'WWVB CMAX Chip?'
Sean Breheny wrote:
> I suspect that your limiting factor may be the effect of pulsed noise
> on your high-Q filter. You might actually want to use a noise
> blanker-type circuit (detects spikes and prevents them from passing
> through into your narrowest filter, to prevent ringing).
Thanks - I'm not familiar with noise blanking so I'll have a look into that.
Switched capacitor filters have also been mentioned to me, but I've never
used those before either; possibly a more compact way than using LC or
op-amp filters, although they do have the disadvantage of requiring a clock
which may contribute noise into the system.
Olin Lathrop wrote:
> You're off by a factor of two and you're oversimplifying. You make it sound
> like a L-C filter passband suddenly cuts off at a particular frequency. In
> practise it has fairly steep sides, but the "width" of the passband depends
> on how much attenuation you need.
I should probably have qualified it with some sort of attenuation figure; it
did sound like a brick wall response now that you mention it. The filter I
currently have on my breadboard has a -40dB bandwidth of about 10KHz according
to the software I used to design it; in practice it has fairly steep rolloff,
but difficult to measure with my equipment - my function generator doesn't
have a frequency read-out which makes it awkward.
> I've never read AN1115 nor do I plan to. This is basic signal processing
> folks, with pretty simple math. You'd be a lot better off understanding it
> than treating it like black magic describe in a sacred scroll. There's no
> need to wave dead fish here.
AN1115 is far from a sacred scroll, but it does detail a method very similar
to what you originally described; so it was a logical document to include in
my further reading on the implementation details.
> They would, but then using a FIR is silly in the first place. How did you
> get FIR filter in your head? I certainly never said anything about using
> one. I was envisioning a simple one or two pole low pass filter with each
> pole realized by:
> FILT <-- FILT + FF(NEW - FILT)
AN1115 again ! The basic architecture they present is the sin/cos
multiplications followed by a FIR lowpass, decimation, and another FIR
lowpass. FIR also seemed logical since they are (generally) linear phase
from what I understand. It seems like I have missed something very simple
As a side note, I did get something working yesterday in Octave using the FIR
filters and successive decimations - 4 FIR filters with orders 12, 20, 30 and
40 (decimations of 3, 5 and 5). Too much processing for a single ISR pass,
but I was looking at implementing it iteratively (as a state machine) which
should have left lots of cycles left over. But that gets very complicated
and a nightmare to update should you change the filter characteristics. It
also takes a fair amount of memory. I'll revisit the drawing board looking
at simpler filters as you have said; it's not so easy to see when you are on
the wrong path if it is not one you have trodden before.
> I agree, but such a filter will need careful tuning. Even then you have to
> think about drift over time and temperature. That may be perfectly fine for
> a one-off hobby project if you're willing to tune it once a year and keep it
> at a constant temperature.
That's not the sort of solution I'm looking for - it should just sit there
and work, day-in-day-out. I had something similar originally but it didn't
seem clean or reliable; there's something about using trimmers in such an
application that is decidedly unsatisfying. This is one reason why I'm
looking into your DSP idea - it promises to keep the hardware simple at the
expense of more complicated (yet more flexible and updateable) firmware. And
I get to learn lots of new and exciting things along the way, some of which
I'm hoping will sink in.
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