Hello Lyle, Jim, and folks,
The uglier this gets the more interesting I find it! I hope no one is getting disheartened. After all, this is why we all got involved in the first place, isn't it? If it was easy it would be boring.
I just cooked up an idea for a noisy power source that could be used during payload development and test. I stuck my write up and a block diagram at the end of my current log.
Here's the link: http://www.juanr.com/pages/hobbies/ham_radio/Eagle/Spur_Suppression.htm
What do you think?
Shielding the CAN-Do inductor obviously won't solve all these problems but it might buy back 2 inches of space and that's worth the effort.
Lyle,
My suspicion is that the two VCOs on this receiver are also the most susceptible to radiated EMI from CAN-Do. I can't say for sure but it looks that way to me. Both are in metal cans soldered to ground. The closest one was about 2 inches away from the CAN-Do inductor. I've more than doubled that distance to 4-1/4 inches to get away from it. That's a lot of EMI!
73,
Juan WA6HTP
-----Original Message----- From: Lyle Johnson [mailto:kk7p@wavecable.com] Sent: Sunday, June 17, 2007 7:44 PM To: juan-rivera@sbcglobal.net Cc: Chuck Green; David Smith; Dave Black (Work); Dave Black (Home); eagle@amsat.org; Samsonoff@Mac. Com; Juan.Rivera (Work) Subject: Re: [eagle] Re: 70 cm Receiver IF Spurs - Problem Solved
Hello Juan!
I have been watching all of this pretty much in the background for a variety of reasons. IN addition to the CAN-Do! noise that you've noted, we are also faced with a few power system unknowns that are likely to dwarf the CAN-Do! conducted noise:
1) The spacecraft power will probably be coming from a distributed set of sources that will be switching themselves on and off bus, with hysteresis in their switching logic.
2) The various power sources may not be synchronized in terms of their switching frequencies.
3) As a result, well have both switching regulator noise from the regulators themselves, as well as varying source impedance as the various power sources attach and remove themselves from the power bus (due to spacecraft spin, varying power demand from transmitters, ...).
4) The power system is not designed, so it is very hard to characterize it.
From all of this, I think that all modules need to have excellent on-board regulation and assume dirty power in terms of both switching noise and source impedance. I don't have numbers for this, but perhaps a source impedance of 0.05 to 1 ohm is not unrealistic. Similarly, switching noise amplitudes on the order of 1 volt are likely.
If we can shield the CAN-Do! inductor to reduce its radiation, we should. This will be a retrofit, of course. The dynamic magnetic environment is not known at this time, and I think we've been assuming that most radiated energy would be from the antennas, along with "leakage" from the power supply switching system.
***
In a previous life, I had to deal with systems that included radios with VCOs that were susceptible to external fields. In the end, we had to the VCOs had to be shielded to make them essentially immune to such fields.
Since we don;t yet know what the power system will be like in terms of conducted or radiated energy, we have to assume a pretty dismal picture for both of these components and try to design our modules to be insensitive to them to the extent practical.
Lou, any ideas on the power system and how it might look/behave/targets for noise, load regulation, etc? Any measurements available on the cap/cell charge/discharge distributed power system modules?
73,
Lyle KK7P