Newer Model Instrumentation Amp

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.

This will be connected to fairly low-impedance things: power busses, current shunts, signal conditioners, transducers, maybe raw strain gauge bridges. The initial use is for aircraft structural testing. Its DC input impedance is 500K to ground on either side. This wouldn't be suited to EKG or electrochemical sorts of things.

John

I think there are a couple thousand Beeson families around (but only two in Canada that show up in the phonebook, anyway). We pretty much all descend from the Edward Beeson who emigrated to Pennsylvania in the late 1600s.

I understand the lowpass filter section now. But don't R120 and R123, in parallel with R417, alter the filter characteristics when you trim R123? Or have you built enough attenuation overhead into the other sections that it doesn't really matter?

have

The frequency rolloff of C113 takes into account the Thevenin impedances resulting from all the stuff that's parelleled; the 25Ks in the INA matter more than the 412K resistors. It's a 5% cap, so there's no need to get too precise here.

Turning the pot has a tiny, probably unmeasurable, effect on the frequency response.

John

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

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.

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.

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

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

---Joel

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.

You're kind. ;-)

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

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... :-)

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:

You can probalby do OK if you get people to weat hardhat-mounted antennas, like this:

---Joel

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

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).

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

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

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

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.

Hiding under your bed.

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

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.

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

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.

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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.

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. ;-)

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

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.

So we are talking about 1-3 MHz, is this the actual data rate or the spread spectrum chip rate ? At 2.45 GHz ?

In a SS system, the spreading chip rate should be several percent of the absolute frequency to be effective against multipath.

Regardless modulation method, some general precautions against multipath:

Is it possible to install the base station antenna on the inside of the dome itself ? This would generate a strong direct (dome, field) signal compared to reflected (dome, field, dome, field).

If the base station antenna can not be installed in the dome itself but it has to be installed at ground level or somewhere in the audience, at least make sure that the radiation pattern above the horizontal plane is heavily attenuated, reducing power radiated into the dome (or received from the dome). A metallic shield (or mesh) above the antenna might be usable, but of course, it creates early reflections from the shield, which may be (or not) harmful depending on the modulation method.

Don't lock at all or sometimes drops out of lock ?

How long does it take to relock ? Is this time longer than an other antenna with separate receiver typically maintain lock with a moving receiver ?

There are several ways to implement spatial diversity, e.g. on the base band level switching between full receivers or simply RF antenna switching when the received power drops on current antenna.

To speed up relocking one might also notice that since at least one receiver usually remains locked, this information can be used to limit search options on the unlocked receiver.