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Re: D700 TNC Calibrate mode etc (II)



Danny,
9600 BAUD , NRZ, is the data rate, which is twice the frequency rate (4800 
Hz) when a 1,0,1,0,1,0, is sent. We look for transisitions not cycles. 9600 
Baud FSK is direct FM ie: a low is the carrier frequency (-deveation), a 
Logic 1 is carrier frequency (+deveation). The transistions should be 
filtered and Gausian in shape ( 1/2 a sine wave at 4800 Hz), also known as 
raised cosine.
This is now called GFSK, and most transmitters have the filtering built in 
to make the transitions gausian. Problems arise when the radios (receive or 
transmit) are not DC coupled. In the case of 0,0,0,0,0,0,0,0,1 the receiver 
data output must not drift back to the center frequency voltage, but must 
remain the same low voltage after the first 0 was sent and stay at that 
voltage until a 1 is sent. Level shifts can be caused by either the 
transmitter or receiver. DSP can be used to process data filtering out these 
problems but this causes a lag, in the processing time.
This is the simple explaination, but in the real world synthesized 
transmitters, the use of  phase modulation in some transmitters instead of 
frequency modulation, and Doppler shift, make this a more difficult problem. 
Some sort of peak detector for 0&1's for Automatic Frequency Control, direct 
connection to the receiver discriminator for received data, and a fractile N 
synthesizer that shifts and holds transmit frequency for a 1 or 0 will solve 
the above mentioned problems.
Good luck with the project,
Art KC6UQH

"Danny Jacobs" <jacobs@physics.montana.edu>
To: <amsat-bb@amsat.org>
Sent: Saturday, June 03, 2006 10:07 AM
Subject: [amsat-bb] D700 TNC Calibrate mode etc (II)


>   Is anyone on this list familiar enough with the Kenwood D700 to
>   describe exactly what the Calibrate function of the TNC does and/or
>   describe exactly how the modulation is done at 9600.
>   I am attempting to close a 9600 digital link between the D700 on Tx and
>   a CC1000 FSK chip radio on Rx.   I have control over many demodulation
>   settings such as FSK seperation frequency and peak detect in the CC1000
>   but I dont have much information on the standard Ham FSK modulation
>   that I think the D700 is doing in 9600 mode. I can easily close the
>   link between two of the chip radios.
>   So here are my questions:
>   1) Is the D700 doing true FSK in 9600 bps mode?
>   2) If it is, what is the seperation between mark and space?
>   3) Is a scrambling polynomial being used, (i am trying 17 bit register
>   with bit 5 as tap)
>   4) Is the Calibrat mode in the TNC a scrambled square wave?  Has anyone
>   gotten this to work?
>   5) I have read in several articles oblique references to standards for
>   FSK links but havent been able to find any.  Where are they?  How do
>   different Ham radio manufacturers build radios that can talk to each
>   other?
>   This is part of a University satellite project. We could use matching
>   chip radios to talk to our satellite but we want very much to provide a
>   friendly packet-style ham interface to our operation, however we are
>   finding that interfacing to the ham equipment quite difficult.  Please
>   Help!
>   ~Danny KE7DHQ
>   ~~~~~~~~~~~~~~~~~~~~~~~~
>   Space Science and Engineering Laboratory
>   Montana State University
> ----
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