
N9DG