First project log

A project log for Handheld 3GHz Spectrum Analyzer

Includes RF detector up to 6GHz and a datalogger

roelhroelh 08/15/2014 at 18:060 Comments

A new pcb version, that I did receive from the pcb service today, has small design changes and corrects several small errors, and will also have a SAW filter that can be switched into the IF path. The filter is wide enough (around 5 MHz) that still two RF-IF combinations can be used for mirror suppression (if not wide enough for a certain situation, the SAW can be bypassed). The intention is that the SAW filter can give better selectivity.

The SAW filter can hopefully also filter the false response of the Si4431 away (at about 1.9 MHz next to a peak, it has a second peak that is 30dB lower than the main one).

The firmware still requires much work. Changes must be done for the SAW filter. It might be good to have a few options for the mirror suppression system. I also want to be able to use stored settings. Some logging would also be nice.

The sound recorder is not working yet, I could not reach a good write throughput to the micro-SD card. On a new card it was good, but it seems that as soon as there are other things on the card, write rate drops to around 3 kByte/second. I use a rather slow cpu speed (4 MHz), but I don’t think this is the problem, because I can playback music files at 22050 Hz sample frequency (and they sound very good). I use the famous FATFS from ChaN, and two 1.5KB buffers. Perhaps someone has an advice ?

The sound recorder might also store the screen contents, and show this screen contents when the sound is played back.

When using the device to debug wireless communication, it will be possible to decode the communication in the Si4431 (it has several functions for that) and show the (decoded) contents on the screen (perhaps in a scrollable window). The Si4431 can also transfer the raw FSK or OOK data to the processor.

The frequency of the spectrum analyzer is quite precise, because the Si4431 derives its frequency from a crystal. The Si4012 oscillator has no crystal, but we could measure its output with the Si4431 and then adjust the frequency of the Si4012 if it deviates. The devices do have some phase noise, that might be problematic in some cases. If we assume an IF around 315 MHz (the intended SWA filter frequency), then the phase noise of the Si4012 will be the most prevalent (at high frequencies and in 3rd harmonic mode). We could add a small add-on pcb with a higher quality oscillator if the phase noise is a problem.