Project shelved conclusion

The project has been shelved. I don't think the frontend of this tuner can be used for a serious spectrum analyzer. The harmonics generated by the frontend itself are too strong.
So checking if a transmitter is clean enough is not possible.
It can be used for some purposes though. Those are:

• Monitoring band
• Indication where signals are

I have bought a simple spectrum analyzer. So no need for this project anymore. 

I have learned a lot making this analyzer.