eBay SA complete, suggestions for improvement?


Erik Kaashoek
 

Now most of the modules are in, I combined everything into the complete signal chain.
The purpose of this post is to document the setup and performance of this configuration.

The +10dB after the ADL5801 should not be needed but the conversion gain of the ADL5801 is -8dB i.s.o +0.8dB. I still don't understand why. 
The Arduino Zero controls the ADF4351's and the 12bit ADC measures the output of the AD8307 which gives a 120dB range and 0.05dB resolution

The settings are (speed is actually in 0.1ms units):

The noise level, 1dB CP and IIP3 performance of the whole chain has been verified by doing the performance measurement including two tone IP3 performance of each stage starting with the last stage and working towards the input
The SW allows any mixer to have any role and all frequencies of all filters can be changed. Unused LO's can be used as signal generators or tracking generator. Even after building the total chain it is stil possible to sweep each filter

Scanning 0 till 1Ghz without input signal gives

The many spurs around 100MHz are from the ADF4351 as it runs with R=10 from a 25MHz reference.
Spur performance of the black ADF4351 eBay module is not acceptable. I still need to investigate the loop filter and improve supply decoupling
The 433MHz spur is not yet explained. At some point I had a terrible spur at 42.2MHz. After some testing I found it to be caused the keyboard of the PC. Shifting the keyboard to the other side of the bench did remove the spur.
The signal around 950MHz is leakage from the mobile base station close to where I live.
The absolute signal level is calibrated using a calibrated generator (which was checked using a scope with a 50 ohm input) up till 12MHz but I still need to check the sensitivity at higher frequencies. 
Scanning again 0-1GHz after connecting a 20cm antenna to the input of the SA gives:

Below 250MHz its AM, FM, DAB and some analog TV broadcast. around 550MHz, 800MHz and 950MHz there are mobile base stations and digital broadcast signals.

Connecting two old analog signal generators with a passive -20dB combiner to the low-pass filter gives the two-tone test

Do I read this correct as an IIP3 of around +10dBm? At least changing the signal strength of one or both signals always results in the IIP3 of around +10dBm and that would be consistent with the performance of the weakest part in the chain, the ADE-25MH.
With the current spur performance of the ADF4351 the active mixers seem to be the best choice, they have more noise but their LO rejection is better so unwanted mixing with the LO spurs (specially with R=1) will not obscures the measurement.

I see the following next steps:
- Investigate why the ADL5801 has so little output, solving this allows me to remove the +10dB amplifier and will motivate me to buy another ADL5801 module to replace the ADE-25MH
- Solving the spur problem of the ADF4351 will allow me to use passive mixers everywhere, I have mixers with an IIP3 at +13dBm, less then the active mixers but with much better noise performance so in total more spur free dynamic range. I ordered a green ADF4351 eBay module that uses a 10MHz reference with possibly a better layout around the loop filter
- Find the cause of the 433MHz spur.
- The 15kHz RBW filter as seen in the two-tone test has way too much BW below -50dB. The phase noise of the ADF4351 is much better (was verified using two ADF4351 modules 5kHz apart and a mixer to produce audio into an audio spectrum analyzer) I probably need to improve shielding

Any suggestions for further improvement or checking apart from putting everything in a box with good shielding?

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