Hi all, I've had some experience putting together a prefab circuit board, like real simple stuff and aiding friends or classmates soldering. However, I am here today because I was picked for a fellowship over summer to get more hands on learning at my university.

The issue: I've been tasked with designing a passive RFPD circuit, and the grad student talked kinda fast and I understood the words he was saying, but I feel completely out of my depth in building this circuit from scratch. I wrote everything kind of jumbled in my phone notes (my mistake) so it's not a lot to go on. Basically, i'm supposed to take input(s?) from a coherent locking field and a 2nd side-band (probably things I should look up to understand what they mean), and the circuit itself is looking for a peak (meaning graphically?) at a specific MHz, ultimately to, from what I could understand, I could be completely wrong, pick up a beat note. I am unsure if the MHz i'm looking for is the beat note or just the input. The circuit is to have it's own volt supply 5-10V. I am just starting today, so research is my main goal for the next week, as well as familiarizing myself with Python - zero and LTspice for modeling. The output will eventually go to a VCO to go a cavity if it is in the specific MHz.

***Side note he did state that what I have to use to build the circuit is restricted to what is in the circuit lab on campus, so I will have to finagle stuff if they don't have what I end up modeling as what would work if that makes sense, though he did say I could build my own inductor (thats not a problem, I am a geriatric student who used to work at a vape shop when mech mods were still a thing, my coil wrapping skills excellent)

Any help, advice, starting points, resources I'd seriously, seriously appreciate. Thank you to anyone who takes the time to read this and respond, I want to know how to do these things and I want to also understand so I could do my own future projects for funsies.

 

and what exactly is RFPD?

 

and what exactly is RFPD?

Radio Frequency Photo Diode? I am about 89% positive

 

I am unsure if the MHz

This is important. "RF" can be almost any number.

 

" presence detector" probably makes more sense in this context.

I suggest asking for clarification of the task before charging into the weeds.

 

Firstly, welcome to AAC.

Sounds like you might have been thrown in the deep end here. I started my career as a RF design engineer and I'm struggling to make a lot of sense out of your description.

We need to know what RFPD actually stands for? RF phase detector, RF peak detector, or ?...I've never heard of a Radio Frequency Photo Diode and secondly, the actual frequency in question is critical as doing something at 1MHz is very different to doing it at 100MHz or at 10GHz!

If the circuit has a power supply it can't be passive.

A coherent lock is forcing an oscillator to track the frequency and phase of a reference so their relative phase difference stays constant (or follows a controlled ramp) as part of a phase-locked loop (PLL).

Not sure where the side band fits in to this unless it's a reference to the necessary rolloff of the filter to focus on the primary signal.

Too many unknowns to start advising you. You need the detail.

 

Found this info that might be relevant:

Optical Phase-Locked Loop (OPLL)

• Beat note between master and slave lasers measured on a fast photodiode.
• Phase detector and high-bandwidth feedback to laser current and fast actuators.
• Used to lock slave laser phase/coherent offset to master for heterodyne, interferometry, and coherent communications.

 

Firstly, welcome to AAC.

Sounds like you might have been thrown in the deep end here. I started my career as a RF design engineer and I'm struggling to make a lot of sense out of your description.

We need to know what RFPD actually stands for? RF phase detector, RF peak detector, or ?...I've never heard of a Radio Frequency Photo Diode and secondly, the actual frequency in question is critical as doing something at 1MHz is very different to doing it at 100MHz or at 10GHz!

If the circuit has a power supply it can't be passive.

A coherent lock is forcing an oscillator to track the frequency and phase of a reference so their relative phase difference stays constant (or follows a controlled ramp) as part of a phase-locked loop (PLL).

Not sure where the side band fits in to this unless it's a reference to the necessary rolloff of the filter to focus on the primary signal.

Too many unknowns to start advising you. You need the detail.

Radio Frequency Photo Diode is the exact description of what I got, from another students project it is possible! It is a detector AND a filter of sorts, the clarification I received today: 6.5 MHz, creating a sort of passive system with inductors and capacitors, trying to dampen the noise below 6.5 MHz, and get a reaction when the beat note is at 6.5 MHz, I don't have to do anything with the next part. But if the beat note is not 6.5 MHz then it passes to a VCO to change the voltage of the system. The grad student is actually unsure how to control the 2nd band, it is some sort of effect after the CLF interacts with a green laser at a certain voltage output? So roll off sounds correct? He's not sure how to calculate or control it, so he's trying to see the effects of it when looking at the beat note. If the beat note is in fact 6.5 MHz it'll pass onto a mixer, of which I also am not going to be working on.

 

Found this info that might be relevant:

Optical Phase-Locked Loop (OPLL)

• Beat note between master and slave lasers measured on a fast photodiode.
• Phase detector and high-bandwidth feedback to laser current and fast actuators.
• Used to lock slave laser phase/coherent offset to master for heterodyne, interferometry, and coherent communications.

These are good jumping off points for research, thank you!

 

This is important. "RF" can be almost any number.

6.5 MHz beat note for exact clarification

 

All you need to know about beat note....

https://www.rp-photonics.com/beat_note.html

 

I read the posts and still it is very confusing as to the actual requirements. In fact, it is actually very unclear as well as confusing.

 

I would edit the title of the post - "RFPD" is almost meaningless.
I could be an expert in this- but I would skip it because... Just spell it out.

 

I would edit the title of the post - "RFPD" is almost meaningless.
I could be an expert in this- but I would skip it because... Just spell it out.

"RFPD" is almost meaningless. Certainly RIGHT!!! My first guess is Radio Frequency POWER DETECTOR! I have seen circuits for those in HAM RADIO publications! They are used for checking to see if a transmitter device is functional. Of course, it could also be a "Reliable Female Puppy Detector, an item used by dog breeders.

Of course, I have also, within the past year, come across an article describing a scheme to measure the difference in frequency of two separate beams of light. It was intended for use with laser devices. That was a college level engineering project that was published a while back.

 

"RFPD" is almost meaningless. Certainly RIGHT!!! My first guess is Radio Frequency POWER DETECTOR! I have seen circuits for those in HAM RADIO publications! They are used for checking to see if a transmitter device is functional. Of course, it could also be a "Reliable Female Puppy Detector, an item used by dog breeders.

Of course, I have also, within the past year, come across an article describing a scheme to measure the difference in frequency of two separate beams of light. It was intended for use with laser devices. That was a college level engineering project that was published a while back.

Apologies for using the acronym, I wrongly assumed from the start it was more common than it is. Measuring the difference in two beams of light from a laser is on par with what I'm tasked to do. They know by calculations what the expected value is, and I believe they want something to certify that.