[amsat-bb] Re: P3E transponder and launch

William Leijenaar pe1rah at yahoo.com
Tue Mar 27 11:21:49 PST 2007


I am supprized to see that my mode-uv work for the P3e
is still remembered :o) I wonder what happened with
the mode-uv transponders designed by the other (New
ealand, Russian, German and ??) groups ? Lately I only
read about the SDX work of NA, UK and DL, but what
about the other parts made by other (none NA,UK,DL)
hams ?

>From the messages I feel that the SDX seems to be the
best radio at all, with a larger dynamic range, better
aligator killer, better HELAPS etc...
SDR technology is very interresting and it should be
explored, but somehow I feel that only its advantages
are named. So I like to "flip the count" to its other
side to have "the pair of scales in balance".

About the dynamic range, I wonder how it can be better
than an analog receiver. I suppose the RX side (UHF)
will not directly be digitalized (it would need an
high speed and high power ADC) so there is still an
amplifier and down-mixer needed in front. Then there
is only the IF part where it can be dynamically be
better. I guess with analogue electronics this can
also be done :o)

I also seen someone mentioned about 1970 electronics,
as if todays electronics is only better in IC form.
Todays 2007 transistors are much better than in 1970,
they are much smaller, and have a better performance.
Even analogue style transponders can be made better
and smaller size than those in Ao-10, with todays
transistors. Have a look at my modern analogue
transponder system at: http://www.observations.biz 
When talking about old technology then HELAPS (or
officially called EE&R) is even more older, from 1952

That the HELAPS works better only due to the "modern"
SDR system is not compairing in a fair way. Then you
compair 2007 SDR electronics with 1975 transistor
electronics. I believe with todays modern discrete
electronics the HELAPS system will work equally or
even better.

The SDR system with zero-IF is a nice system which is
very flexible. However zero-IF can have problems with
DC-drift over time what gives unefficient ADC use or
even ADC clipping. It is also more sensitive for low
frequency 1/f noise, and the high frequency digital
processing signals are a serious threat on sensitive
receiver inputs... (also a problems in modern
cell-phone systems)

A Near-Zero-IF system might be a solution but is more
complex to integrate.
The filters at such low frequencies (zero-IF) can be
made very sharp, but the same trick can be done in
analogue way, with an RC low pass filter :o) Several
years ago I present such a bread-bord system as
"Quadrature transponder" where I could easily change
the transponders bandwidth and change from inverted to
none-inverted :o) The difference is that it takes far
less power than a DSP does, and it was extreemly

I agree that digital DSPs are more flexible which is
good when you like to use the same radio for other
modes also.

This was my vote for the analog transponder systems
I hope the digital SDX works well and hope that its
analog sister will also have a chance to do its work
and gets its technical respect...

73 de William

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