[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
> 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
> 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.
> Sent via AMSAT-BB at amsat.org. Opinions expressed are those of the author.
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