# need help in connecting Lock in amplifier

Discussion in 'General Electronics Chat' started by matsci, Feb 4, 2009.

1. ### matsci Thread Starter Member

Feb 4, 2009
10
1
Burnse005 likes this.
2. ### KL7AJ AAC Fanatic!

Nov 4, 2008
2,189
421

I'm not sure how you plan on controlling a signal with Hyperterm, but I can tell you all about Lock-In amplifiers. What sort of signal are you looking at?

A few more details would be nice.

eric

Burnse005 likes this.
3. ### matsci Thread Starter Member

Feb 4, 2009
10
1
Thanks eric,
I am supposed to measure third harmonic voltage (3ω). To give some detail, I would like measure thermal conductivity of film by 3-omega technique. We lay a metal strip over the film which will act both as heater and thermometer. We apply current of omega through this wire when the heat will generate at 2 ω and the temperature oscillates at 2 omega and hence the resitance at 2 omega as the metals resistance is proportional to temperature. Finally the output voltage which is a product of current ω and resitance 2ω making up 3ω is measured which has the information of thermal properties of the film. We have this lock in amplifier to measure this V-3ω.

I hope you got some idea what I am to measure. I do have some main circuit. Anyway first I would like measure voltage at fundamental frequency with this amplifier using some dummy circuit with some resistors. I really do not know how do I make one and where to connect. Once connected, I can then learn to operate with this hyperterminal so I get used to it. I would be happy if could help me with dummy circuit and connections so I can measure the voltage at fundamental frequency.

In the website posted above, we can see connecting points. I can give details of these connectors from manual:
1) A and B/I signal input connectors. A connector is the sugnal input connector for use in single-ended and differential voltage mode. B/I for use in differential voltage mode (A-B) and also when current input mode is selected.
2)OSC out connector-for internal oscillator
3) REF IN connector- for general purpose external reference signal

The above connectors in the front panel and at the back of the panel, I have the following connectors:

4) REF MON connector-the signal here is a TTL-compatible wavefor, synchronous with the ref. This output monitors correct reference channel operation.
5)REF TTL connector-is provided to allow TTL compatible pulses to be used a the ref input
6) SIG MON connector-the signal at this connector is that immediately prior to the main analog to digital converter and after the pre-amplifier, line filter and anti-alias filters (these are componets- I dont know if this helps to understand the connector)
7) CH1, cH2 Connectors- the signal here is an analog voltage corresponding to a selected output, such as X, Y, R, θ etc
8) TRIG connector-accepts a TTL-compatible input and can be used for triggering the auxiliary Analog to Digital Converters. The input operates on the positive edge only
9) ADC1, ADC2 connectors-The input voltages at these connectors may be digitalized using the auxiliary ADC's and read either from the front panel or by the use of a computer interface command
10) DAC1, DAC2 connectors-the output voltages can be set here
11) FAST X, FAST Y connectors-the signals at these connectors are X channel and Y channel O/P signals derived from the point after the first stage of O/P low pass filtering.

We have then this RS232 connectors, GPIB connector, digital outputs connector (for controlling auxiliary apparatus like lamps, shutters etc) and PREAMP power connector (used for poweing remote preamplifiers)

Is it possible to help me.

4. ### KL7AJ AAC Fanatic!

Nov 4, 2008
2,189
421
Am I guessing correctly that you're using an SR-830 Lockin amplifier? If so, now I understand where Hyperterm comes in. You might find that, in the long run, it's easier to use GPIB than the serial bus. But, in any case, the commands aren't too complicated. (We used Labview to control our Lockin amps in the UCLA plasma lab...very nice combination!)

The biggest problem you're going to have is to make sure you're ONLY getting the 3 Omega frequency back out. Is there any chance the 1 omega reference frequency is going to be mixed in with your result? I imagine your experiement is going to be in the millihertz frequency range or less.

Measuring your fundamental frequency is a snap, if you just want to test your amplifier. You can just take your reference output and loop it right into one of the signal inputs. Are you interested in phase as well, or only magnitude?

You can run the inputs in either single ended or differential mode...I don't know if that's an issue for your experiment.

Also, if you have the manual on the SR-830, it's very good. All the commands are in the appendix.

Hope this helps some.

eric

5. ### KL7AJ AAC Fanatic!

