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