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RE: LONG and long overdue report



Hi Drew

> As someone primarily looking in from a user/member point of view I would
> like to offer two comments. First, I'd hate to think we are going to hang
> our entire mission on just the SDR transponders. Wouldn't it be prudent to
> include at least a bare bones analog backup transponder? Please tell me
this
> is the plan.

Coincidentally, this is what we're doing over here in Blighty with David
G0MRF's transponder http://www.g0mrf.freeserve.co.uk/transponder.htm and my
optional DSP IF http://www.g6lvb.com/stella.zip (13Mb) that we're presenting
at this weekend's AMSAT-UK Colloquium. The conventional IF is switched in by
default, with the DSP IF optional. Like Bob's design, it's also based on the
original SDR-1000 design, using the quadrature oscillator, quadrature
exciter and detector I plagiarised from Gerald Youngblood's SDR-1000 design
here (thanks Gerald!)... http://www.g6lvb.com/images/img_2945.jpg (700Kb).

The main differences are that in order to maintain compatibility with
existing 10.7MHz transponder IF's, and to reduce power consumption
requirements, it uses a fixed 10.7MHz LO rather than a DDS. It also uses a
low power TMS320VC33 DSP with an expanded 180kHz passband bandwidth,
although for demo and development purposes it can be plugged into a PC's
soundcard. Drivers for PC soundcards and their Windows drivers generally
have maximum 3dB points around 20 to 23kHz, irrespective of the sampling
rate, limiting passband bandwidth to about 46kHz if using quadrature
detectors and exciters. Depending on the hardware CODECs themselves, many
will support 3dB points upto 0.45Fs to 0.48Fs where Fs is the sampling rate.
Sampling in quadrature using the the Cirrus Logic CS4272 device with a
dedicated DSP will support a full duplex dual channel 192kHz sampling rate
giving a ~180kHz passband bandwidth.

> Second, I'd like to encourage that software development not lag
> too far behind the hardware. I'm not a big fan of the idea of writing the
> code once it's in orbit. One does not have to look too far to find where
> AMSAT has oversold or overpromised features based on writing the software
> after launch. Please take these comments in the spirit with which they
were
> offered, with only my desire to see Eagle as succesful as possible.

I agree - although it is often difficult to know all the necessary
parameters until it flies, in particular the nature of Radar style pulses.
My personal opinion about devices such as RUDAK is that they're only
accessible and usable by a small inner circle of developers, and there is
not enough information available for third parties to be able to develop
something on their own hardware on the ground for possible inclusion later.
I raised this a few years ago, asking what was in RUDAK for Joe Ham, and
based on the responses then, and in practice on AO-40, I still believe that
the possibilities for experimentation for your average Ham are limited to
say the least. My transponder DSP IF code is actually developed on a PC
first and then ported to the TI TMS320VC33 DSK (although with an upgraded
CODEC), and as such should provide accessibility to those wishing to try out
new code for possible inclusion at a later date.

Neither Bob nor I realised that we were both working on similar goals until
yesterday, so I am sure that we will have a lot to learn from each other!

73, Howard G6LVB
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