Hello
I am trying to simulate compact geophone model SM-6.
I have seen example model from Analog Devices document titled "Understanding the Fundamentals of Earthquake Signal Sensing Networks".
Page 5 shows model as below, full document is attached here.


I am trying with LTSpice but transient analysis gives below figure. Please assist me to resolve it.
Does anyone has a geophone model.

Thank you.
Lhagva

 

You implemented a 29:1 instead of a 1:29 transformer. The transformer used in the model should be ideal, which means you only need to set the turns ratio (1:29) with K=1. L2 (112.8mH) takes care of the correct inductance of the primary winding.

 

hi i99,
This LTSpice shows the BW response of the geo model.
As noted by Ori0n the transformer was reversed.
E

 

Lp and Ls are arbitrary values erroneously introduced by the OP and are not part of the shown geophone model. The AC sweep shows that the transformer is not usable in the frequency range of interest (1...10Hz). Either use an ideal transformer or set the primary to 112.8 mH and get rid of L1.

 

hi Ori0n,
I take your point, but I always try initially to use the circuits posted by the TS, so that they understand any shortcomings in their circuits and hopefully appreciate any problems.

All too often, helpers replies just criticise the TS circuit, and then state the TS should be using an alternative circuit/method the helper may post.

IMO, this doesn't help the TS understand why his circuit will not work as he expects.

BTW: The TS has used the model test circuit as posted in the Geophone datasheet.

E

 

hi i99,
This is the BW response of the Geo basic circuit d/s compared to LTSpice.
E

 

Recalculate the transformer inductance and add R3.
The transformer probably has a lot of inductance (just a guess).

 

Hi i99
This is a basic test simulation using online quake wav file.
E

Delete the txt extension on the wav file, used txt to enable a download

Added the 2ng image, includes the TXR as shown in post #7....

 

R5 is not needed since the transformer output is gounded.
It was installed in post #7 to prevent singular matrix error because the transformer output was open ended.

 

Hello all
Thank you very much for assistcance. I can run simulation now. What is the role of transformer here?
@ericgibbs Thank you for earthquake.wav file, it gives very nice simulation.
I would like to simulate negative impedance stage to extend the bandwidth of geophone below its natural frequency.
https://www.academia.edu/40257314/B..._5_Hz_geophone_for_seismic_monitoring_purpose
Unlikely no any difference coming up.

 

This is the complete 2 stage.
When I place transformer the signal is distorted.

 

hi i99,
Please post your asc files.
E

 

hi i99,
Please post your asc files.
E

Please check attached files. Thank you.

 

hi i99,
The transformer has a 1:29 step up, so the Vout is limited by the OPA supply rails. [ used a 2k feedback]
The transformer does have an advantage in that it lowers the0dB point from 2.5Hz to 0.25Hz.

E

 

Hello
Thank you very much.
Now I understand the role of transformer. As datasheet explains the sensitivity of the geophone is 28.8 V/m/s. So the ratio is 1:29.

I expect the extended bandwidth as the paper in the post #10 explains.
But my circuit does not extend.
Please assist me to find an issue with my circuit, I attach here my latest .asc.

As you can see on transient analysis result, phase of the current through resistor Rg is opposite to voltage Vgeo, from this point
I think the current is flowing back to geophone, am I right?

 

I think the current is flowing back to geophone, am I right? phase of the current through resistor Rg is opposite to voltage Vgeo

Hi, i99.
In LTSpice the Resistors are polarised.
Press F7 and place the cursor and hold it on Rg , then press and hold Ctrl key as you press the R key.
This will rotate Rg, recheck the current direction.
E

 

Hello
Very interesting, the current direction has changed. Thank you @ericgibbs .
Please note that "F7 is no longer the shortcut in LTspice 24. Rotate is available in any of the "place" modes, i.e. Place Resistor (R)". So I used "place" mode.

But the frequency response is still same, not extending the bandwidth below natural frequency.

 

I expect the extended bandwidth as the paper in the post #10 explains.
But my circuit does not extend.

The circuit in the mentioned paper is not able to extend the bandwidth of the geophone. Yes, you could compensate the winding resistance of the transformer with some negative input resistance (< -375 Ohm) but as the resistive component of the geophone impedance at it's natural frequency is many times larger than the winding resistance it has no effect in moving the lower frequency corner down.

I tried the method described in the following paper to increase the bandwidth down to about 1 Hz, keep the amplitude response flat and also limit the upper frequency corner to below 200 Hz.

https://www.mdpi.com/1424-8220/23/6/3082

 

Hello @0ri0n
Thank you so much for your kind assistance.
The network is performing perfectly! I also simulated it using LTspice, and the results were excellent.
I will input it to fully differential amplifier and then delta sigma ADC.

Best regards,
Lhagva

 

Hello @0ri0n
I would like to ask about phase difference.
The paper shows flat phase of 90 degree from 1Hz to 50Hz.
Please recommend how to improve phase shift.

Best regards,
Lhagva