All:
If there's any interest, I WILL be on at 2100 EDT. I see one other
check in.
Agenda would be Symposium and general status update.
I've just been too busy to send out agenda before now, sorry. Way too
many balls in the air right now.
As always, anything else anyone wants to discuss...
73,
Jim
All,
The 70 cm Receiver has been stuck in a rut for too long. Now that I've
completed my Symposium presentation I'd like to get it back on the tracks
and moving forward again. Here's the way I see it:
Passband Ripple - we want to understand this problem before proceeding with
the next upgrade revision so we don't end up with another prototype that
still has excessive ripple. John will design a new PCB that will act as a
test fixture for the 199 MHz 1st IF string. Parts will be distributed to
John, Bill Ress and to me. We will all work this problem in parallel.
The PCB will also contain test fixtures for any other portions of the
receiver that we want to characterize independently of the receiver. The
various fixtures can be cut apart once the PCB is fabricated. I'd suggest
that the proposed analog replacement 10 MHz reference be included. I'll run
that in my temperature chamber.
CAN-Do Module - No one has volunteered to work the power supply issue so
I'll do it. It's my belief that the existing step-down converter is running
at a much lower efficiency that anyone realizes. I'll test this theory and
report back. It appears to be a generic characteristic of this type of
converter that the efficiency drops off towards zero at very low output
currents. As I've discussed in my logs, this one is only putting out 11
milliamps. This could also explain the motorboating that I have observed.
So far I have not found any other switching step-down converter that is
optimized for this low output current. This problem needs to be resolved
now since it impacts the design of the enclosure. If we go to an analog
regulator the CAN-Do conducted and radiated EMI problem will be completely
eliminated. If we stay with a switching step-down converter then we may
either need to move to a two-compartment enclosure or vacate the front third
of the existing enclosure to escape the CAN-Do radiated EMI.
The Enclosure - I put forth a recommended enclosure requirement calling for
a worst case PCB flex of 0.0084" in any one-inch segment. No one has made
an argument against this. I do not believe that the current sheet metal
enclosure with attached self-clinching standoff mounts and three separate
heat sinks can meet this proposed requirement over the proposed temperature
range which extends down to -60*C. With the current design that attaches
the CAN-Do module to the main PCB via the 40-pin header and then fixes the
CAN-Do module to the front panel, there is no room for any flexing or
twisting between the base plate and front panel since those induced stresses
will be transferred directly to the solder joints on the 40-pin header. The
sheet metal vs. milled enclosure questions needs to be resolved.
Low temperature Operation - The minimum operating temperature of the SAW
filters is -35*C and the minimum storage temperature is -40*. For the IC's
the minimum operating temperature is -40*C and the minimum storage
temperature is -65*C. These temperatures are not just guesses on the part
of the manufacturer. They are a function of the difference in thermal
coefficient of expansion between the various internal parts of the devices.
Operation below the minimum operating temperature will cause premature
failures to develop in short order. The worst case scenario would be to
power up the Receiver after is had cold soaked to -60*C. The silicone would
heat up very quickly and begin to expand. The thermoplastic case material
would lag behind and expand at a different rate causing large internal
stresses. I have not seen a final word on the minimum temperature that the
Receiver will see in orbit. If it really is -60*C then I propose that a
space-rated thermal switch in placed in series with the DC power to prevent
activation below -40*. Even this thermal switch proposal side-steps the SAW
filters which cannot withstand -60*C. The proposed work around of splitting
the receiver into two sections adds complexity which reduces reliability. A
way needs to be found to keep the receiver from dropping below -40*.
External EMI from the primary power source - The next revision of the
Receiver needs to include EMI filtering and possible shielding to deal with
radiated and conducted EMI from all sources. These sources need to be
identified and characterized so that the Receiver can be designed properly.
It makes no sense to press on with the next revision upgrade until this is
sorted out.
Receiver Requirements vs. ATP - The ATP contains tests for which there are
no requirements. We need to harmonize these two documents and do a new peer
review for both.
Let's get these items resolved!!
73,
Juan - WA6HTP
Team:
A few weeks back, I requested updates for the Journal article, due now.
I've received info from Juan Rivera, Bill Ress, Bdale Garbee, Marc
Franco, and Bob Davis.
