On 7 Aug 2024 at 15:30, Black Michael via wsjt-devel wrote: > We noticed a phenomena on the IC-7300 where the AGC is interfering > with SuperFox decoding. I'm interested in seeing if anybody else sees > this on other rigs. May explain why RR73's are being missed.With AGC > on slow we see large variations in the signal strength.With AGC off > these variations disappear. > > Top one is AGC off -- bottom is AGC slow which is 3 seconds for the IC-7300 > by default.
Operating with a rig's AGC off is fine as long as you can be ABSOLUTELY SURE that medium to strong incoming RF signals won't drive some part of the receiver chain into overload, which in turn could/will cause the generation of unwanted and undesirable artefacts on the audio signal that's being fed into WSJT-X. For example, I know by rigorous measurement that, with AGC off, the audio output from my present IC-7610 and also its IC-7600 predecessor begins to overload and becomes distorted when the RF input signal reaches a level of around -85dBm, which equates to an S3-S4 signal. Measurements made by Rob Sherwood NC0B on his IC-7610s have shown almost identical results to mine. The presence of distortion on the audio signal going into WSJT-X is presumably undesirable... On the HF bands the minimum discernible signal (MDS) of my IC-7610, as measured by the late Adam Farson AB4OJ and also by me, is around -126dBm with the preamp(s) off so an incoming signal of -85dBm is only about 40dB above that MDS level. Assuming the audio levels are set in accordance with the suggestion made in the WSJT-X user documentation, ie. to give a reading of 30dB when receiving only band noise (which could easily be 10-15dB higher than the rig's MDS), then it is entirely conceivable that the rig's audio output could become distorted when the level shown on the WSJT-X audio level 'thermometer' is around the 60-65dB mark, ie. well below the clipping threshold for the audio ADC used by WSJT-X. The PDF document created by Rob NC0B that's available via the following link makes for interesting reading: http://nc0b.com/wsjt/ Based on my own findings and also those by Rob NC0B as documented in his paper I have for the past several years operated my IC-7610 with AGC on, with a fast time constant set to 0.8secs. Furthermore, I adjust the audio levels to give a WSJT-X 'thermometer' reading of 68dB when receiving only band noise. The rig's AGC ensures that even on the strongest incoming signals the reading on the 'thermometer' never exceeds about 78dB which is below the level at which ADC clipping is indicated by WSJT-X. Also, the IC-7610's front-panel 'OVF' annunciator, which indicates clipping of the rig's front-end ADC, never illuminates because the gain of the overall receiving chain is being controlled by the AGC. Finally, I haven't yet copied N5J since they started but using the settings I've described above seemed to work entirely satisfactorily when I was copying and working the K8R SuperFox. -- 73, Martin G0HDB -- This email has been checked for viruses by Avast antivirus software. www.avast.com _______________________________________________ wsjt-devel mailing list wsjt-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/wsjt-devel
