[amsat-bb] Re: Automatic doppler tracking of DSB

John B. Stephensen kd6ozh at comcast.net
Wed Feb 6 00:45:07 PST 2008

The reason that people looked at SSB with a pilot carrier is that DSB has a 
3 dB penalty compared to SSB.



----- Original Message ----- 
From: "James Whitfield" <n5gui at cox.net>
To: <amsat-bb at amsat.org>
Sent: Wednesday, February 06, 2008 00:57 UTC
Subject: [amsat-bb] Re: Automatic doppler tracking of DSB

> Many thanks to each of you who have replied to my inquiry.
> VE7LDH:  I am curious what got you to thinking about Doppler frequency 
> tracking on the BPSK signals.  Did you consider a specific approach on 
> how?  Had it occurred to you that it might have application beyond BPSK or 
> AO-16?  If you would rather not post the reply, you may contact me 
> directly.
> KD6OZH:  Your reference to pilot carrier technique allowed me to find some 
> interesting web reading, mostly IEEE and patent stuff.  QEX had nothing of 
> interest and the AMSAT journal does not seem to be available prior to 
> 1995.  Pilot carriers seem to have been proven to be workable, but not 
> advantageous in the long run.  I do not have the expertise to demonstrate, 
> but I suspect that a pilot carrier required a minimum power to acquire and 
> maintain lock on the signal and that the more modulation power being sent, 
> the more pilot carrier power needed to maintain the lock.  I will comment 
> more below, but with a Costas Loop, the more power in the modulation 
> information, the easier it is to acquire and maintain lock.  As a result, 
> a pilot carrier consumes power without benefit, while at the same time, as 
> you suggest, accurate satellite data allows the shift to be predicted. 
> Tuning the radio becomes something that can be learned by the "handy" or 
> built into the radio.  From my limit!
> ed experience it seems intimidating and more than a touch expensive. 
> Nothing suggests that DSB will work, or that it might become popular if it 
> did work.
> If DSB had ever been considered, it seems to have been cast aside pretty 
> quickly.  That may have been due to the double bandwidth of DSB.  I was 
> somewhat puzzled that AO-16 had used DSB downlink.  I can only assume that 
> the weight penalty for an SSB transmitter was traded for what was intended 
> to be single channel BPSK.  If the downlink design had been multiple BPSK, 
> it would have required a more powerful transmitter and DSB bandwidth would 
> have been a bigger problem.
> KD6BD and ZL1TYF:   I am glad that I sparked an exchange of ideas.
> My first thought concerning CTCSS is that it shouldn't be necessary.  The 
> time for the Costas Loop to lock should be no worse than the time for a 
> VOX circuit to key a transmitter.  That is an opinion based on faith in 
> the concept and the building skills of someone much better than I.  But I 
> suggest that it would do no harm.  At least no more harm to the equipment 
> than someone using very long 
> between words.  The Costas loop would be able to maintain lock.  It fact 
> it seems to me that the level of CTCSS should be specifically set low, to 
> barely maintain lock.  The engergy to acquire the signal should come from 
> speech, particularly intelligent speech rather than someone whistling into 
> the mike.
> As for a standardized tone, and again this comes from my personal bias 
> that a tone should not be required to make the system work, I fail to see 
> the purpose of standardizing the tone.  The Costas Loop will not care what 
> tone is used.  It could even be a BPSK8 signal at 50 Hz with your callsign 
> ID and latitude / longitude co-ordinates.  I would not suggest using a 
> 1KHz tone unless you expect everyone listening to have a sharp filter, or 
> just listen to it.  Adjust the tone for "minimum lock maintenance signal 
> strength with no audio" to reduce the annoyance.
> You asked about multiple overlaping uplinks.  I am not sure that I 
> understand.  If we are talking about AO-16 with an FM uplink, is this 
> really a problem?  If this was a translating repeater ( sorry if I used 
> the wrong term, I go back before OSCAR, but I am pretty new to this stuff. 
> It took me several hours to find a clear reference to TCA.  Once I did it 
> was obvious that is the point that has zero doppler shift and maximum 
> doppler rate... ) then two signals in the uplink passband would result in 
> two signals in the downlink.  With an FM receiver providing audio to a DSB 
> transmitter, you are going to hear what the FM receiver locks onto.  If 
> that is one strong signal, or the interference between two weak signals 
> that cannot maintain lock, you are going to get one signal to downlink. 
> Here is a slightly ficticious example:  I live in Kansas and tune my FM 
> radio to a frequency unused in the local area, but that is used in both 
> Denver and Dallas.  Most of the time, I don't hear!
>  any music from the radio.  If a meteor streaks over Cheyenne, I hear 
> Denver until the meteor trail fades.  If a meteor comes in over Austin, I 
> hear Dallas.  If the meteor comes overhead, I hear the overlap of Denver 
> and Dallas which is probably unintelligible.
> Using the measured downlink Doppler shift to derive the proper uplink 
> shift.  By theory, the shift is proportional to frequency.  So multiply 
> the downlink shift by the ratio of the uplink frequency to the downlink 
> frequency.  For AO-16, it is a waste of effort since the uplink is FM.  If 
> on the other hand you had a DSB uplink that you were monitoring to correct 
> your shift through a translating repeater, you would a much more complex 
> situation that would depend not only on the uplink and downlink 
> frequencies, but the inverting or non-inverting type of repeater...  The 
> control function math is getting me confused so I think you would need to 
> break the loop somewhere to prevent instability or oscillation.  What my 
> years of flight control development tell me is to design the system from 
> the ground up with each transmitter fixed and just compensate at each 
> receiver.
> If AO-16 is manifesting a residual "pilot carrier" it may be an indication 
> of an imbalance in what I can only presume is a double balanced mixer.  On 
> the other hand if the uplink FM receiver does not have the functionality 
> of a squelch, there may be a "real" DSB downlink of the audio received by 
> the uplink.  The difference would be obvious to a synchronous detector.
> Thank you again for your input.
> James
> n5gui
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