Plans for Receive only board (suggestions needed)

#1

Hi All,
We plan to use the LIMESDR-USB for Radio Astronomy and therefore need only the RX part of the board. We want to modify the board for better performance (making it less noisier). Can you suggest if they will work?

We plan to do the following modifications:

  1. Removing the components on the Receive side (Transformers Caps, Resistor on 3 Inputs).
  2. Removing the SPDT switch used for TX RX calibrations.
  3. Removing the DDR2 IC’s and its Power supplies.(Not sure if this will harm the FPGA itself)
  4. Removing the 0 ohm resistor at the output of the LMK00105 IC used for generating the TxPLL_CLK.

External precautions are:
Use a Linear Power Supply to feed the external power Input, don’t use the USB supply.
Recommendations are needed for the best Input Voltage, 6,9,12V ? (the one that generates the least EMI as there are switching power supplies at the input stage)

Thank you very much in advance…

#2

@Crater - Alexander,

Before you do anything like that, please check out this post from the RASDR team:

…and this post that specifically talks about board mods:

It may speak only to an enclosure for the Lime used for Radio Astronomy, but that group focuses exclusively on that using the LimeSDR. You may want to see what they’ve done prior to doing anything too extreme on your LimeSDR. It always makes my skin crawl when I see anyone suggesting that they’re going to take parts off their LimeSDR board, so PLEASE don’t do that until you’ve spoken to members of that RASDR team that have got this running on their Limes. Doing a search on ‘Radio Astronomy’ in the search tool for the forum will pull up numerous posts by Hams and researchers who have already blazed this trail.

Hope this helps - 73 de Marty, KN0CK

#3

Thank you for the suggestions.
The LIMESDR Case in the link is no longer available. (The Link is dead)
This link describes the settings to turn off the TX:

Their observation was: Even if you turn off the TX there is still a signal around -55 dBm.

Thats why we were considering severing the TXPLL clock to LMS7002m.

#4

Best is to just supply the board with USB3.0.
If you need to use a supply, no more than 8VDC.

Ed

#5

If you can nolonger calibrate the receive path I’d have thought this would have an adverse effect on performance.

Maybe @Zack can comment on your proposed modifications.

#6

We plan to use the RX1_L input with the EasyFIX2 applied.
Reading the schematics, we are not sure if RX1_L has a calibartion path using the SPDT switch.
If RX1_L is on the calibration path can we apply the mods described in the link below?

#7

These mods are only for the limesdr-mini. I don’t know if the limesdr-usb board have the same issues as the lime-mini.

#8

@hobbyrf,

The Easy Fix was employed on the LimeSDR USB model only and does not apply to the Lime Mini at all…I know…I was the first one to apply it in late 2017 and the LimeSDR Mini didn’t show up until early 2018.

73 de Marty, KN0CK

#9

@Martywittrock , you are right for the easy fix (which improves HF reception on lime usb), but I was talking about the mods for reducing 100MHz spurs on lime mini as Crater quotes. :slightly_smiling_face: