Hi, I want to share some mods that help reducing spurs (especially on 100Mhz) on limesdr-mini :
- first : increase C17, C18, C28 and C29 from 10pF to 10nF (values are not really critical) : these are decoupling capacitors for DC controlling voltages of RF switches (TX and RX) (sky13411).
According to datasheet the typical value is 10pF to provide fastest switching speed
(70ns). In lime-mini, we can increase these capacitors to higher values to provide
better RF filtering. It seems that these DC lines also carry a lot of interferences and severely degrade RF signals that are travelling across theses switches. These 4 lines are coming from the FPGA Chip.
Before modding, the lime mini RX port transmit many spurs (connect the RX lime mini port to any SDR receiver to check this). After the mod, spurs at RX port are fewer and weaker. Spectrum panels of sdr softwares show less spurs when nothing (or a 50ohm load) is connected to RX port.
- second : add 2 metal shielding covers : to enclose the ftdi and fpga chips on the upper side. On the lower side the cover enclose the 2 switching power modules (these work also with 100MHz frequency) and the fpga clock generator.
On the pcb cover limits markers are printed and there are some grounding pads.
Perhaps this shielding isn’t really necessary if your lime-mini is in a metal case.
Theses enclosures get quite hot but everything seems OK after hours of operation at 10MHz sample rate.
Note : be very carefull if doing theses mods because the lime mini is a very dense board with many tiny components.
I soldered 10nF smd capacitors in parallele with the original 10pF capacitors.
I use a thin soldering tip and a good magnifying glass to check solder joints.
You can buy original shield metal covers and clips.
(RFI shield clips mini tin smd = HARWIN S0971-46R = Farnell ref. 2361677
RF shield cover = Wurth Electronik 36003250 = Farnell ref. 1909662)
I have made my own cover with iron metal sheet. It’s really not easy to do. Be also very careful because the size must be precise to avoid short-circuits ! I use kapton tape to insulate inside and around the cover.
I post below some pictures of my lime-mini and screen capture to show improvements of this mod. (note that the noise floor have decreased)
C17-18-28-29 modding capacitors :
DIY shielding covers :
Some screen captures :
with sdr#
now can see and hear near 100MHz broadcasting fm stations :
same without antenna at rx port :
now when settings all gains to minimum :
with gqrx without antenna :
with sdrconsole no antenna :
