Newer Model Instrumentation Amp

[[email protected]]

In my ideal world this is the sort of thing the purchasing guys would notice.
"Hey Mr. Engineer -- is there something special about this cap that makes it
worth 10x this seemingly superior one over here?"

With lead times going out into forever, we're just lucky they've not had any
purchasing disasters this year. OTOH, we have inventory of some components
that go out three years, or more.

Although in my ideal world the engineers at some point went over the BOM to do
a little sanity checking on prices as well, and would also notice it. Before
hiring you, I mean, since I imagine it's a product that existed before you
were hired.

Actually it did. ;-) It's used in the football version of the unit you
played with and came out a year before. The good news is that there is only
one per board, the build numbers are in the low tens per year, and the product
has *reallY* fat margins (5-10x that of what you saw).

The real problem is that engineering never saw prices after the original
look-see in DigiKey during design. I asked our admin to dump the purchasing
database by part number and I'm going through the BOM manually (actually, I
placing the cost info into the schematic properties pages). I'm trying to get
the IT folks to feed back the purchasing database into our engineering
database at least monthly, but so far no one is very interested.

If one's not going to use 'K', 'RLY' is a lot better than RL or REL.

I suspect the kids have never used them so make things up as they go. No
reason to use any such abortions. Relays get 'K', period. ...might just as
well use that commie 49R9 and 3K9 crap. ;-)

Abbreviations are kinda strange, though. We occasionally refer to spectrum
analyzers as "SpecAns" for short, but I have a radio that refers to them as
"SpeAnas." Weird...

That's the problem with such abbreviations. Everyone does it differently.

As far as the "SpecAns" goes, how about "FreqDomScope", or better, "that gizmo
over there"?

 

[Joerg]

On Mon, 21 Jun 2010 17:17:02 -0700, "Joel Koltner"

[...]

If one's not going to use 'K', 'RLY' is a lot better than RL or REL.

I suspect the kids have never used them so make things up as they go. No
reason to use any such abortions. Relays get 'K', period. ...might just as
well use that commie 49R9 and 3K9 crap. ;-)

Commie? That's different, got a schematic here but my PC can make those
na'sdarovje letters

I am sometimes tempted to use 4R99. Especially after another call from
someone where it went *PHOOMP* .. *SPLAT*. "I thought it's 4.99ohms" ...
"No, that must have been a speck of toner from your printer, it is 499ohms".

That's the problem with such abbreviations. Everyone does it differently.

As far as the "SpecAns" goes, how about "FreqDomScope", or better, "that gizmo
over there"?

"them thar thang over yonder" (for the guys from the west).

 

[Joerg]

I suppose with amateur radio guys "panadapter" counts too...

But the really tranditional guys would never connect that to the IF,
they'd have a Q-multiplier on there. With a tube, of course

 

[[email protected]]

Does it matter diddly-twat what designators you use, as long it's a
standardization within your own company? Though I would think you'd
adhere to some semblance of an international readability.

I disagree. Schematics are intended to convey information. If everyone
chooses their own language, information is lost. Even if a company
"standardizes" on REL, or some nonsense, it doesn't help their communication
and adds to the learning curve for anyone outside.

For Spice, all that matters is the "template" (to identify
Spice-type). The part designator doesn't have to relate to the
template at all.

You don't see what's going on inside the model. Why should you care? Flat
schematics are somewhat different.

 

[[email protected]]

That sounds like a pretty good strategy.

Not really. Banks hate it.

I didn't realize there were many markets that supported fatter margains than
the pro audio guys. Wow!

Multiply an 87000 seat stadium by a $50 ticket, with free labor and come back
to me. ;-)

Well, maybe jewelry or sporting goods...


That's a useful idea, and if it's automated you could easily run it, e.g.,
every night or so. We're still just working on getting ORCAD CIS tied into
the Big Expensive MRP program; purportedly all the hooks and configuration
bits already exist, but someone needs to sit down and define the actual fields
we want to use, (more importantly) standardized methods of filling in those
fields, etc. -- and then actual start filling in all that data for the parts
that already exist in it.

Only the layout guy has CIS. What good it is for him on 16.2 CIS when
everyone else in on 15.7 is beyond me. The other hardware designer has his
own copy of Orcrap tied to his spice license and I use the floater.

Yep, I think you're right.


We'll occasionally refer to, e.g., "the 8594" or "the 9020a," but if someone
isn't remembering the particular model number, it turns into "the really
expensive specan" or "the el-cheapo specan."

We only have one, and it's in the service department. Well, we do have FFTs
on the scopes, witch is usually good enough for what I need.

 

[[email protected]]

On Mon, 21 Jun 2010 17:17:02 -0700, "Joel Koltner"

[...]

If one's not going to use 'K', 'RLY' is a lot better than RL or REL.

I suspect the kids have never used them so make things up as they go. No
reason to use any such abortions. Relays get 'K', period. ...might just as
well use that commie 49R9 and 3K9 crap. ;-)

Commie? That's different, got a schematic here but my PC can make those
na'sdarovje letters

Yep. Euro-commie crap. ;-)

I am sometimes tempted to use 4R99. Especially after another call from
someone where it went *PHOOMP* .. *SPLAT*. "I thought it's 4.99ohms" ...
"No, that must have been a speck of toner from your printer, it is 499ohms".

We do everything on a plotter (HP DesignJet, actually) so the printing is
pretty good.

"them thar thang over yonder" (for the guys from the west).

Yep, they only talk like that here when there is a Yankee in the room. ;-)

 

[[email protected]]

Is that aluminum? $7 is amazing. We stock a 220u, 25v that's 19 cents.
Tantalum.

I was just reviewing my ADC board and noticed a half dozen 47 uF 25V
tantalum caps, $2 each. We have a 33 uF in stock for 18 cents. Good
enough. Some of the regulators, like 337s and 1117s want tantalums on
their outputs.

We don't use alluminums. They don't like the RoHS process much. Damn
Euro-commies.

 

[Joerg]

That's why I have the "ziggy" line beside my diode symbol when used
for LED's

Same here. But "D" can still throw younger purchasers a curve. "Hey, the
vendor data says it's an LED and not a diode" ... "LED means light
emitting diode" ... "Oh!"

 

[Tauno Voipio]

Hi, Doug,

Those parts do two things:

1. R417+R413+C113 make the first pole of a 5-pole lowpass filter. The
other 4 poles and the ADC are on the next sheet of the schematic. I
used the free TI FilterPro software to design a 5-pole Sallen-Key
lowpass filter and then ripped out the first section and made C113 do
the equivalent rolloff.

2. R417 and R120 and R123 (and their symmetric mates) allow fine trim
of the common-mode rejection ratio (CMRR) of U118. It's spec'd at
something like 80 dB, and we tweak it up to about 120. And hope that
will last.

Turns out that the CMRR trim is mathematically interesting/annoying.
The amount of trim is very nonlinear on the value of R119/R120. I hate
circuits like this, trimmed dividers, bacause they are often messy to
design with available parts.

Gain and offset trims are done digitally, in an FPGA, based on cal
tables stored in an eeprom.

Note that the ADC, AD7699, measures voltage relative to a 2.048 volt
pseudo-differential input pin. So the reference pin of U118 is set to
VCM, namely that same 2.048 volt rail, to shift the ground-referenced
signal up. There's also clamping downstream, so the possible +-16 volt
swing of U118 doesn't blow anything up.

None of this is remarkable. It's just connecting up boxes into
conventional circuits... carefully. A lot of engineering is like that,
declaring performance specs and then implementing it carefully. Once
in a while you get to design something really new and clever.

My first summer electronics job was working for Ed Beeson in microwave
spectroscopy, in New Orleans. I guess there are a lot of Beesons
around.

John

Please pay attention to get a very high input impedance,
to avoid common-mode problems from varying electrode
series impedances. If there is any difference in the
electrode contact impedance, some of the common-mode signal
will be converted to differential signal if it has an
opportunity to create a voltage divisor with the input
impedance.

My experience with ECG and EEG front-ends is that the
contact impedances behave wildly. The experience is
from 30 to 40 years ago, but I suspect that the test
subjects are still of similar construction and design.

