Re: Open HPSDR Hermes (14 bit ADC, EP3C25) Transceiver Card
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Try listening in the 80m DX window in New England – AG1J uses a ~60 year-old SSB rig. Wide hardly starts to describe it, I think it’s a phasing exciter with obligatory after-burner.
Simon Brown, G4ELI
From: main@SDR-Radio.groups.io <main@SDR-Radio.groups.io> On Behalf Of Conrad, PA5Y
Sent: 25 February 2022 09:56
Subject: Re: [SDR-Radio] Open HPSDR Hermes (14 bit ADC, EP3C25) Transceiver Card
By clean I mean - can I hear anything of sufficient magnitude to disturb the noise floor anywhere in band, and hence inhibit my ability to receive signals close to the noise floor. The noise floor in a semi-rural location on 28, 50 and 70MHz is not particularly low so I do not feel that this is unreasonable. The biggest problem with the IC7300 is the AM noise bump at 20kHz and 130kHz which is only 60dB down on the carrier when running at 30W on 28MHz. This power level is typical when driving an amplifier. When modulated with an SSB signal this occupies a considerable amount of bandwidth. The close in PN -s only -120dBc/Hz at 1kHz and -130dBc/Hz at 10kHz. The composite noise is -115kHz at 10kHz, this is not what I would consider clean, even close in. The K3S( and a few others) is far better in this respect although the PA has quite poor linearity, especially on 6m.
The Hermes will do a much better job with any decent PA, even without pure signal. There are some spurs @ -65dBc which improve with a higher sample rate. I have not checked these with the V2 firmware but I will do immediately I receive my Hermes from Apache.
I do not think that a transceiver at the IC-7300 price point has fixed pre-distortion. As you quite correctly say keeping it under control over temperature and voltage variation would be difficult. Also Icom would almost certainly have mentioned it in their advertising. What the IC7300 does well is manage audio overshoots by using a ‘look ahead’ algorithm. This is maybe where the impression of it having a clean TX comes from.
I think that the problem here is that so many transceivers are quite bad, so I can just about accept that the IC-7300 is less bad.
OK, please refresh my mind what you mean by "clean". It certainly appears to be sending something much closer to pure signal concentrated within its intended bandwidth than most other transmitters. I also understand that it is not as good as it can be with full active predistortion. Both might generate increased noise at some significant separation from the intended signal frequency. I am not sure of the mechanism by which this would take place.
On 20220225 01:13:11, Conrad, PA5Y wrote:
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