[eagle] Re: 70 cm Receiver IF Spurs - Problem Solved

Lyle Johnson kk7p at wavecable.com
Sun Jun 17 19:44:22 PDT 2007

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?


Lyle KK7P

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