[eagle] Proposed 70 cm Receiver plan - Let's get this project going!
juan-rivera at sbcglobal.net
Fri Aug 10 12:43:54 PDT 2007
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
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!!
Juan - WA6HTP
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