[eagle] Re: IMD, Phase Noise, a New Spur, and EMI suggestions
John B. Stephensen
kd6ozh at comcast.net
Thu Jun 28 12:25:39 PDT 2007
The receiver wasn't designed to be linear with a full-strength PAVE PAWS signal in the passband. The effect of strong signals needs to be measured so that the in-band dynamic range (blocking and two-tone) can be determined, but the receiver will only operate linearly with radar pulses that are not aimed directly at Eagle.
PAVE PAWS signals offset by 750 kHz or more from the center frequency should have little effect (less than 1 dB increase in noise). This does require that the signal generator have low phase noise at this offset. The receiver first LO phase noise at a 750 kHz offset must be less than -132.4 dBc/Hz to acheive this so the phase noise of the signal source must be less than -138.4 dBc/Hz to acheive a 1 dB measurment error. It should be much lower so that we can see the true characteristics of the receiver.
The spur at -134 dBm could be a problem for text-mode uplinks so its source needs to be determined. Is the internal switching regulator on?
Table 4 in the new requirements document provides a good summary of expected signal levels at the input and output of the receiver. Spurs may need to be lower than originally assumed as the text-mode uplinks are very low power.
----- Original Message -----
From: Juan Rivera
To: 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
Cc: Juan.Rivera (Work)
Sent: Thursday, June 28, 2007 04:36 UTC
Subject: [eagle] IMD, Phase Noise, a New Spur, and EMI suggestions
I've started a new log here.
I've repeated some preliminary phase noise tests using a simulated PAVE PAWS signal, along with a simulated Ham uplink signal. In the process of looking at those signals with the SDR-IQ I discovered another spur directly in the middle of the IF. I don't know the exact source but it is generated inside the receiver, and definitely not from the CAN-Do module.
I've also come to one general conclusion that I would like to bring up for discussion... I believe all switching power supplies on Eagle should be running at frequencies as high as possible - around 1 MHz would be a good ball park. That would put most spurs outside the passband of the payloads, and make EMI filtering much easier. The 5 kHz CAN-Do switching power supply needs to be completely redesigned in my opinion. I believe that if it is not cleaned up the impact will be devastating, and the full scope will likely not be known until integration and test when it is too late to do anything about it.
Having said all that, I also believe that we need to come up with EMI standards for everything on Eagle. Specifically we need:
1) A definition of the noise amplitude and spectral content to be expected from the Eagle power distribution point
2) A repeatable way to reproduce that noisy DC source in our labs for testing
3) A simple and effective way to test to the spec that we create
I think the way to make this work is to create a noise source that consists of several representative switching power supplies that are intended to be flown on Eagle. The exact makeup of this test fixture would be specified in the EMI requirements. Then, all payloads would need to be designed to operate without impairment while powered by this noisy power source. The EMI requirement would also spell out exactly what constituted impairment. They also need to keep conducted noise leaving the payload down to some specified level. More on that in a minute...
Here's a possible layout for a noise source:
The one ohm resistors account for lead resistance and allow the three switching power supplies to all modulate the power bus, along with whatever conducted EMI comes back from the devise under test (your payload.) It would be easy to crank out three or four of these noise sources for distribution to groups that need them. That way everyone would be testing using the same conducted EMI.
The gold standards for EMI are MIL-STD-461 and -462. They break EMI into four categories:
· CS - Conducted Susceptibility (the 70 cm Receiver suffers from this)
· RS - Radiated Susceptibility (the 70 cm Receiver suffers from this)
· CE - Conducted Emissions (The CAN-Do Module is guilty of this)
· RE - Radiated Emissions (The CAN-Do Module is guilty of this)
This noise source would provide a way to test for compliance with our CS requirement. RS and RE can be dealt with by shielding. That leaves CE. We'd need a way to insure that noise coming out of a payload is within spec. We'd need another test fixture for this.
I hope this stimulated some discussion. I'm eager to hear what you all think...
Juan - WA6HTP
Via the Eagle mailing list courtesy of AMSAT-NA
Eagle at amsat.org
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