[IC-7000] ALC question
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Excessively high ALC on SSB will produce IMD, which can't be seen in the envelope displayed on a scope. You need a good spectrum analyzer to see the distortion products. Or have a nearby ham with a good receiver tune down the band an listen for your splatter.
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ALC does introduce a non-linearity into the amplitude and phase of the signal, since it is changing the amplitude of the waveform (delayed by whatever the ALC circuit characteristics are). Ignoring high ALC meter readings on SSB is not a recommended practice on any radio. As previosuly mentioned, in constant-carrier modes, the ALC meter reflects how the overall TX loop gain is being controlled to maintain the TX Power Output setting, so full scale ALC meter readings is not necessarily a problem. The ALC circuit is also used to limit TX power during several fault conditions: SWR > 2.5:1, forward power > 105 watts or current draw > 22 amps(due to low inpout voltage or a failing FET). These condtions will increase the ALC voltage bias to the IF stage, which reduces the power to the driver stage, to prevent damage to the PA FET's. So if you are always seeing a high ALC level in SSB, regardless of mic gain settings, it may be due to one of the above condtions. Steve, W3AHL --- In ic7000@..., "ludoradio@..." <ludoradio@...> wrote:
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I don't dispute what you are saying (other than "the ALC display is wrong"), but what you are doing does NOT measure the problem with many modern radios' ALC circuit. You are measuring a static level two-tone signal and seeing that the ALC prevents over-modulation (clipping).
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The problem I am describing results from the DYNAMIC performance of an ALC circuit with too short of a time constant (plus other things), that results in a rapid amplitude modulation of the IF stages (and thus driver and PA). This ALC signal modulation introduces higher harmonics (not related to the baseband audio signal frequencies) that results in a form of IMD that significantly increases the bandwidth of the desired signal, out to tens of KHz. I'm focused on several projects for the next week that limit my time to collect more data and graphs, but for an example of how running one of the currently best rigs (FTdx5000D) in class A mode with no ALC versus 50% (3 dB) ALC, download: This is a 45 minute (39 MB) presentation by Rob Sherwood for the PVRC Webinar series. At time marker 23:11 he begins discussing TX bandwidth measurements using white noise audio, instead of two-tone, to better represent complex waveforms. At 25:52 is a spectrogram of the effect moderate ALC has on one of the best radios. What I'm seeing on the 7K is much worse. If the ALC level is greater than 50%, the TX bandwidth increases to 20+ KHz with some components only 40 dB below PEP. What I haven't looked into is why turning on compression to only 3-5 makes it so much worse, compared to other radios (such as the K3). This isn't just a theoretical problem seen on a lab analyzer. It has been confirmed multiple times in on-the-air contacts, by having the receiving station tune slowly off frequency (using a narrow roofing filter) and listening for spurious emmisions. This problem isn't unique to the 7K either. Many operators run their radios with excessive ALC and they are easily spotted on a spectrum analyzer if their signal is at least an -70 dBm and the band noise isn't too high. The 7K's ALC indicator isn't broken in SSB mode. Steve, W3AHL --- In ic7000@..., "ludoradio@..." <ludoradio@...> wrote:
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Steve W3AHL