Hi Geoff, 
The transformer idea could be made to work , however, you have to take it
apart a bit (you need to change the magnetic assembly from a closed field
(which is how a transformer works ) to an open field (which is how a
solenoid works)... 

If you pull the "Es" and "Is" apart and reassemble the transformer with just
the "Es" all pointing the same way it will sort of work.
"C" core type transformers don't work well!
hope this makes sense to everyone reading this.....

Transformers are typically used with ac signals to change the level or give
isolation to the signal (ac voltage) which incidentally is why they are
called transformers, and while seismic signals are an ac signal, the
frequency of seismic signals is so low (typically 0.01 hz to 20hz) that any
transformer off the shelf just wouldn't have enough core in the right
configuration to respond at these frequencies as a transformer.

Just on the side....

Typical mains (power) transformers in Aust are designed for 50Hz and have
more core and inductance and are physically larger and heavier than the ones
you have in the US which are designed for 60Hz, and likewise motors designed
for Aust conditions have more core (the soft iron metal) than ones designed
for the US.
US transformers and motors don't perform well in Aust, they get hot and
suffer from poor efficiency. Likewise , when Aust motors and transformers
are used in the US at 60Hz they don't like it either..and it's got nothing
to do with northern verse southern hemispheres and that water goes down the
sink in a different direction.....


(Good) Audio transformers typically have enough core in them to work from 20
or 30Hz upwards. They also use different core material.

If you look at a low current relay coil with a pole piece, it has very large
amounts of wire and reasonable amounts of metal core (which is again
different metal to that used in transformers).

These could be used as a sensor for seismic but there are issues with
linearity etc if you have large amounts of movement between the magnet and
the core of the winding, again they are only able to respond to ac signals.

To test this, hook a coil up to a multimeter on dc volts and slowly move a
magnet to and fro, you will see a varying dc signal, which is of course an
ac signal.

Now place the magnet on the end, once the meter settles down, you will see
no voltage produced as the magnetic flux isn't changing. 


It is a problem for all magnetic type sensors, poor dc (or low ac frequency)
signal response.

To a certain extent, the more core the better the low freq response, but
there are practical limits.


Getting back to Instrumentation Amps, why the decision to go down the IA
path?
Do you or do you think you have a common mode noise issue that has to be
addressed?

Blair

-----Original Message-----
From: psnlist-request@.............. [mailto:psnlist-request@...............
On Behalf Of gmvoeth
Sent: Sunday, 3 July 2011 10:52 PM
To: psnlist@..............
Subject: Re: Damping CDR for HS10-1

I tried to use a centertapped step up transformer
but was unable to get that to work.

It was like no signal at all.

I guess you cant just grab a transformer off the shelf
to make such a thing work.

Also tried a centered tapped coil with a magnet
which did work but too complex to make
it is better to make two coils and use then
together. or so it seems to me.

regards,
geoff

----- Original Message ----- 
From: "Mark Robinson" 
To: 
Sent: Friday, July 01, 2011 7:16 AM
Subject: Re: Damping CDR for HS10-1


