FM Demodulator

Client

ECE 1472: Analog Communication Systems

Location

Pittsburgh, PA

Year

2023

Resources

Design Files

The University of Pittsburgh's Swanson School of Engineering's ECE 1472: Analog Communication Systems features a lab assignment in which we create an FM modulator-demodulator pair using a digital PLL integrated circuit and discrete external components. To avoid the parasitics of breadboards, we use custom PCBs. They work, but I wasn't happy with the board layout. In addition to some electromagnetic considerations, I wanted to design a board that was more intuitive for students to use.

Schematic

I used schematics previously designed by the course professor, Dr. Seteven Jacobs. The FM demodulator consists of a digital PLL IC from Texas Instruments [1] and looks like so:

The circuit regulates a +12V input down to +5V to power the PLL IC. Pins 3 and 14 of the PLL IC serve as inputs to a mixer that acts as a phase comparator between the PLL's VCO and the RF input. R3, C3, and R4 make up a passive low-pass filter, the output of which is fed into the downstream amplifier. One can see that this circuit is a more concrete instantiation of a PLL block diagram [2]:

This circuit demodulated an FM signal by locking onto the incoming signal and mixing it with a cosine wave with a frequency equal to that of the incoming signal. This will eliminate the FM carrier frequency by shifting the incoming modulating signal down to baseband and up to a frequency that is twice that of the original carrier frequency, the latter of which is filtered out by the loop filter. The VCO ensures that we continue to mix the incoming signal with a cosine wave of equal frequency, as the frequency of an FM signal varies continuously with time. We take the output of the loop filter as our demodulated signal.

OLD PCB LAYOUT

REV 01 of the FM Demodulator PCB is shown below:

Front

Back

While REV. 1 of the FM Demodulator PCB works, I found a few things that could use some improvement:

No Planes: We're working with relatively low frequencies (hundreds of kHz - tens of MHz,) but with all of the available space on the board, I was stumped by the lack of power and ground planes to improve power and signal integrity.
Non-User-Friendly Layout: I found the layout to be a bit confusing and difficult to separate the PLL circuit from the amplifier circuit conceptually, as these two circuits are built and utilized in different sections of the lab assignment.
Insufficient Silkscreen: A bright silkscreen would aid greatly in labeling circuit sections, test points, and phase comparator inputs. For example, it's easy to mix up which pins of the 3-position screw terminal correspond to +12V, -12V, and ground. Plugging these wires incorrectly can fry the board and can be prevented by simple silkscreen indicators.
Boring Aesthetics: A rectangular board with green silkscreen is simple and cost-effective, but something else would look much cooler.

Front

Back

From a copper perspective, I used wide signal traces and filled the rest with power and ground powers. I placed thermal relief on all component pads, as students will hand-solder these boards.

NEW PCB LAYOUT

Front

Back

My new PCB design further separates the PLL and amplifier circuits, both spatially and with silkscreen polygons. I also labeled relevant test points and moved them to the right side of the board to make them easy to plug in to a single oscilloscope or multimeter. I also labeled the power screw terminal and the phase comparator pins to allow students to quickly connect headers without constantly referring back to the lab assignment document. I also included a silkscreen box to allow lab groups to label which PCB belongs to them, as I experienced a few mix-ups with other groups when I took the class. The yellow-on-black aesthetic also looks incredibly slick.

Manufacturing by PCBWAY

I'm tremendously grateful to PCBWay for sponsoring this project and manufacturing these boards for me.

Even before our partnership, I'd always opt to use PCBWay if I had my choice of PCB manufacturer and assembler. PCBWay offers high-quality PCBs that are shockingly inexpensive. I like that they're not just a prototyping service for simple boards - PCBWay can handle incredibly complex designs involving 16+ layers, HDI via types, unique surface finishes, and RF dielectric materials.

What stands out to me is their commitment to satisfying all customers, no matter how small an order is. I've had PCBWay manufacture rigid and flex PCBs, both of which turned out wonderfully. My favorite service is their turnkey assembly service; PCBWay has always done a spectacular job of sourcing the parts required for my design and communicating with me through every step of the process. They always send photos of the finished assembly for my inspection, which is a very nice touch.

The FM Demodulator circuits look even better in person than they do in the 3D model. The matte black solder mask is very smooth, and I'll be ordering every PCB I can with this solder mask from now on.

References

[1] Jacobs, Steven - "Laboratory Experiment #2 Frequency Modulator/Demodulator Using Phase-Lock Loops." In use with ECE 1472 - Analog Communication Systems, Swanson School of Engineering, University of Pittsburgh.

[2] PLL Block Diagram, uploaded by Sanjay Sharma, used in "Frequency Synthesis Techniques for High-Speed Communication System" by Goving Singh Patel et al., June 2011.