Automatic DC Correction.

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Automatic DC Correction.

gm3sbc
Hi Jim,
In Version 4.1.36 is is listed that there is now a DC Correction Facility. Is this function to remove the Centre Hump ? I am using a Softrock Radio configuration. I had posted elsewhere that if I use HDSDR I get vitually no disturbance at the Zero Frequency display position. On Quisk I am getting up to 25 dB Notch disturbance at DC?
I am unable to see a difference at the Zero Frequency area of the display compared to an earlier version I am comparing it with ( 4.1.21 ) . Do I need to do anything to enable this new function ?

Regards
Ed GM3SBC
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Re: Automatic DC Correction.

ahlstromjc
Administrator
Hello Ed,

I added the new DC removal to the Soapy devices but I am still using the old DC removal for Softrock. I will work on this for the next release. The new release is being held up by problems making Soapy device transmit work.

Jim
N2ADR
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Re: Automatic DC Correction.

gm3sbc
Thanks Jim,

The DC removal will make a big difference. Thank you for your efforts on providing this application. It gets better all the time.
 
My colleague is working on functionality in other areas and we hope to present this to you at some point in the near future.  It will be based on the Softrock requirements.

Regards
Ed GM3SBC
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Re: Automatic DC Correction.

ahlstromjc
Administrator
Hello Ed,

I tried the new DC corrections on SoftRock but it did not work. The DC was removed but a band of low frequency noise remained. My sound card seems to have very low frequency noise and that produces a peak centered at zero hertz. My previous DC filter was suppressing this noise because it had a width greater than zero.

I am not sure what to do about this. I could keep the old filter and make it variable width. I am currently setting up a different sound card to see if it is different.

I do not use SoftRock in my shack. I replaced it with DDC/DUC hardware like the one at hermeslite.com. There is currently a group buy in progress if you are interested.

Jim
N2ADR
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Re: Automatic DC Correction.

gm3sbc
Ok Jim,

Well there could be mileage in having a variable filter bandwidth, at least for trial. Certainly that technique works well on the HDSDR.
It would be good if you did implement that function as I could give it a test and report back to you on my findings.

Fine on the Hermes. Yes I made one from bare board. It is somewhat limited in that the upper frequency is 30 MHz which means I miss out on the Magic Band (50 MHz) and 70 MHz. I get about 3.8 Watts output, that's with NO low pass filters fitted. I hope to fit your LPF design once delivered. With the additional Insertion Loss the power will be a little lower.

I am using my Scottie Globemaster SDR design. It covers 1.8 to 1300 MHz. 1.8 to 70 MHz at 38 Watts PEP. 144 and up to 1300 MHz at 2 Watts PEP.
It runs at 16 Bits, so great dynamic range. BTW it also has a built in CW Keyer with all the Modes. Speed up to 85 WPM.

It's a pity that the Hermes Lite is a little limited, however for some it will just be the ticket. I certainly enjoyed the build. I may build another as I have another spare PCB.

Regards
Ed GM3SBC

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Re: Automatic DC Correction.

ahlstromjc
Administrator
Hello Ed,

I will add a variable bandwidth zero hertz notch filter to Quisk for the next release.

Just out of curiosity, since you have much more capable hardware, why are you working with SoftRock? I always felt that the amplitude and phase corrections needed to reduce the image on transmit were a problem. I don't know how to adjust for them without a spectrum analyzer.

Jim
N2ADR
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Re: Automatic DC Correction.

gm3sbc
Hello Jim,

Thanks , I shall look forward to testing the DC correction code.

Re Softrock, I am using a DG8SAQ - I2C interface within my Globemaster SDR hardware. The hardware has an embedded 24 Bit TX/RX Audio System. The DG8SAQ interface was originally associated with the Softrock hardware. Hence the link in name only.

Regarding the TX Image Rejection, typical figures are around -70 dBc. This is maintained over a wide range of frequency of operation.
The TX IQ Mixer design uses devices that operate up to 6 GHz thereby providing a very accurate IQ relationship, especially within the DC Bands to 70 MHz. The PCB IQ transmission line propagation delays are matched to a very high degree providing a wide frequency range over which an accurate IQ phase relationship is maintained.
 
Of the small adjustment that is required to achieve the absolute maximum Image Rejection, the Quisk TX Image Rejection adjustment facility is used. Only one stored correction is required on any specific band.
Indeed a correction done on one band will provide more than adequate Image rejection on the adjacent frequency bands. This stability of performance cannot normally be achieved by the likes of standard Softrock type hardware. The same applies to Receiver Image performance.

The two methods I use to detect the Image is either a Monitor Receiver or a Spectrum Analyser. The procedure requires only to be done once.

With Quisk having the functionality to store any required IQ adjustment on both RX and TX, this makes Quisk a very attractive application. Not to mention all the other functionality it has embedded.

Jim, we all thank you for that.

Regards
Ed
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Re: Automatic DC Correction.

ahlstromjc
Administrator
Hello Ed,

Thanks for the information. And congratulations on the -70 dBc rejection, an excellent result. I am going to delay my work on SoapySDR Tx so I can release the other changes I have been working on.

Jim
N2ADR
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Re: Automatic DC Correction.

gm3sbc
Hello Jim,

I have installed Version 4.1.37 and checked the DC Correction facility. I used 10 Hz BW and it is excellent. No sign of any display disturbance  at Zero.

Fine job. Thanks for your effort in providing this. It has made a vast difference, especially when using the waterfall.

Ed GM3SBC