On 05/21/2018 01:59 PM, Dan CaJacob wrote:
Hey Marcus,
This may not be what you are looking for, but these are awesome. They
may be going to space soon :)
http://www.newedges2.com/products/SAX239-R2_Datasheet_February-27th-2018.pdf
Interesting. Not quite what I was looking for.
Basically just doing a design-tradeoff study for grins.
Fixed sample rate and filter/decimate in a "sized-for-economy" FPGA, or
variable sample-rate, and a chain of tuneable analog filters in front of the
ADC.
A chain of 3 or 4 2nd-order active-lowpass filters using video op-amps
would probably work OK. Use digipots to set the R values, and use a fixed
value for C, or perhaps selectable C as well (probably only two or
three values).
Or, a small number (4?) of selectable L-C-R low-pass filters, and fix
the sample rates to a small number.
On Sun, May 20, 2018 at 10:13 PM [email protected]
<mailto:[email protected]> <[email protected]
<mailto:[email protected]>> wrote:
I used to work at Maxim, but my dealings with those RF guys dealt more
with coffee and the quality thereof.
I looked at the Maxim chip. The filter is relatively steep but not
complex. Looks like two or three poles maximum. I don't know
specifically about that chip, but that group had SiGe technology, so
I'm leaning towards variable transconductance to do the tuning.
You quickly learn just how bad google is at doing a search for this
technology. The buzz phrase you need is continuous time, so I suggest
"variable continuous time baseband filter". Sadly OTA gets links to
"over the air." (And yet, they claim artificial intelligence will take
over the world.)
The thing to note in any highly integrated analog chip is that you
don't see the sausage being made. Once you have a system on a
chip, the
metric is the system performance, not the performance per se of any
individual block. So those filters may not be as great as you think.
Note the Maxim part shows a tempco on the corner frequency, which
could
imply variable transconductance. One you have bipolar elements in the
process, variable transconductance is just a matter of tail current.
(SCF performance was relatively temperature independent.)
Getting back to seeing the sausage, take the old analog modem market.
Initially the SCF tech was used to make the official Bell 212 filters.
Once the modem was fully integrated, the on-board filters were
simplified for a number of really good engineering reasons, not just
cost. One was the harmonic distortion of the band split filters. The
harmonics of the in-band signal were far greater than the out-band
signal that you were trying to reduce.
Getting back to these baseband filters, if you use a ladder
design, the
filter is relatively immune to component error, well as opposed
(contrasted) to a chain of biquads. If you go full differential, the
variable transconductance amp are reasonably linear. But probably they
limit the number of poles for the same harmonic distortion problem.
Filters should subtract, not add.
Most of the transconductance based filter designs probably are
derivatives of integrated video filters. Plenty of papers online for
those designs.
If your goal is to roll together your own filter, TI has app notes on
how to make analog tracking filters.
On Sun, 20 May 2018 21:19:47
-0400 "Marcus D. Leech" <[email protected]
<mailto:[email protected]>> wrote:
> On 05/20/2018 09:13 PM, [email protected]
<mailto:[email protected]> wrote:
> >
> > Can you be more specific about the corner frequency?
> Corner frequencies step-tunable from perhaps 20Mhz down to perhaps
> 2MHz.
>
> Many chips, like the R820T2, the MAX2112, and the
higher-integration
> devices like the AD9361 and LMS7002M have
> programmable analog low-pass corner frequencies, for bandwidth
> tailoring of the complex baseband.
>
> Just not sure how its done. The chips have very low external parts
> count, so whatever it is, it's got to be done
> internally...
>
>
> >
> > Cell phones use chips that have switchable banks of capacitors for
> > antenna tuning. st.com <http://st.com> IIRC is a source.
> >
> > I used to design switched capacitor filter chips in the
80/90s. The
> > technology was killed by oversampled converters and DSP. The SCF
> > players went into continuous time video filters using
> > transconductance amps and such.
> >
> >
> > _______________________________________________
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> > [email protected] <mailto:[email protected]>
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>
>
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--
Very Respectfully,
Dan CaJacob
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