Hardware modifications to Rev 1.0
Improvements to strong signal robustness
Compared with other SDRs, all values are good to very good with the exception of TOI. The relatively poor value of -10 dBm is, in my opinion (DK5DN), down to the preamp. It is optimized for good sensitivity and short antennas, but not however for strong signal robustness. You can't have everything at the same time! If the FiFi SDR is driven with powerful antennas, the strong signal robustness can be improved by modifying these component values.
|Component||New value||Old value|
|R15||200R or 220R||was not fitted|
The reception sensitivity of -105dBm is not altered by these changes, but the IP3 (TOI) improves from -10dBm to +3dBm. These changes alter the input impedance to around 50R. This will limit the usability of short rod antennas.
The capacitor C62 provides symmetry and thereby to suppression of unwanted sidebands. If sideband suppresion with C62 = 15pF is worse than 40Db, it should be replaced with a smaller value or left out altogether. A little experimentation may be necessary here.
C58 is the wrong way round.
The tantalum electrolytic capacitor C58 is already fitted the wrong way round in the circuit diagram. The error was noticed by coincidence by DG1YFE, although there have been no problems in practise to date. The problem will be solved with Rev 1.2 of the production series.