Digilite Transmit with low SR

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F6DZP
Posts: 217
Joined: Fri May 27, 2011 9:59 am

Digilite Transmit with low SR

Post by F6DZP » Sat May 05, 2012 8:29 pm

Hello,
I wish to use my Digilite with low SR.
I can go down to SR1000 without problem, but for values between 1000 and 500 I can't find how to get a good sync.
With values below 1000 I always get constant resync.
I don't understand why it runs very well at 1000 and can't run at SR 900, two values very near.
Has someone tested these values?
I wish also to go below SR 500 and the software doesn't allow that. Perhaps in the future?

Jean Pierre F6DZP

G4EWJ
Posts: 748
Joined: Wed Feb 17, 2010 10:11 am

Re: Digilite Transmit with low SR

Post by G4EWJ » Sun May 06, 2012 12:04 am

DigiLite Transmit will work at any symbol rate between 0.5MS and 8MS by hitting CTRL-U on DigiLite Config and entering the required value. It's just mathematics. I will remove the lower limit of 0.5MS in the next version.

The serialiser cannot work at all symbol rates. The cycle period of the symbol rate (e.g. 250ns at 4MS) has to be a whole number (3 to 23) of dsPIC clock cycles. The dsPIC clock frequency has to be one which it is possible to achieve using the multipliers and dividers in the dsPIC phase locked loop.

There are a number of symbol rates programmed into the serialiser. Not all of them are on the list in DL Config, but there are none below 1000. If the serialiser receives a command for a symbol rate below 1000, the LED will blink in groups of 3 to indicate that it does not understand. The serialiser symbol rate will stay at the last valid value. That is why it resyncs continually.

I will look at putting some lower symbol rates into the next version of the serialiser. 466.66666kS and 500kS are both possible.

Brian

F6DZP
Posts: 217
Joined: Fri May 27, 2011 9:59 am

Re: Digilite Transmit with low SR

Post by F6DZP » Sun May 06, 2012 4:47 am

OK Brian, thank you.
But I don't understand your 2 sentences :
1>DigiLite Transmit will work at any symbol rate between 0.5MS and 8MS by hitting CTRL-U on DigiLite Config and entering the required value
2>If the serialiser receives a command for a symbol rate below 1000, the LED will blink in groups of 3 to indicate that it does not understand. The serialiser symbol rate will stay at the last valid value. That is why it resyncs continually.

The first sentence says that I can use SR between 500 and 1000.It is what I have done :I have tried yesterday afternoon, using CTRL U, SR900, SR800, SR500 and always found resync problem.
The second sentence say that if I ask for an SR below 1000, the serialiser does not understand and stay at the last valid value.
All my tries yesterday confirm the second sentence. I firstly tried SR 500 but impossible to work, always resync.

Does that means that, for the moment Digilite transmit can work with SR below 1000 but the serialiser cannot?

Jean Pierre

G4EWJ
Posts: 748
Joined: Wed Feb 17, 2010 10:11 am

Re: Digilite Transmit with low SR

Post by G4EWJ » Sun May 06, 2012 11:09 am

>Does that means that, for the moment Digilite transmit can work with SR below 1000 but the serialiser cannot?

That is correct. I should have made the lower SR limit of DL Transmit to be 1000. That would be more consistent.

DL Transmit can do ANY symbol rate between 500 and 8000. The serialiser can do ONLY symbol rates that have been programmed into the dsPIC software. E.g. the serialiser can do SR1000, but it does not know how to do SR1100.

It would be more flexible to be able to send to the serialiser, the parameters to do any possible symbol rate and I plan to do that in future version.

In the meantime, If you (or anyone) can tell me which symbol rates you would like to be available in the next version of the dsPIC software, I will see if they are possible.

Brian

F6DZP
Posts: 217
Joined: Fri May 27, 2011 9:59 am

Re: Digilite Transmit with low SR

Post by F6DZP » Sun May 06, 2012 12:38 pm

Ok it's clear now.
With low SR there is a dark side:

When we take lower SR we reduce bandwidth and get more efficiency considering analog HF transmission.
But when we take lower SR, demodulators chips have more problems to decode QPSK and digital SNR(MER) go down.
In reality there are to graphs : if we take very low SR, one graph show that we get better CNR and the other one show that we lost digital SNR.
That's why some demodulator can't decode even SR 1000.

For low SR values, I have tested two kind of demodulator chips : 2d generation STB0899 and 3d generation STV0903/STV0900

With STB0899, absolute lowest SR value that can be decoded is between SR 400(OK) and SR 380(not OK).(my result after testing 3 PCI cards)
If we consider the 2 graphs, we can see we earn something until we reach SR 750 after that if we take lower value, we lost more (MER) that we get (CNR)

With STV0903, absolute lowest SR value that can be decoded is between SR 270(OK) and SR 260(not OK).(my result after testing 2 x TT S2-1600)
I have not create the two graphs in this case, but I suppose we can go down to about SR 500 with more efficiency and lost if we continue to go down.

So for the low value I will be happy to find : a value around SR 750 and another : SR 500.
These 2 steps under SR 1000 will permit to test and practice in this part of the SR possibilities.
SR 500 if we use receiver with 3d generation demodulator chips like STV0903, and SR 750 for 2d generation like STB0899. SR 500 can be received by both kind of demodulators

Jean Pierre F6DZP
(future Tutioune version will work with STV0903 with TT S2-1600 PCI card, I have begun to develop with this new chip))

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