FT8 is digital communication protocol used in amateur radio bands, most prominently from 7 to 70 MHz. It's use is rising in popularity due to its reliability in weak-signal conditions, low bandwidth, and simplicity. A minimum amount of hardware is needed to get an FT8 transceiver working, and this makes it appealing to application such as military and maritime usage.
FT8 relies on a primarily digitally-driven architecture due to it's modulation scheme; 8-GFSK. However, due to its robust technical specification, a strong analog front-end is needed for successful operation.
[image of frontend] [caption]
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Proper characterization of the PDK devices is paramount for accurate circuit design in future steps. Once values such as $g_{m}$ and $V_{TH}$ are obtained, processes like gm/Id design can be utilized to derive circuit topologies and values.
nfet_01v8[image of circuit] Start by placing a sky130_fd_pr__nfet_01v8 device with the default parameter values into a new schematic in Xschem. Attach a voltage source V1 to the gate and another V2 to the drain. Ensure that the bulk and source are grounded. Also ensure that V2 or $V_{DS}$ is held at $V_{DD}/2$ or 0.9V. Create a new code block and run a dc sweep of V1.
.control dc V1 0 3 0.01 .endc .saveall
Once the simulation has finished, run plot -i(v2) to view the drain current vs. $V_{GS}$ graph. This graph helps to give us the transconductance $g_m$ of the MOSFET, which indicates how efficiently the device can convert a voltage to a current. To derive this value from the simulation, you can either run the command print @m.xm1.msky130_fd_pr__nfet_01v8[gm] or use the typical analytical expression: $$g_{m} \ = \ \frac{\partial{I_D}}{\partial{}V_{GS}} \ = \ \mu_{n}C_{OX}\frac{W}{L}(V_{GS}-V_{TH}) \ = \ \frac{2I_D}{V_{GS}-V_{TH}}$$
To find the threshold voltage $V_{TH}$ of the device, you can simply run the same command as above for the parameter: print @m.xm1.msky130_fd_pr__nfet_01v8[vth]
[image of circuit] Using the same circuit as before, sweep V2 instead of V1 at varying V1 values. This aids in finding the saturation point for a given $V_{GS}$ and the behavior of $I_D$ beyond $V_{DSAT}$. The code for this may look like this:
.control alter @V1[value] = 0.7 % start at Vth dc V2 0 5 0.01 plot -i(v2) alter @V1[value] = 1 % step to new Vgs value ... % continue changing Vgs alter @V1[value] = 3 dc V2 0 5 0.01 plot -i(v2) .endc .saveall
For a given DC sweep, one can obtain the $V_{DSAT}$ value by running print @m.xm1.msky130_fd_pr__nfet_01v8[vdsat]. Or, use the expression $V_{DSAT}=V_{GS}-V_{TH}$. Now that the key values of the device have been extracted, one can now determine some other Figures of Merit, such as on resistance: $$R_{on} \ = \ [\mu_{n}C_{OX}\frac{W}{L}(V_{GS}-V_{TH})]^{-1}$$ And to determine the behavior of drain current past saturation: $$\int_0^LI_D\mathrm dx \ = \ \mu_{n}C_{OX}\int_0^{V_{GS}-V_{TH}}[V_{GS}-V_{TH}-V(x)]\mathrm dV$$
$$ \therefore \ I_D \ = \ \frac{1}{2}\mu_nC_{OX}\frac{W}{L}(V_{GS}-V_{TH})^2(1+\lambda V_{DS}) \ \ \ \ \mathrm{for} \ V_{DS}>V_{DSAT} $$