 

[[email protected]]

Same here. But "D" can still throw younger purchasers a curve. "Hey, the
vendor data says it's an LED and not a diode" ... "LED means light
emitting diode" ... "Oh!"

How? Parts are ordered by part number, not by reference designator. I doubt
our purchasing people know what an LED looks like.

 

[[email protected]]

Hi Keith,



Hmm, good point.

I've always figured that guys manning the hot dog stands are making pretty
much minimum wage too.

Yep, that $50 doesn't include $5 hot dogs or $7 beer, either. ;-)

[Spectrum analyzers]

We only have one, and it's in the service department. Well, we do have FFTs
on the scopes, witch is usually good enough for what I need.

We have some big boxes that are effectively just fancy RF switch matrices with
filters and amplification included and we're always sniffing around them, it
seems, with spectrum analyzers trying to improve isolation, make sure things
aren't compressing, measuring intermods, etc. For the wireless stuff we
design, as long as I've been here it's always been a "roll your own" approach
using mixers, amplifiers, filters, switches, etc., and often the scopes remain
powered down for days at a time in deference to the spectrum analyzers -- we
haven't had a project yet where we felt the wireless radio modem "modules"
were the best option, even though they certainly have many compelling
features.

Unfortunately, it's becoming harder to find general-purpose radio parts like
this -- particularly if you're after low-power operation. If you look at,
e.g., I/Q modulators, most of them are around a watt! -- Fine for
basestations, not so great for anything strapped to your hip. The iPhones and
Razrs and whatnot of the world are driving this sort of thing to a
system-of-chip approach...

We don't do our own RF, so our needs in that area are much smaller. There was
a proposal to do a hip-strapped base station but RF exposure was an issue.
IIRC, we're just under the limit.

 

[[email protected]]

When I've placed orders with, e.g., Newark in the past, over the phone, I'd
rattle off a list of part numbers, and then they'd read back part numbers and
descriptions. So if your BOM just says, "D3, Avago AV672" and the order-taker
reads back, "Avago AV672, green LED," it might raise an eyebrow.

The inventory control system will have all of the ordering information. Parts
aren't ordered from the schematic or even the BOM. The BOM (and production
forecast) just feeds the inventory control database as a gazouta.

I'm confident out purchasing people know what LEDs look like.

You're kind. ;-)

These days most parts purchasing is done over the Internet, but a few things
still end up being ordered by phone.

But not from a schematic. The reference designator is simply a key to the
various databases.

 

[[email protected]]

Gee, some day perhaps you could become a customer then, if you guys decide to
aggressively pursue certain markets where at least some people believe the
"all-digital in the ISM bands" approach isn't the best choice. Heh, heh...

The problem with that idea is that it indeed seems to be the *right* choice.

Do you guys have any fancy digital audio matrixing products? I was amazed
just how sophisticated some of the commercial ones out there appear to be,
e.g., this one by Rane: http://rane.com/mongoose/ .
http://rane.com/jpg/mongoosetracker.jpg

Nope, but those are slick. Our partner does the wired stuff. We're almost
all wireless (there is some wired stuff at the bottom end).

You can probalby do OK if you get people to weat hardhat-mounted antennas,
like this: http://www.ka7exm.net/Dayton2007/IMG_3791.JPG . (From the "odd
headgear" section here:
http://www.ka7exm.net/Dayton2007/hamvention_photos.htm )

I suggested a beanie with an automatic antenna rotator to solve our multi-path
problem in domed stadiums. ;-)

 

[[email protected]]

Given some time you might decide that there are additional markets out there
that it isn't currently the right choice for.

I doubt it, at least while I'm there. 2.4G isn't the only game in town,
though.

Heck, nothing you have nor anything we would is probably the right choice for,
say, a small venue like a church that has limited funds and is going to be
looking at more budget-priced options. I'm kinda surprised that companies
like Shure -- who seems to make endless varieties of wireless microphone
systems at very attractive price points -- don't get into wireless intercoms.
Besides the pro-audio guys, the only other major players seems to be
restaurant-oriented where you have the likes of HME and 3M.

