Below 30Mhz on LimeSDR Mini - Full of garbage

#1

From my use of my LimeSDR Mini I have found that FM garbage is everywhere. Even when almost no FM stations are viable on the FM Broadcast band. (I have an FM Stop Filter and also they are bearly visable)

Yet there is still a ton of them from about 50 Mhz and lower.

Why does the LimeSDR Mini have such a bad signal imaging problem?

#2

Use good hardware BPF (or LPF) with high attenuation above 30MHz. Also, try RX GFIR filters.

#3

I have tried with an MFJ 30Mhz lowpass filter and it seems the LimeSDR Mini is pretty deaf. Compared to an rtlsdr with direct sampling?

Seems also that unlike the rtlsdr it required this filter to not see all the nosie.

#4

Unlike RTLSDR LimeSDR has much wider receive bandwidth (30MHz-3.8GHz) and sample rate can go as high as 80MHz. It has built in Rx LPF (up to 60MHz) and 3xGFIR for additional filtering. Sometimes it is not enough! Because of very strong broadcast station with ERP in MW you will need BPF with high selectivity (low insertion loss) and high attenuation outside band.
HF part is a little different story: you can’t receive or send directly on HF frequencies before upconversion (for Rx) or downconversion (for Tx). For example: 20m band is upconverted using NCO (NCOfreq = Losc30MHz-Rx14MHz) with Rx LPF set to 40MHz (sample rate can be low as 1.536MHz).
Many people forget this LPF settings and as a final result you will have deaf receiver!


This is example how 20m band looks with BPF+GFIR set to 0.4MHz.


This image is without GFIR filtering. Notice big bump on the right side of the screen! It is from GSM signals in the 840MHz region because I don’t have enough attenuation above 30Mhz.

#5

Okay? Still disappointed in MyriadRF about that up converter stuff to. I was sold an SDR that only supports down to 30Mhz with a upconverter attached. Which is NOT what was advertised.

But atleast I know it works for anything up to 3ghz. (Which is sort of boring. I bought this for Ham Radio HF stuff mostly.)

#6

“Upconverter” is feature inside LMS7002M, not additional hardware. It uses NCO for HF but NCO can be also used above 30MHz (for example set NCO to 10MHz, Tx center frequency to 154MHz and your Tx PA LPF will suppress LO leakage for 2m band). Ham radio is not massive market as 3G,4G and others multiRx-multiTx modulation scheme. SDR software is usually with many options which are not set automatically and can cause frustration sometimes.
Anyway, Mini is advertised as 10-3800MHz SDR Rx-Tx (in practice without modification input circuit can receive 40m band) not as HF (1.8-30MHz) and that is what works well. Take a look at:
https://pskreporter.info/pskmap.html?preset&callsign=yt7pwr
Not bad for dipole antenna (4m high inside attic, oriented N-W).

#7

How are you getting to 7mhz for 40m? I can bearly get to 10Mhz with out having to push the sampling rate out of spec. (Annoyingly NCO is limited based off sampling rate)

#8

Currently I’m using 1.536MHz sample rate which allows reception down to 30-(1.536*16)= 5.424MHz (1.536MHz because it can downsample very easy to 48KHz audio VAC sample rate, 16 is maximal decimation factor, 30MHz is on SXR page Rx center lowest frequency).

#9

Maybe I don’t understand the NCO correctly. Isn’t it the freq being feed into the mixer that makes up that up converter?

#10


(page 3 and much more…)

#11

I guess I am to use to how RF mixer stuff works as a physical device and how I can click 1 button with my other 3 SDRs and instantly get HF stuff (lower then 30mhz)