[hpsdr] ALEX - Call for Comments - III

[Graham Haddock]

All:

Thanks to all for good continuing input and discussion.


General:
Interesting reading suggested by other emails earlier today:
"G3SBI H-Mode Mixer as tested by PA3AKE"
Specifically - preselector comments/measurements on Page 9.
Best I can tell, we are using the improved topology he refers to.



1.) Name...
I'll take offline with Don.



2.) Control...
Lyle's KK7P comments on control lines allowing coupling to the
input and output of the filters and allowing "filter bypass"
are noted.

A question for Lyle or Phil C or someone familiar with the CPLD parts...
If I don't care about output rise and fall time, what is the
largest capacitor (I am thinking 0.01 uF or 0.1 uF, .01 ohm ESR)
that I can throw on the output line of a CPLD?  Will it current
limit and protect itself until the capacitor charges, or will
it trigger life issues after repetitive cycling?

Phil C. - I will research the EPM240 CPLD.  

Also, I am thinking about inverted signals from the decoder, versus
distributed inverters for the RF switches, versus a distributed
SPI decoder and respective impacts on PCB layout and
potential RF coupling.



3.) Filter Design and selection...
I posted the individual filter performance plots on the bottom of the ALEX
page on the Wiki, for those of you that wanted to look at them.

These look like a "contest" or "field day" grade set of preselectors
appropriate for use in "Overload Alley".

Read Phil H's comments on the robustness of the native
Mercury receiver (yesterday).  It does not sound like a preselector
is always necessary on a Mercury.  The story may be different
on other receivers.

I also played around a bit with opening this filter topology out
to about 40 percent bandwidth, (you have to relax the ripple
spec from 0.01 dB to 0.1 dB) and posted a sample of that to the
Wiki, too.

It looks reasonable, which means that a ten section overlapping
filter continuous coverage preselector is also possible, with just
L and C value substitution.  I assume that MW and down would
be a single low pass filter, and 1.7 to 30 MHz would be 9 overlapping filter
sections of about 40 percent bandwidth each.



4.) Physical design...
Nothing to report



5.) Preamp...
The preamp guys came alive last night.  I need a recommendation
for a 6M preamp of known performance/layout to add to the board.



6.) Transient Protection
I don't think that the neon light will work in this application.
The weak link is the Hittite RF switches.  Their spec
sheet does not say, but I can't imagine a GaAs FET
in this part surviving anything over 30 volts.
I think we need to consider biased diodes or something.
Does someone have a circuit of known performance/layout to
add to the board?


--- Graham / KE9H

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