> Good thread.
> 
> I seem to remember Geoff's rig has a centre tap on the coil.
> 
> How should he connect that?
> 
> Mark
> NZ
> 
> On 02/07/11 00:51, Blair lade wrote:
>> Geoff,
>>
>>   Blair here in Aust,
>>
>> I do lots of (work) stuff with instrumentation amps, matching is
certainly
>> easier using some adjustable components...0.01% resistors are about $30
each
>> here!
>>
>> However, the increase in noise from using a 3 opamp IA config compared to
a
>> traditional single input amp has to be weighed against the better common
>> mode rejection with a true instrumentation amp.
>> If you don't have to deal with Common Mode noise, then don't go down the
IA
>> path..
>> There is much literature on the pluses and minuses of single opamp input
>> verse 3 opamp IA input designs from a seismic point of view.
>>
>> With instrumentation amps (infact with all opamps), there has to be some
>> bias current path from the inputs back to ground, usually provided by the
>> excitation circuitry (strain gauge stuff) but if you are just 'going in'
>> with a floating signal from a coil, you will need to provide some bias
>> resistance / reference to ground, probably a pair of 1meg resistors would
>> suffice , one from each input to ground (0 volts) depending on the IA
input
>> resistance, ofcourse, these could be your dampening resistors, the total
>> value being split and the centre going to ground...
>>
>> I prefer the 1 megs as any mismatch in low resistance values here will
>> affect your CMR a lot.
>>
>> So the dampening resistor is across the inputs with a couple of 1megs to
>> ground.
>>
>> Need also to be careful to balance the capacitance of your input leads to
>> ground as well, but if you are using good quality 2 core screened
microphone
>> cable (with the screen tied to grounded only at the amplifier end) that
>> should be fine for the frequencies of the signals you are looking at.
>> The other end of the screen (seismo end) would normally be tied to the
>> sensor's ground and should not be 'hard grounded'(eg. earth staked)
unless
>> you are expecting lots of trouble from lightning.
>>
>> I'd not bother with having a guard amp driving the screen for seismic
>> signals.
>>
>> With careful design and correct choice of components, CMRRs of 150db at
low
>> frequencies with respect to output are achievable without trimming but if
>> you have that much common mode noise you should do something about it!
>>
>> The 1.25 hz periodic noise... what is your sampling rate and do you have
>> some sort of low pass filter that lobs everything off before you get to
the
>> nyquist frequency (approx 1/2 your sampling rate)?
>>
>> Also note that induced magnetic fields at the coil end are not common
mode..
>>
>> Blair
>>
>> -----Original Message-----
>> From: psnlist-request@..............
[mailto:psnlist-request@...............
>> On Behalf Of Geoff
>> Sent: Friday, 1 July 2011 3:59 AM
>> To: psnlist@..............
>> Subject: Re: Damping CDR for HS10-1
>>
>> I think I just wasted a lot of time
>> trying to get rid of a signal which
>> is real differential and not
>> common mode,
>> there seems to be a machine
>> owned by a neighbor which is not
>> always used.
>>
>> I am totally unable to rid the artifact of
>> about 1.25 Hz which is periodical.
>>
>> Creating the multiple resistor pairs
>> was a waste of time.
>> A pot, most likely, is the beat way
>> to balance the two against the ground.
>> Like 100 Ohm or 10 Ohm 15 turn
>> between two resistors matched already.
>> 1 ohm is the best my DMM can do
>> without help.
>>
>>
>> I have trouble keeping my website
>> to be real time. So its not
>> 100% reliable.
>>
>> I think my website shows the
>> instrumentation amp with the geophone
>> on the negative leads of the instrumentation
>> amp, will have to change that
>> with a new schematic.
>>
>>
>> Regards,
>> geoff
>>
>>
>>
>> -----Original Message-----
>> From: Geoffrey
>> Sent: Saturday, June 25, 2011 8:39 PM
>> To: psnlist@..............
>> Subject: Re: Damping CDR for HS10-1
>>
>> Interesting Bob,
>>
>> But I'm using an instrumentation amplifier.
>> In such an arrangement of three op amps
>> you are using two positive inputs which means
>> the input impedance is mega ohms to giga ohms.
>> The only input is the the resistors which are
>> split against ground. So in my case the you
>> have verified my numbers to be basically correct.
>>
>> I have learned something new to myself in the past
>> few days about this input.
>>
>> There seems to be common mode signals
>> of an electrical nature coming in on the
>> geophone input. The only way to balance out
>> this unwanted signal has been to
>> make several pairs of identical split resistors
>> and see which pair will after installed eliminate the problem.
>> It seems my test equipment can not resolve the measurements
>> fine enough to properly match these two resistors.
>> Therefore it is a matter of chance that the right
>> combination can be achieved.
>>
>> I have never been able to do this balancing
>> act with any configuration other than an instrumentation
>> amplifier.
>>
>> It is my ignorance in combination with
>> people who simply refuse to talk about this
>> which has caused me years of headaches.
>>
>> In my case the Ge seems to reduce to
>> (2.99 * 1302)/1742 or 2.234 v/(in/sec)
>> But this is not how I handle this figure.
>> I treat it as an overall loss of 20log(2.234/2.99) or -2.53dbv
>> when calculating the final amplifier gain.
>>
>> Thanks for your feedback.
>>
>> Regards,
>> geoff
>>
>>
>>
>> -----Original Message-----
>> From: Bob McClure
>> Sent: Saturday, June 25, 2011 6:11 PM
>> To: psnlist@..............
>> Subject: Re: Damping CDR for HS10-1
>>
>> For whatever it is worth, here is my computation of the shunt resistance
to
>> be applied to the HS-10 geophone to obtain a
>> damping coefficient of 0.707. It confirms Geoff's latest results, but
also
>> allows for the loading provide by the amplifier itself.
>>
>> HS-10 properties
>>
>> Sensitivity, E = 2.99 V/ips = 117.7 volts per meter per second
>> Natural Frequency = 1 Hz = 2*PI radians per second
>> Natural damping = 0.031
>> Inertial Mass = 33 oz = 0.936 kilogram
>>
>> Erhard Wielandt, in his chapter "Seismic Sensors and their Calibration"
in
>> the Manual of Observatory Practice
>> presents a formula for electromagnetic damping.
>>
>> The formula is h = (E^2 / 2* M * wo * Rd) , where
>>     E is the output in volt-seconds/meter,
>>     h is the damping coefficient (0.5/Q),
>>     M is the effective pendulum mass in kilograms,
>>     wo is the natural frequency of the pendulum in radians/sec, and
>>     Rd is the total shunt resistance.
>>
>> The recommended total damping is 0.707. Since the HS-10 has an open
circuit
>> damping of 0.031, we want the electromagnetic
>> contribution to be 0.707 - 0.031 = 0.676.
>>
>> so,
>>
>> Rd = E^2 / (2*h*M*wo) = (117.7)^2 / (2 * 0.676 * 0.936 * 2 * PI) = 1742
ohms
>>
>> Let us say the coil resistance is 440 ohms. The input resistance of the
>> amplifier and its applied shunt resistor must then
>> equal 1742 - 440 = 1302 ohms. The 1302 value is that of the external
shunt
>> resistor in parallel with the amplifier input
>> resistance.
>> Say the amplifier input resistance is 10K ohms.
>> 1/Rext = 1/Rt - 1/Ramp
>> 1/Rext = 1/1302 - 1/10000 = 0.000768 - 0.000100 =  0.000668
>>
>> Rext = 1497 ohms
>>
>> The applied load will reduce the sensitivity of the geophone. The output
>> will be Rshunt/(Rcoil + Rshunt) times the open
>> circuit value.
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