Electronics > RF, Microwave, Ham Radio

Hobby radar design

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radar_macgyver:
@buta: the calculations don't include the LO to IF isolation of the mixer. The mixer, being double-balanced, will have some isolation between LO and IF ports. This will be degraded somewhat by coupling between the traces, but will still be higher than 0 dB.

@sckzor: Sorry if this is coming in a bit late, but your layout looks much better with good coupling between ground planes. Consider rotating U6 so that the LO port (pin 2) faces up towards the board's TX port. This will eliminate a couple of bends from the RF traces. I would say that if you had to pick one part only for size reduction, it would be the RF coupling capacitors. You could easily make these 0402 or even 0603, with enough room around the parts, these aren't too hard to hand solder or rework. You could also drop the value to 1 nF or lower and make sure they are C0G dielectric, to minimize losses. On the layout surrounding U3 and U4 (the RF amps), you have a short stub on the RF traces connecting the amplifier outputs to the biasing inductor. You could make these "inline" to avoid the stub. Check out NXP's AN11902 to see how they designed a board for this part. AN11416 also has more detail on the completed board, look closely at how the inductor is placed relative to the part. Also note the BOM shows the exact parts used.

buta:

--- Quote from: radar_macgyver on February 07, 2023, 04:16:47 pm ---@buta: the calculations don't include the LO to IF isolation of the mixer. The mixer, being double-balanced, will have some isolation between LO and IF ports. This will be degraded somewhat by coupling between the traces, but will still be higher than 0 dB.

--- End quote ---
The calculation considers the TX path not the RX path, it shows the LNA U3 (use as a TX amplifier) operates close or above to its 1dB compression point in typ and max conditions.  The TX signal will be distorted and will contain additional harmonics, noise or ...
Usually amplifiers should operate few dB's below the 1dB compression point.

radar_macgyver:
Oops - missed that you were only looking at TX. The OP could replace U1 with a different part to bring down the gain. Then again, it's FMCW, so the amplifier can operate into saturation without sacrificing performance other than generating more harmonics.

sckzor:
You are not too late @radar_macgyver.  I am still dealing with a tremendous amount of work and have stalled continued design unfortunately.  If you think smaller coupling caps and inductors would be better I can probably spare the funds to replace them, and while I am at it I can order the larger attenuator.  Later this week, probably Saturday or Sunday I will get around to making further revisions the PCB and and parts list and post about them.

At the risk of redundancy, thank you again @buta and @radar_macgyver this would not be possible without your help.

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