I'll add a specification for spurs such
that that they don't exceed the noise level in the text-mode
uplink bandwidth.
Maximum VSWR can be calculated given the I/O
impedances specified in the document. I could reword this as minimum return loss
in a 50 ohm system.
The image is at 833-836 MHz and this frequency
range is allocated to "Public Mobile" or cell phones. There is no
system-level requirement, but I could create one by assuming that the
maximum interfering signal level will be 1 kW EIRP plus a path
loss 6 dB greater than that at 435 MHz plus at least a 6 dB loss in the
antenna. This makes the interferer 18 dB stronger than a 1 W text-mode
uplink so 24 dB of image rejection should be adequate.
LO leakage is at 634-637 MHz and won't interfere
with on-board systems. This range is allocated to terrestrial TV broadcasting so
it would take a very strong signal to cause interference. We can
set the allowed level to 1 mW.
I have a long-term frequency stability requirement
when the internal reference is used. Otherwise, it's determined by the external
frequency reference. The short-term stability could be added. This would be
determined by the requirements for the ranging system. Who knows what's required
there?
The power system designers should come up with a
conducted EMI number. I'd guess that it won't exceed 1 Vrms over a range of
0.01-100 kHz and 0.1 Vrms above 100 kHz but I could be wrong.
The receiver should be tested for susceptibility to
the three transmitters. The 70 cm antenna will be effective at picking up the 2
m transmitter. Since no one has simulated the antenna coupling, it's hard to
guess signal levels. Measuring the response to 2 W at 145.8-146 MHz might be a
good test. The third harmonic needs to be attenuated as much as possible
and 80 dB should be acheivable so testing with 20 nW (-47
dBm) at 437.4-438 MHz should be adequate. Note that the uplinks have to be
in the lower half of the band, or 435-436.5 MHz, as with previous P3
satellites.
The interference levels from the S1 and C downlinks
should be lower so testing with 200 mW signals at 2400-2402 MHz
and 100 mW signals at 5830-5850 MHz should be adequate. This assumes a
20 dB loss between the transmitting antennas and the receiving antenna.
I didn't realize that I specified two types of SMA
connectors. I should have specify\ied gold-plated brass bodies with gold-plated
contacts for all connectors. This makes soldering easier.
73,
John
KD6OZH
----- Original Message -----
Sent: Saturday, July 28, 2007 04:23
UTC
Subject: Receiver Spec vs. ATP, a few
Suggestions and a Question or Two
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