[Moon-Net] RFI from laptop via USB cable

James Morris jmorris at namei.org
Thu Apr 22 20:57:44 CEST 2021


Thanks for this info! I think I had the attenuator at the wrong end of the 
chain (if needed at all).


James W7TXT


On Thu, 22 Apr 2021, Charles wrote:

> Hi James
> 
> On 10GHz, and really any EME application on the higher bands, dynamic range
> isn't an issue.  Putting an attenuator between the preamp and transverter
> (unless you need a small value one to prevent oscillations, as has been noted
> I think by Kjeld) will increase your system noise figure.
> 
> I do have an attenuator in my system, but it is located just before the main
> receiver.  I certainly need an attenuator ahead of my RSP1A (which I use
> together with Spectravue for sun/moon noise measurements) to prevent ADC
> overload. But i only put in just enough so that the system noise is not
> worsened to any significant extent.
> 
> 73
> 
> 
> Charlie G3WDG
> 
> On 22/04/2021 09:06, James Morris wrote:
> > Yep, I'm sure it's at IF.
> >
> > I have an attenuator between the preamp and the transverter, which I'll
> > try removing, thanks.
> >
> > I'm finding it difficult to avoid driving the transverter rx path into
> > saturation when measuring sun noise. I also have the rx gain pretty far
> > down in the transverter (this is a Kuhne G4). I've spent quite some time
> > experimenting with attenuators and rx gain to get the best dynamic range
> > here.
> >
> > And I will also try directly connecting the audio.
> >
> >
> > James W7TXT
> >
> >
> > On Thu, 22 Apr 2021, Charles via Moon-net wrote:
> >
> >> Hi James
> >>
> >> It sounds like the QRM is coming in at the IF.  Do you have any attenuation
> >> in
> >> circuit between the transverter and the 817?  If so, I would take it out so
> >> that the noise from the transverter (and preamp if you have one) might be
> >> at a
> >> higher level so the QRM is then sufficiently masked.
> >>
> >> Another alternative might be to try to connect the 817 directly to the
> >> audio
> >> input of the PC.
> >>
> >> 73
> >>
> >> Charlie G3WDG
> >>
> >> On 22/04/2021 06:09, James Morris via Moon-net wrote:
> >>> I'm wondering if anyone has experience dealing with RFI from a laptop (or
> >>> other computer) when connecting audio via USB?
> >>>
> >>> I have a Kuhne 3cm transverter, an FT-817, and a Rigblaster (which uses a
> >>> USB audio connection to my laptop). This has been working fine for
> >>> terrestrial contacts, but I'm running into issues trying to receive the
> >>> DL0SHF beacon.
> >>>
> >>> When the USB cable is connected, there's a bunch of interference on ~144.0
> >>> MHz, with sidebands extending up at least 25kHz, which makes a mess of the
> >>> spectrum at the beacon frequency (144.025 at the IF radio, +/- doppler).
> >>> I've tried adding ferrites to the USB cable but with no effect.
> >>>
> >>> The laptop is a Thinkpad T470s which has otherwise worked fine with weak
> >>> signals at higher frequencies in the band. This is running on batter
> >>> power, also.
> >>>
> >>>
> >>> Any ideas / suggestions?
> >>>
> >>>
> >>> - James W7TXT
> >> _______________________________________________
> >> Moon-Net posting and subscription instructions are at
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> >>
> >>
> 
> 

-- 
James Morris
<jmorris at namei.org>


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