Re: Open HPSDR Hermes (14 bit ADC, EP3C25) Transceiver Card


I am not clear what you mean by "between the modulation".

The full feedback process for improving modulation characteristics attempts to make the transmitted signal as close to a perfect rendition of the ideal waveform as possible, This will probably reduce noise as well as signal distortion. It may even show a close to the carrier phase noise improvement depending on the design. When the SSB modulation goes silent the transmitter should go silent within the capabilities of the feedback loop. Outside the loop bandwidth there MIGHT be a small increase in the transmitter's noise floor.

With fixed predistortion (such as learning the precise D/A transfer function) the signal generation process is improved which will reduce IMD products and such. It may even allow for digitally compensating for the transmitter's linearity. This MAY need frequent calibration as it may be sensitive to supply voltages and the like. Temperature changes will almost assuredly alter the curves. But it will not make any material changes to the local oscillator noise characteristics.

I've noticed in the past that the average ham transceiver has nothing near the signal level to noise floor level characteristics of the receivers. That is largely a separate issue that will take some serious design efforts to change it. The goal of the whole exercise should be to make at least the next adjacent channel usable from a co-sited antenna with the transmitter running full power and as small a spacing between the two antennas as possible. There is a lot of room for effort here. And at present to my somewhat dated knowledge the transmitter is the most fertile ground for this effort at this time.


On 20220225 00:55:01, sm6fhz wrote:

Simon and Conrad.
It seems to me that you maybe are talking about two different parameters when it comes to a clean Tx-signal from the IC-7300.
I get that Simon is referring to splatter from the intended modulation.
I interpret that Conrad is looking at composite noise from a carrier.
I guess you are both correct.
If some kind of static Pre-Distortion is improving the the splatter levels from a SSB transmission it is very nice.
Still, the composite noise will be there in-between the modulation and also outside the modulation BW.
Splatter and composite noise will for sure have very different QRM characteristics on the band.
Can it be that the composite noise seen, is an artifact of the Pre-Distortion process?
Just a I have never been deep diving into Pre-Distortion engineering. But usually there is a price to pay, if you are not very cautious with the details, when engineering a solution.
I am sure someone here is more into this and can explain.
73 / Ingolf, SM6FHZ

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