[eagle] Re: Receiver Spec vs. ATP, a few Suggestions and a Question or Two
John B. Stephensen
kd6ozh at comcast.net
Sat Jul 28 16:00:57 PDT 2007
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.
----- Original Message -----
From: Juan Rivera
To: 'John B. Stephensen'
Cc: eagle at amsat.org ; Bill Ress ; Dave Black (Home) ; Dave Black (Work) ; Dave hartzell ; David Smith ; Don Ferguson ; Juan. Rivera (Home) ; Juan.Rivera (Work) ; Samsonoff at Mac. Com
Sent: Saturday, July 28, 2007 04:23 UTC
Subject: Receiver Spec vs. ATP, a few Suggestions and a Question or Two
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.
Juan - WA6HTP
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