Nov 4, 2008
2,189
421
Hi Matsci:

I did a bit of research on the 3 Omega method, and learned that frequencies in the kilohertz range are often used. I had NO IDEA thermal phenomena could happen that fast! Well, this makes our job MUCH simpler.
What you WILL need, however, is to create a third harmonic of your heater current; this will be used as an EXTERNAL reference for the Lockin amp. (You cannnot use the internal reference, because this must be tuned to 3 omega, and there's no way of getting a precise "sub-harmonic" of that from the Lockin-amp itself).
If you are using a function generator as a fundamental oscillator for your heater, you can derive the third harmonic from that. (You can just use a bandpass filter, since the function generator reference is a square wave, full of odd harmonics).
I wish I were in the lab with you...this sounds like great fun!

Eric

6. ### KL7AJ AAC Fanatic!

Nov 4, 2008
2,189
421
Hope this helps some.

Eric

7. ### KL7AJ AAC Fanatic!

Nov 4, 2008
2,189
421

Oops. "Reverence Input" should actually say "Reference Input." Of course, we don't want an IRREVERENT input, now, do we?

8. ### matsci Thread Starter Member

Feb 4, 2009
10
1
Thanks Eric. So kind of you for responding. I too really wish you are in lab as its fun to you and I am sure thats because you know about these devices well and I can learn from you. For a person like me who doesnt have good knowledge of these components, its something crazy of myself.

Well its not SR830 but DSP 7225 Lock in amplifier.

9. ### matsci Thread Starter Member

Feb 4, 2009
10
1
Function generator.... hmmm we made a circuit for this and I have taken various connections from this. May be I can send it to you tomorrow. Anyway for the time being, I have to make a dummy circuit. The commands are all ok as it is in manual. I mean my Prof. asked me to make a dummy circuit with resistors and connect to lock-in amplifier and try to measure voltage and stuff like that. I am really as I know nothing where to take connections and so on. I will try to figure out your attached document tomorrow. Thank you so much.

Feb 4, 2009
10
1

Nov 4, 2008
2,189
421

12. ### KL7AJ AAC Fanatic!

Nov 4, 2008
2,189
421
How is the project coming along?

13. ### matsci Thread Starter Member

Feb 4, 2009
10
1
Hmmm, I am still working with it. I connected the BNC cable from A input to 'ref in' in the front panel, I then checked the output magnitude in computer through this hyperterminal program. Everytime I measure this, I get different values!!? Same with measuring phase. Also, the oscillator freq was set to 50 Hz, I was expecting the ref freq (at which lock in amplifier is working) displays the same value but it shows just 19 Hz. Where am I wrong?

14. ### KL7AJ AAC Fanatic!

Nov 4, 2008
2,189
421
You might need to force the Lockin-amp into the EXTERNAL reference mode.

eric

15. ### matsci Thread Starter Member

Feb 4, 2009
10
1
Hi Eric,
Yeah its working now.
Borrowed a signal generator too from another lab but the problem is the frequency that I set in signal gen. by knob is not stable. As its keep changing, I couldnt lock the phase. I hope I will get through it somehow. Thanks for all your valuable suggestions.

16. ### matsci Thread Starter Member

Feb 4, 2009
10
1
Hi,
Here I go with the circuit and the way its connected to lock-in amp to measure third harmonic voltage.

Ok, I tune the tuner till I get low V1-V2. Then from there how do I measure the third harmonic voltage.
Thank you.

Last edited: Jul 7, 2009
17. ### matsci Thread Starter Member

Feb 4, 2009
10
1
Anyone?
I have taken the BNC connection from the tuner circuit and given it to B/I input of lock-in amplifier. Shoud I have to do any more connection like with ref-in or osc out of lock-in amp.

18. ### scalpas New Member

May 14, 2012
1
0
Hi

I just found this page on google, I'm also trying to set up a 3 omega bench. I don't if you're still around KL7AJ, but I'll give it a try !

I'm using a SR 830 and a Keithley 6221 current function generator. Up till now, I was sending the function generator signal to REF IN on the lock-in, then using the REFERENCE HARM# button, I set the lock-in to detect signals at the 3rd harmonic of the generator output.

Now, if I understand correctly, you're saying this won't work because the lock-in isn't precise enough ?

Thanks !

19. ### razheys New Member

Dec 30, 2013
1
0
is dual phase lock-in amp is a must or single phase lock-in amp is adequate for measuring V3ω?