Does the waterfall display of your digi-mode software have a lot of 50/60 Hz harmonics and other noise in it - that doesn't come from your receiver or antenna? Chances are, if you see such noise and harmonics garbage in your waterfall display, it will also be on the audio output of your PC! This means that you may actually be transmitting it with your digi-mode software. You don't want that!
The screenshot below shows what my 100-2500 Hz waterfall display used to look like - not anymore! For a long time, I blamed it on QRM generated in/by the apartment building that I live in, and big-city QRM levels in general.
Lots of noise and 50 Hz power harmonics in my receiver's audio spectrum
All the way up to about 8-9 MHz, my receiver's S-meter showed a noise level of at least S8 - try working DX with that!
S-meter without bonding wire (noise = S8)
Then I installed a multi-strand copper bonding wire (#14 AWG, 1.6 mm Ø) between the chassis of my laptop-PC and the ground/return of the external 12 VDC power supply of my transceiver. My laptop is an older model Dell, so it has several D-sub connectors: a DB9 serial-port, and a DB15 display output. Their shells are connected to the laptop's chassis. The metal shell of some other connectors (USB, FireWire) is also connected to the chassis. The DB-connectors have two nuts, for the lock-screws of the mating plug. I made a bonding wire with a small ring lug on one end, and a large ring lug on the other. The small lug is screwed onto one of the D-sub connectors. The large lug goes on the "-" binding post of my transceiver's external 12 VDC power supply.
The bonding wire is connected to the chassis ground of my laptop
This made quite a difference!
My receiver's audio spectrum - after installing a bonding wire between PC-chassis and 12 VDC power ground
To determine where the remaining noise traces came from, I disconnected the laptop's external power adapter (from the wall outlet!!!) and ran the laptop on its internal battery. Ref. 1. The audio spectrum is now basically clean above about 300 Hz:
Installing the bonding wire and running on battery completely cleaned up the audio spectrum
On 80m, the S-meter of my receiver also shows the noise reduction: as much as 5 S-points!
S-meter with laptop on battery (power adapter disconnected), with bonding wire (noise = S2+)
Comparison of audio spectrum with laptop on battery vs. laptop on external power adapter
(bonding wire installed in both cases)
The particular laptop power-adapter that I used with this laptop, is a cheap, simplistic switched power supply (not OEM). It has totally inadequate filtering on both the input side (ref. 2, 3) and the output side (ref. 3). Note that not all laptop power adapters (or desktop internal power supplies) are this bad!
The circuit card of a cheap laptop power adapter
Note that interference from the power adapter is not just conducted to the transceiver. It is also radiated. If I unplug the 19 VDC from the laptop and touch the barrel connector (and not touch anything else), most of the power supply noise returns!
I have a second laptop, on the same desk as the other one. It is not connected to my transceiver, other than via the filtered 220 VAC power strip. I have now also added a bonding wire between this second laptop and the power-supply of my transceiver. This has reduced the receiver noise a tiny bit more. Every little bit helps! As this laptop is in a docking station, the bonding wire is screwed onto one of the D-sub connectors of the dock.
S-meter when second laptop is now also bonded to the power-supply of the transceiver
- Ref. 1: "Computer to Rig Interfacing - You Don't Need to Buy an Interface", Jim Brown (K9YC), 91 slides presentation [pdf]
- Ref. 2: "Reducing switching supply racket (RF interference)", Clint Turner (KA7OEI), blog, December 8, 2012 [pdf]
- Ref. 3: "Completely containing switching power supply RFI", Clint Turner (KA7OEI), blog, August 18, 2014 [pdf]
External links last checked: October 2015
©1999-2016 F. Dörenberg, unless stated otherwise. All rights reserved worldwide. No part of this publication may be used without permission from the author.