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R_Misra
R_Misra
02:30
Is there anyone here who is willing to help me with an Eaglecad problem? I would make a thread, but it's something very simple that I don't think is worth a thread
But I can't figure it out for the life of me
Shalvenay
Shalvenay
throw it out there
R_Misra
R_Misra
Also, how do I scroll up on this chat?
Oh, the arrow keys
W5VO
W5VO
scroll wheel?
R_Misra
R_Misra
02:48
Also, I remember seeing a room where I could message the moderators. Does anyone know where I can find it?
 
12 hours later…
Asmyldof
Asmyldof
14:20
@PlasmaHH Tell that to Lao me.
 
3 hours later…
Branden Boucher
Branden Boucher
17:04
Hey all.
Is it possible to get a +200mV offset on an op-amp if I used an op-amp with null offsets?
 
4 hours later…
PlasmaHH
PlasmaHH
21:26
Lights flickering... didn't see that in a long time here...
KyranF
KyranF
22:16
question - if I have three possible signals (of known fixed frequency) 250kHz apart (channel spacing I guess) and need to extract from the input signal which of these three are currently being received at any time, and sometimes even simultaneously, how would you do it?

My current idea is to gain + buffer the incoming signal (modulated light received by photodiode) and then pass it through 3 parallel 2nd order active band-pass filters, designed for the three expected frequencies and channel spacing
then the output of these three band-pass filters will just go to a low ( few hundred mV) comparator for becoming digital enough for an FPGA to read which channel is active
Shalvenay
Shalvenay
@KyranF -- you are almost there! what you need in front of the comparator is some means of envelope detection -- you are basically building 3 AM radios in parallel ;)
KyranF
KyranF
anything I should be doing here? I'm new to this kind of hands-on analogue design. Should I be trying to use a Frequency-To-Voltage converter IC and just do some comparator stuff straight out of that? Perhaps feed the signal into a Frequency Divider chip and then into the F-to-V converter chip
Shalvenay
Shalvenay
I wouldn't faff with F-V conversion here
KyranF
KyranF
@Shalvenay i see,
@Shalvenay what do you mean envelope detection? like a form of hysterisis on the input to it?
Shalvenay
Shalvenay
@KyranF basically, AM detection/demodulation -- if you want to be sophisticated about it, you can use a calibrated RF power detector stage there even
KyranF
KyranF
22:23
k i just read the wiki on that topic
looks like a great idea
thanks mate
that would make a much better signal for the comparator too
than what I was planning on lol
Asmyldof
Asmyldof
22:52
@KyranF starting to sound to me like a DSP issue
KyranF
KyranF
well once it's in the FPGA there will be plenty to do with timing and logic, but I need to get the signals to the FPGA as digital, and reliably (hehe) and have the photodiode circuitry as sensitive as possible (within reason)
im sure there are many ways it can screw up :(
SPICE simulating is only getting me so far too, SPICE only works realistically up to a point and even then its not that reliable. It's faster for me to prototype in SPICE than it is to work with hardware anyway, while i'm tinkering around with various approaches to this problem
Asmyldof
Asmyldof
nobody is more a fan of doing stuff in analogue hardware than me... well not many people.
But tuning drift and such... I'd prefer tearing the signal apart with a DSP
KyranF
KyranF
well, i can get the signal to a strong 0-3.3V signal easily already, im up to the band-pass filtering but as you say, it's reasonable enough to just do inter-pulse timing or i suppose an FFT and check for freq with highest power
another of my issues is there are up to 15-20 of the circuits I am designing, in parallel, and all need to be processed simultaneously (that is why so much hardware, and an FPGA)
@Asmyldof do you have any suggestions on how i'd do an FFT for signals of 4.5-5.5MHz? Is there a useful IC or particular MCU that you've used that can do this? The signal would be something like +-1V in a kind of sinusoidal shape, but that's just a shit quality square wave at 4.5, 4.75, 5, and 5.25 Mhz
Asmyldof
Asmyldof
23:38
It depends too much on the specific signals to be able to really say
Most likely I'd go for a Floating Point / DSP enabled MCU, or if your FPGA has enough pins equip it with a bunch of MSPS ADCs
Case of the MCU, at 32bit, I'd say you'd want 80 ~ 120MHz with the right architecture and DSP or very good floating point operations, or 180+MHz for just floating point and we'll see how good
DSP might be able to go down to tens of MHz if the DSP core is very good at its job
bed time now
Spent all last night dancing my poop hole off at an event in Windsor and foolishly strolled around London all afternoon and evening... I'm spent
KyranF
KyranF
@Asmyldof good night old chap
I can get hold of an ARM Cortex M4 with DSP, 120+MHz (STFM4 has that covered right?)
I guess my main issue is i need 15-20 of these in parallel, i really can't have an ARM M4 looking at each sensor output, that's absurd
also only 2.4MSPS on the built-in ADC, so that's not fast enough :(

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