Everybody else, please send me something before 1800 9 August, when I
will start writing. This is the last Journal before Symposium, so it is
our chance to provide reasons for members to come.
Thanks 7 73,
Jim
wb4gcs(a)amsat.org
John,
Per your request I have modified the input coupling to the 2nd Mixer as
follows:
* Remove C37
* Replace L15 and L16 with 16 ohm resistors
* Install 0.27 µH inductors from pins 2 and 3 of U8 to ground
I then retested the IF passband ripple by sweeping a 1 MHz span centered on
10.7 MHz. The output level was reduced by 3 dB but there was no change in
the ripple. Click HERE
<http://www.juanr.com/pages/hobbies/ham_radio/Eagle/passband_ripple.htm> to
view my log.
I think we need to look outside of that coupling area since weve pretty
much replaced every component. Lets replace both SAW filters next. What
do you think?
73,
Juan
John,
I took a few minutes to look over your new specs and compare them against
the Acceptance Test Procedure. I've got a number of tests in the ATP for
which there are no specs:
* Image rejection
* Internally generated spurs
* Local oscillator leakage
* Input and Output VSWR
Items that need to be addressed that aren't in either document:
* EMI conducted susceptibility
* EMI radiated susceptibility
* Short and long-term frequency stability
I also take issue with the -60C minimum power-down temperature. I think
this is unrealistic just from a CTE mismatch perspective. The reliability
of anything subjected to that wide a temperature spread is going to suffer.
A way must be found to raise that temperature.
I have a few thoughts... The CAN-Do switching step-down converter is only
supplying 11 milliamps. If we take a slight efficiency hit we could just go
to a simple linear regulator and completely eliminate the radiated and
conducted EMI emission problem from CAN-do. That eases the EMI filtering
and shielding requirements for every single payload. That seems like a good
trade-off to me.
Rather than worry about trying to conduct heat through a PCB why not just go
to externally mounted regulators for the CAN-Do and the Receiver right on
the case itself. We're not that pressed for space. That eases the heat
sink and the associated thermal gap filler issues. EMI will still be an
issue, but only for external sources instead of one that is inside the
enclosure itself. That strikes me as a huge bonus. Why not design a
single-sided PCB with the regulators hanging over the edge and the whole
assembly, PCB and regulators, mounted directly to the enclosure. Stick it
in with the CAN-Do module in a separate cell. Filter all the signal and
power through the common bulkhead.
I'm also curious as to why you are specifying two different types of SMA
connectors.
73,
Juan - WA6HTP
This year's CSVHFS meeting wound up tonite in San Antonio. Among the
many AMSAT folks in attendance were W4MO, W7LB, W5IU, W3XO, N4HY, K5SDR,
W5SXD, N6TX, KL7UW and K3IO. There were no satellite specific papers
presented this year, but the world of DSP and SDR was very much present.
K5SDR presented his new SDR5000 radio. KL7UW (with the SDR-IQ) and W5LUA
(with the Softrock-40) reported on the use of SDR receivers for various
VHF/UHF/Microwave tasks.
AMSAT highlights of the meeting came at Saturday Night's banquet. K3IO
accepted a financial contribution from CSVHFS for AMSAT. And then the
crowning glory: Bob McGwier, N4HY was presented the 2007 John Chambers
Memorial Award for technical achievement, recognizing his many SDR and
AMSAT activities. Bob joins previous AMSAT Chambers Award recipients
W0LER, W0PN, W3GEY & K3IO.
Congratulations to Bob -- 73 de Tom, K3IO
Gents:
I need your input NOW. I have to write an article this week for
submission.. Please tell me what you have done, what you are doing, and
what you are going to do. I can speak from memory to U-rx., S2-Rx, and
CAN-Do!. In need inputs from IHU, Power, structure, and lab.
Thanks & 73,
jim
wb4gcs(a)amast.org
Yes, we are talking with Intelsat. They have told us there is no reason
to keep this is a big secret. Just do not spread it around all over the
place until we have our deal set.
Bob
--
AMSAT Director and VP Engineering. Member: ARRL, AMSAT-DL,
TAPR, Packrats, NJQRP, QRP ARCI, QCWA, FRC. ARRL SDR WG Chair
"If you're going to be crazy, you have to get paid for it or
else you're going to be locked up." Hunter S. Thompson
