Its set at a fixed frequency of 433MHz and does the the logarithmic signal strength measurement. The bottom blue module is the receiver SI4432. 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$ The one directly connected to the mixer acts as a tunable LO with 20dBM output between 433MHz and 860MHz 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$ 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 This is all you need to measure signals between 0 and 400MHz at levels between -80dbm and -20dBm 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.īuilding 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. 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 impact is easily seen as most spurs and mirrors almost disappear giving confidence the RXTX is not that bad. Luckily I implemented this wobbling for the 2GHz spectrum analyzer and I could enable it by switching on 'Spur Reduction'Ĭombined 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. This is possible because the 434MHz IF filter is about 1MHz wide, much wider than the currently selected RBW bandwidth of about 20kHz. 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. 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 non harmonic) signals such as at 0.8MHz, 9MHz, 12.8MHz and 13.8MHz When measuring the harmonics from my Softrock Ensemble RXTX I noticed a number of strange (e.g.

Nanovna as spectrum analyzer