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Re: [linrad] OP amps and antialiasing was...a simple front end
- Subject: Re: [linrad] OP amps and antialiasing was...a simple front end
- From: "John Harrison, NI1B" <jmh6@xxxxxxxxxxxxxx
- Date: Mon, 11 Aug 2003 06:30:05 -0400 (EDT)
Gotta get some of them :)
One other problem I discovered.
If the in-band signal gets too loud into the audio board you get
overload and distortion. You can easily tell this by monitoring the audio
samples. Linrad could tell the front end to put in more attenuation.
What about slightly out of band signals? It turns out that there is,
as far as I can tell, *NO INDICATION* from the sound card that the input
is overloaded by one or more out of band signals :((.
Thus, a peak type level detector is needed between the front end and
the audio board.
Another possible "solution" is to just use more bits. i.e.- 24 verses
So far I don't have an elegant solution to that problem.
In practice, it only seems to be a problem on afternoons when 20M is
open and out of band commercial broadcast stations are very loud. However,
one also has a potentially serious problem if their are any strong local
signals, such as at a field day site.
On Sun, 10 Aug 2003, w3sz wrote:
> Hi, John et al!
> Thanks for the note.
> The OPA 2227 is rated at 3 nV/root Hz.
> Note that that is nano and not micro.
> The AD797 is rated at 0.9 nV/Hz, or slightly less than 1/3 the noise of the OPA 2227.
> The AD797 also has THD of -98 to -120 dB.
> Other 'common' op amp specs I am aware of include:
> LM833 4.5 nV/ root Hz
> OP275 6 nV/ root Hz
> TL07x 18 nV/ root Hz in ascending order of noise.
> So the OPA 2227 is good, but the AD797 is excellent.
> I found the 797's very easy to use, and had no problems or instability.
> I had no significant aliasing problems for two reasons:
> 1. I used IF filters before going 'wideband' with the Delta44.
> 2. I originally used ISA Soundblaster cards and then went to the Delta44, which has
> a reasonable [not perfect] anti-aliasing filter built right in.
> Here are Leif's comments on the Delta44 anti-aliasing filter, from:
> ----start quote
> The builtin filter of Delta44
> The -3dB point of the builtin filter is at 46.3kHz and the attenuation is only 10dB
> at the Nyquist frequency 48kHz. Adequate attenuation, -60dB is obtained at 52kHz
> making the useful range 0 to 44kHz.
> Low pass filter with notch
> Since the builtin filter is perfectly ok above 53 kHz the additional filter does not
> have to attenuate above this frequency. To get a very steep filter around 48kHz
> notches are used, see fig.1. Having the components at hand, the easiest way of
> producing notches is to use LC series links. Filter design in the audio range is
> conventional engineering and many other ways are possible to realise a filter with
> notches slightly above 48kHz. The useful frequency range is increased by about 2kHz
> by this filter and it is certainly questionable if it is worth the trouble of adding
> it to increase the total bandwidth from 88 to 92kHz (4.5%)
> -----end quote
> So you can do reasonably well with a Delta44 without additional filtering, but do get
> a bit of extra bandwidth by adding external antialias filtering.
> So, again, start simple and add on as you go. This makes getting everything working
> easier and less intimidating, and you get to see the benefits of the added complexity
> as you create it. And when a problem crops up, you have a pretty good idea of where
> it is. I find that fact particularly helpful!
> For practical purposes, there were additional references to anti-aliasing filters on
> Leif's site on my most recent previous listing, and the QRP site URL in that post
> also had data that would enable one to start playing around with low-pass filters.
> These URL's were:
> Below are some low-pass URL's that might [or might not] be of interest.
> ----- Original Message Follows -----
> > Hi all,
> > Regards low noise. I have an I&Q with diode mixers running on 40/20m. I
> > finally wound up with a TI part IIRC 2227 p/n or so. It has 1.3uv/root
> > hertz noise. Was 10-20 db quieter than til082.
> > Regards Bandwidth into audio card. This is a MUCH more serious problem
> > than it first appears.
> > Older ISA audio cards, no problem. They all seem to have good
> > anti-aliasing filters.
> > On many PCI (3 or 4) that I have tested, there is *NO* anti-aliasing
> > filter on the board, *AT ALL*. Motherboard audio seems to be particularly
> > bad this way.
> > Given the difficulties of building discrete audio filters, finding a
> > board with real filters in whatever chip is probably the easiest approach.
> > That said, at the higher rates (96k and 192k) the chip filters have been
> > designed with *MUCH* too high a cutoff frequency for use in I&Q designs.
> > This limits a 192k part to quite a bit less.
> > So far I have not found an easy workaround, though an additional digital
> > filter on the output of the ADC appears possible.
> > warm regards,
> > john, ni1b
> Roger Rehr