Differentiating real signals from spurs in the tiny spectrum analyzer

When measuring the harmonics from my Softrock Ensemble RXTX I noticed a number of strange (e.g. non harmonic) signals such as at 0.8MHz , 9MHz, 12.8MHz and 13.8MHz
Is the output of the RXTX really that bad, or are these spurs and mirrors? Below is a 4 times averaged scan of the output of the TXRX through a 60dB attenuator with RBW set to automatic



When wobbling the first IF around 433MHz, real input signals should stay at the same frequencies but mirrors or internally generated spurs may change to different frequencies or disappear altogether.
This is possible because  the 434MHz IF filter is about 1MHz wide , much wider than the currently selected RBW bandwidth of about 20kHz.
Luckily I implemented this wobbling for the 2GHz spectrum analyzer and I could enable it by switching on "Spur Reduction"
Combined with 4 times averaging this positions the 434MHz IF at 4 different frequencies, still within its 1MHz bandwidth but twice the RBW apart and the averaging should reduce the signals that change position in the scan.
The impact is easily seen as most spurs and mirrors almost disappear giving confidence the RXTX is not that bad.



There is some different in the amplitude of the real signals at 7.11MHz and 14.22MHz as the IF filter at 434MHz is not really a flat top filter and the calculated IMD changes a bit but the difference (about 1dB) is acceptable
The SW spur reduction nicely compensates for the absence of good HW filtering and helps to keep the tiny spectrum analyzer small and easy to build.

Further tuning of the 2GHz cavity filter

Some more tuning resulted in a nice flat top of the pass band




At least flat within 0.5dB
Center frequency at 2020.7MHz, 
S21 -5.1dB
-3 dB width 3.3MHz
-40dB width 14MHz

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