[eagle] Re: Eagle 10 MHz Clock
bill at hsmicrowave.com
Wed Apr 11 10:41:24 PDT 2007
Thanks for the clarification on the past command receivers.
I agree with you fully about the frequency agility issue. If it's not
needed for a mission function, then it could be more of headache than a
feature. But then some of the receivers on AO-51 are agile and I'm not
aware of any issues.
On the S2 Receiver - the LO is "not" agile and likewise for the C
Transmitter LO. The prototype is using a synthesizer chip (Peregrine
PE3341) but it's EEPROM programmed before installing onto the PCB. Yes -
I know about the EEPROM radiation issue but I have a fall back position
with the "hardwired" version if it becomes a real problem.
Lyle Johnson wrote:
> Hello Bill!
>> Good points about keeping the command receiver on 100% - no issue
>> there. But I think the matter is somewhat confused (at least for me)
>> since I believe the command receiver that John has designed is also
>> the data/analog receiver. I don't recall that being the case is the
>> past satellites. If command and data function stay combined, then
>> that's all the more reason for considering John's suggestion.
> In the past, the "command receiver" used the same front end as the
> transponder receiver. The difference was an IF tap that was then
> downconverted to baseband to drive a hardware command decoder.
> Thus, all uplink that had command capability had receivers that were
> always on.
> One rule learned the hard way is you *never* turn off a command
> receiver. It's also why command receivers, at least, were never
> frequency agile. It's bad enough to recover a spacecraft if you are
> commanding it in the blind - it's even hardware if you have no idea
> where the receiver might be tuned. So, we always used crystal
> oscillator/multipliers, or PLLS with pins-selection of dividers.
> Using a serial-load PLL adds a lot of risk -- not sure what the reward
> is for the risk.
> Not to say we need to always do what worked in the past, just that we
> should have compelling reasons to change things that relate to
> spacecraft safety and health.
> Lyle KK7P
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