Weird imaging from 500 MHz++ on 100 MHz++

#1

I have a weird problem with my v1.4 LimeSDR. I see DVB-T and DAB signals all over on 2m band (100-200 MHz). The picture shows the original signal on 725 MHz at the bottom and the mirrored image on 145 MHz. Around 187 Mhz I even have GSM signals from ~950 MHz.
The original signals are very strong - is there any way to apply filter settings to eliminate this mirroring?
The problem appears at any bandwidth, sample rate and RX input.

Version information:
  Library version:	v17.02.0-myriadrf1~yakkety
  Build timestamp:	2017-03-08
  Interface version:	v2017.2.0
  Binary interface:	17.02-1

  Device name: LimeSDR-USB
  Expansion name: UNSUPPORTED
  Firmware version: 3
  Hardware version: 4
  Protocol version: 1
  Gateware version: 2
  Gateware revision: 6
  Gateware target: LimeSDR-USB

#2

I dont see 187Mhz in this picture … what was the Lo set at?

think about this for a moment …

145Mhz X 5 = 725Mhz, im not sure what you mean “at the bottom”

I made a video about using the LimeSDR on harmonics of the Lo (Lo is a square wave signal)
Im sure your seeing the same thing … Harmonic operation

You should place a bandpass filter on your Lime for the frequency span your interested in …

I have these in hand … one for 2m, 1.25 and 70cm
https://www.hobbypcb.com/products/uhf-vhf-radio/144mhz-band-pass-filter

#3

Here are some further screenshots, that should show the problem in detail:




#4

Lol … 003 and 004 are almost the same …

Your just seeing the Mixing of the 5th harmonic of the Lo …

#5

I am just wondering, why this happens? I have a HackRF too, which doesn’t have this problem at all.
Building or buying hardware filters for every band would be just insane. The board is unusable with this imaging…

#6

I don’t know what hardware a HackRF has … what kind of Lo and if it was direct sample or 0 If …

#7

Oh …

Put your hackRf in bypass mode … it has filters.

#8

Yes, it has adjustable filters, the LimeSDR doesn’t? This is a pain in the a** then…
Do you also see signals that strong imaging somewhere?

Anyway, i’ve written an email to limemicro concerning this problem.

#9

I even see the signals mirrored at another frequency

#10

Your going to get a blank look …

I think its a great thing …

Im 100% sure that the LimeSDR can be used well into the 47Ghz range …

I found why the HackRf does not have hte mixing image of the Lo – honestly it should, just not nearly as strong as what the LimeSDR has … in the PDF for the RFFC5072 programmable Lo AND mixer …
“The LO signal is buffered and routed to the integrated RF mixer which is used to convert frequencies ranging from 30MHz to 6000MHz.”

HackRf has a buffered(filtered) Lo and i dont think the LimeSDR does … it is a Hard Square …

#11

935Mhz divided by 3 is? – think harmonics of a square wave …

#12

It might be a great thing, if there would be an on/off switch for this ‘feature’ :smiley:

yep, i got the point with the harmonics, but i would rather want to see the signals, where they should be.

#13

Ha !!! yea, probably …
So would a single antenna port for each section … but its 6 Rx antenna ports and 4 Tx … with no Rx / Tx switching …

#14

Too bad the HackRf didn’t have a 120Mhz ADC … that would push it down to 1Khz Rx too …

#15

It also just has 8-bit ADC & DAC. RX performance is similar to RTL-SDR dongles.

#16

What are you using for an antenna anyways ?

#17

A wide-band antenna for 25-2000 MHz http://www.thiecom.de/eurostick.htm

I disassembled it once - it is basically a copper wire with traps.

#18

Ha … yea i was just thinking what it was under the plastic …

Iv seen some overstated stuff for the wireless market …

Traps … humm … Not for Tx … ohwell …

I just have an Arrow dual band J-pole…

#19

Yep, it is also stated in german on this website, that it is not suitable for TX, but it is a good receiving antenna when mounted outdoors. However, we’re going off-topic, in case somebody else wants to help me solving the harmonic ‘fea-ssue’ (feature/issue)

#20

Ummm … I don’t think its going to happen … probably use an antenna for the band you want … (probably not enough filtering)

I even use a pre-selector in an Atlas350XL to limit the Rx to around 30Khz around the specific HF frequency i want to listen to.