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I want to generate a 5 kHz microwave pulse to induce a sample signal. This microwave pulse has an underlying microwave frequency of 2 GHz. In order to investigate very tiny signals, which should arise within my setup with a frequency of 5 kHz repetition, I want to use a lock in amplifier to differentiate my 5 kHz signals from the noise floor.

Currently I have a Hewlett Packard 83640B signal generator 10 MHz-40 GHz and a Zurich Instruments lock in amplifier HF2LI.

Can I use the lock in for generating the 5 kHz reference signal, put it in the signal generator which is then modulated with 5 kHz and generates a 2 GHz signal by its own which is sent to a microwave antenna?

The detection signal of my sample is of course used as input for the lockin, it compares the input signal to its internal reference of 5 kHz, and shows me how big my sample signal is.

Is this possible? Which obstacles do I need to handle? Is there another way if this setup wont work?

I am no physicist and no engineer, please keep your comments rather simple if you have a tip or good advice.

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    \$\begingroup\$ I'm not familiar with the HP sig gen, and the practicality of this is mostly going to hinge on getting the sig gen and LIA to trigger properly. But in general, it sounds like a good approach. What is the nature of the signal coming back from the sample? more microwaves? And what aspects of it are you interested in? delay? amplitude? you will lose most (but not quite all) of the internal structure of the return pulse. \$\endgroup\$
    – Jack B
    Commented Aug 15, 2017 at 11:17
  • \$\begingroup\$ I'd like to agree with Jack - especially if you're not an engineer it physicist, I'd presume that there's a solid chance that the solution that you have in your head doesn't solve your actual problem. So, to avoid that, please also describe what you need to achieve in the bigger picture! \$\endgroup\$
    – Marcus Müller
    Commented Aug 15, 2017 at 11:35
  • \$\begingroup\$ Thank you both for your opinion! The nature of the signal coming back is a voltage originally oscillating constantly at 75 kHz. But if my sample is interacting, it interacts of course with 5 kHz, so I will have a voltage with 5 and 75 kHz oscillation. If my sample does not interact, the 5 kHz signal is missing. So I am interested in the occurence of the 5 kHz, not the exact amplitude and not the delay. \$\endgroup\$
    – Jacccy
    Commented Aug 15, 2017 at 13:05
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    \$\begingroup\$ It would be really useful to see a scope capture of the signal coming from the sample, showing what you want to measure. How quickly will the 5kHz signal change amplitude/appear/disappear? no quicker than 500Hz I hope. Either that or say what your sample is and what it is doing to the microwave pulse. \$\endgroup\$
    – Jack B
    Commented Aug 15, 2017 at 16:31
  • \$\begingroup\$ It is a completely new setup under construction. I never have measured the signal of the sample, I cannot show you any kind of output. And I dont now the rise-/fall-time neither the amplitude. But I expect the signal to be smaller than my noise floor, therefore I need the LockIn. The samples' properties will be changed upon microwave exposure, and this change I want to detect. The detection process generates a voltage, which without interaction shows 75 kHz oscillation. With interaction I expact an additional 5 kHz oscillation. Unfortunately that is all I can say. \$\endgroup\$
    – Jacccy
    Commented Aug 16, 2017 at 7:59

3 Answers 3

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I'm quite sure it is appropriate and practicable.

For modulating the generator, I have, however, some concerns:
In optics setups employing a lock-in often use shutter-wheels to modulate the (light) source; especially if modulating the source itself would be problematic or impossible (indandescent bulbs, daylight, starlight). That way continuously working (light) sources can be used.
The shutter wheel provides also the sync signal for the lock-in (e.g. from a separate fork light barrier).

Switching the signal after generation by shutter- (or reflector-) wheel (or other means more appropriate for microwave) has the advantage that the generator can work continuously avoiding artifacts by modulating it at generation (e.g. frequency/power instabilities).

Maybe this could be also important in your microwave application. Maybe not; if modulating the generator is not a problem.

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Yes, it makes sense in general terms. The lock-in you mention is a rather exotic thing with 50 MHz bandwidth. It is overkill for a 5 kHz signal. It might help to say a bit more about what you are doing. Is your experiment a linear system excited by the RF signal? Or is the 5 kHz pulse essential to create the phenomenon you are interested in?

In general, the best SNR will be obtained when you concentrate all the signal into a very narrow frequency band, and that is the whole point of a lock-in. It is a low-noise preamplifier followed by a narrow-band tracking filter. Rather than make pulses, you may want to think about narrow-band modulation. For example, you could put 5 kHz sidebands on the RF signal, send it through your experiment, demodulate the response to recover a 5 kHz sine wave, and then send that to the lock-in.

Here is an example.

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You might use a function generator to provide the 5KHz modulation.

Then use a NE567 tone-decoder, with center-frequency set to 5KHz.

Just an idea.

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    \$\begingroup\$ But than I lack the features of a lock in which filters out my signal from the noise =( \$\endgroup\$
    – Jacccy
    Commented Aug 16, 2017 at 7:24

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