Sideband difference circuit - Page 1
Forum section: electronics / projects

author: twistar
author_rank: Newbie
timestamp: March 23, 2020, 10:43:11 pm
content: Hello, I am starting design for a particular circuit and I am looking for recommendations for components and comments on the design. This is for a project in an academic physics (atomic and optical physics) research lab. The requirements are as follows.- I have a signal which has a carrier at f_C = 20 MHz. The carrier has two sidebands and f_C +- f_M where f_M is the sideband modulation frequency. The value for f_M is not known yet but it will be around 100 kHz.- The goal of this circuit is to measure the power in the two sidebands, take the difference of the two powers, and then output a bipolar DC voltage proportional to the difference in the sideband powers.We are considering digital and analog methods to implement this circuit. On the digital side we will be programming a red pitaya which we have purchased to do the demodulation, power calculation, and voltage output. However, our team is new to FPGA technology so while this is being developed we would also like to explore analog approaches.For the analog approach this is what I have imagined:- Split the original signal into two paths.- In each path mix the signal with a demodulation tone at frequency f_C+-f_M+f_IF where f_IF is an offset intermediate frequency, say 20 kHz or so? The reason we don't mix the sideband down to DC is I think it will be better for noise purposes to stay away from DC until necessary. We can generate the demodulation tone from some DDSs.- In each path we install a tuned bandpass filter (essentially an IF filter) centered at f_IF. This filter should have a bandwidth of around 10-20 kHz.- We next measure the power in the 10-20 kHz output using some sort of power detector.- We can then use an amplifier difference circuit to take the difference in the two powers.My questions are as follows:1) Comments on the scheme. Is it a good idea to mix the sidebands down to a non-zero IF frequency? Any gotchas?2) Mainly I would appreciate recommendations for components. We typically work with BNC or SMA cables so any "plug-in" modules for the above components would be desirable. We may be sensitive to the calibrations of the power detectors, mixers, filters etc. so I'm curious what to look for here. In particular I think I need the right keywords to search for each of these components. I see that I am essentially building a mini-spectrum analyzer so I'm sure there's a lot available, I'm just not aware.3) Is there a single commercial product which may be able to do all of this or much of this at once? Some sort of lock-in amplifier or IQ mixer?Thank you for any assistance and tips you can provide. Please let me know if I can provide any more information!

author: bob91343
author_rank: Super Contributor
timestamp: March 24, 2020, 04:23:08 am
content: You said the magic words, spectrum analyzer. A good analyzer will give you the desired answer immediately.Basically, a tunable receiver with narrow bandpass will do it. Any ham radio transceiver will do.Some approaches will give you errors due to different sensitivity for each side frequency due to the fact that they are not very close together.

author: twistar
author_rank: Newbie
timestamp: March 24, 2020, 04:56:56 am
content: I guess I failed to mention but I need the difference in the sideband powers as a DC analog voltage. The sideband difference signal is being fed as a control signal elsewhere in the experimental apparatus.

author: bob91343
author_rank: Super Contributor
timestamp: March 24, 2020, 07:24:12 pm
content: That is very different. Of course, two receivers will do it; the AGC voltages will give the different strengths and can be subtracted after proper calibration.But that's probably not what you want. Another approach using one receiver can be set up to switch between upper and lower sideband to get the analogs.That's not what you want either, I am sure. But the only compact solution I can imagine would be a detector and use it much as the upper/lower switch except detect both sidebands at once with two circuits.