Right, churches don't have a large crew that needs to communicate behind the
scenes. In fact we do compete with HME at the low end. With 2.4G digital.
Wireless microphones, per se, aren't of interest though.

 

[JosephKK]

K is the ANSI/Mil designation for a relay. OK, I'm old fashioned and
don't just make up stuff like LED2, TR5, RLY12, CON2, RV7, RN8, ZEN15,
or other abominations.

John

And a lot of it comes from translations from Mandarin or Cantonese.

 

[Paul Keinanen]

On Tue, 22 Jun 2010 18:27:51 -0700, "Joel Koltner"


I suggested a beanie with an automatic antenna rotator to solve our multi-path
problem in domed stadiums. ;-)

In that picture, there appears to be three different length antennas,
so probably for different frequency bands and hence frequency
diversity could be used to combat multi-path.

An array of four antennas if often used in electronic direction
finding application, so mechanically rotation is not needed.

A helmet is sufficiently large for spatial diversity or even MIMO
systems down to lower UHF and possibly even upper VHF.

 

[[email protected]]

In that picture, there appears to be three different length antennas,
so probably for different frequency bands and hence frequency
diversity could be used to combat multi-path.

An array of four antennas if often used in electronic direction
finding application, so mechanically rotation is not needed.

A phase array wouldn't have nearly the visual effect of a beanie on the head
of a 350# football lineman.

A helmet is sufficiently large for spatial diversity or even MIMO
systems down to lower UHF and possibly even upper VHF.

Spacial diversity doesn't help.

 

[Paul Keinanen]

Spacial diversity doesn't help.

If you have severe multipath problems, one of the worst modulations is
narrow band AM/FM, since it is quite likely that a single antenna is
at the multipath null, where the multipath signals cancel each other
completely.

The area at which the signals cancel each other is quite small and
gets smaller with higher frequencies. Having two or more antennas at
some distance from each other (e.g. on different part of your body),
it is unlikely that _all_ those antennas would be in multipath nulls.
With multiple antennas and multiple receivers, just sum the audio
outputs, but squelch those receivers that have a weak (noisy) signal
at the moment.

Of course some more sensible modulation method, such as DSSS, TDMA or
OFDM would make more sense in an environment with heavy multipath
even with a single antenna.

 

[JosephKK]

Given some time you might decide that there are additional markets out there
that it isn't currently the right choice for.

Heck, nothing you have nor anything we would is probably the right choice for,
say, a small venue like a church that has limited funds and is going to be
looking at more budget-priced options. I'm kinda surprised that companies
like Shure -- who seems to make endless varieties of wireless microphone
systems at very attractive price points -- don't get into wireless intercoms.
Besides the pro-audio guys, the only other major players seems to be
restaurant-oriented where you have the likes of HME and 3M.

---Joel

None seem to be trying to cover very cost conscious markets, but for
smaller venues; traditional PA vendors, manufacturers of Assisted
Listening devices (that is the first stage search string, it gets the
manufacturers, then you look at their full lines) seem to be plentiful.

 

[[email protected]]

If you have severe multipath problems, one of the worst modulations is
narrow band AM/FM, since it is quite likely that a single antenna is
at the multipath null, where the multipath signals cancel each other
completely.

The time difference off the ceiling and the direct route is almost the same as
our symbol rate. Things get ugly fast. Without the ceiling things are great.
;-)

The area at which the signals cancel each other is quite small and
gets smaller with higher frequencies. Having two or more antennas at
some distance from each other (e.g. on different part of your body),
it is unlikely that _all_ those antennas would be in multipath nulls.
With multiple antennas and multiple receivers, just sum the audio
outputs, but squelch those receivers that have a weak (noisy) signal
at the moment.

It's not a matter of nulls, rather the receivers can't lock onto the signal.

Of course some more sensible modulation method, such as DSSS, TDMA or
OFDM would make more sense in an environment with heavy multipath
even with a single antenna.

I think the radios are DSSS, with TDMA networking protocol. The receivers
can't stay locked to the transmitters so they constantly drop off line.
Doesn't happen at 900MHz.