Tiny home build spectrum analyzer

Building and tuning GHz cavities is not everyone's hobby but if you need a zero till a couple of hundred MHz spectrum analyzer there is a much simpler build possible.



This is all you need to measure signals between  0 and 400MHz at levels between -80dbm and -20dBm
Top blue module is a mixer. Can be found on eBay either as complete module (ADE-1) for 10$ of you only buy the mixer (AD-25MH (much better level 13 mixer), 5pc for 4$) and put it on a small PCB
The two small identical blue modules are SI4432 modules (do NOT use SI4463 modules as these use non overlapping bands) that can be found on eBay for less than 2$
The one directly connected to the mixer acts as a tunable LO with 20dBM  output between 433MHz and 860MHz
The copper module is a 433MHz band pass filter you either buy from eBay for 25$ or build yourself from two EPCOS SAW filters and two SMD inductors for less then 10$
Datasheet here
The bottom blue module is the receiver SI4432. Its set at a fixed frequency of 433MHz and does the the logarithmic signal strength measurement. The officiel range is -120dB till 0dB but the range is limited in practise between -100dBm and -20dBm as above -20dBm the SI4432 will start to produce all kind of intermodulation products. 
The module with the USB plug is an Arduino zero compatible to provide the 3.3Volt and to control the SI4432 modules (these need 3.3V MISO/MOSI/CLK)

Some real measurements
Scan from 0 to 5MHz



The phase noise of the LO SI4432 is very visible but the 3kHz RBW results in a nice sharp peak. The noise floor is not very flat but no spurs
The AD9851 clearly delivers a nice clean signal at 1MHz through 40dB of attenuators

Switching to 0-100MHz you get



RBW now set to 300kHz. Output of the same AD9851 at 46MHz
Some small spurs but contrary to cheap "spectrum analyzers" you can buy on eBay which are basically nothing more than a LO, a mixer to DC, a LF RBW filter and a log detector, this SA does not have the many spurs from the harmonic modes of the LO due to proper filtering. The second harmonic from the AD9851 at 92MHz is clearly visible

And for the full range



This is the output of a ADF4351 at 75MHz. The harmonics at 150MHz and 225MHz are visible but sensitivity quickly reduces above 200MHz as I did not yet remove the low pass filter at the output of the LO module so the mixer loses its LO at higher frequencies. The spur at 30MHz is probably an alias from the ADF4351 signal as there is no low pass filter at the input yet.

The datasheet of the SI4432 can be found here
And this is the module used

For SW you can go to github as there are several repositories that contain usable libraries.

Hope this inspires some creative use of the SI4432 module