[Lowfer] SDR RX Suggestions

[wa3tts at verizon.net]

Brian & All:  It may be worth taking a look at how much switched mode supply noise is on the USB cable shield between
the RSPdx and the laptop.  A few years ago I found I had considerable common noise between my external audio USB dongle 
plugged into my laptop and the audio line from my IF radio.  A USB filter helped a little bit, but there was a noticeable improvement
when I switched all three of my Asus T101HA PCs over to a commercial grade 5V 9A linear supply. I built up an isolation and filter
board to do this for the 5V supply.   There was a noticeable improvement in wspr2 decodes in the -25 to -34 range with the linear supply powering the PCs
for use at LF and MF.

https://www.dropbox.com/s/42w9t3ecwkb9z0b/cmcheck.jpg?dl=0


Note the 3mv peak to peak measurement on the DSO-112A pocket scope.  Half of that value is likely background noise in the scope.  This is hardly a lab grade instrument, but it does
offer a way to check for CM noise with more accuracy than an AM pocket radio as a noise sniffer.  The pickup is 10T # 24 wire on a 
Fair-Rite Type 31 snap-on bead. The 10T 1T ratio of the inductive pickup can then be used to estimate the measured CM voltage on a cable shield. 
I have also made up some inductive pickips with Type 75 Fair-Rite snap-on beads that have somewhat better LF performance, but those Type 75 Snap-on beads are like $12 each...

Neat thing about the DSO-112A is that it has 3Volt test signal built-in at 10kHz, 100kHz, 1MHz.  So with two of the inductive pickups I can
inject energy into one side of a rf cable shield on one side of a common mode choke and use the second pickup on the other side of the choke
to see the voltage difference. This is compared to the two inductive pickups side by side on a cable shield about 6 inches apart ( I see about 1 volt pickup to pickup).  The ratio of those two voltages
are then used to ball park the in-service choke performance in dB using  20*Log (V1/V2)...  Best to use a termination load instead of a receiver front end when
doing such tests.....

The little pocket scopes are also great for chasing CM noise on AC branch circuits or elsewhere.  I cant imagine dragging my Tek 465B around the house and the long test probes pick up too
much noise to be useful anyway...

Once I find a new CM noise source, I either kill it, filter it, or divert it away from my receivers and PCs...

73 Mike wa3tts





-----Original Message-----
From: Brian via Lowfer <lowfer at mailman.qth.net>
To: Discussion of the Lowfer (US, European, & UK) and MedFer bands <lowfer at mailman.qth.net>
Sent: Sun, Jan 31, 2021 6:28 pm
Subject: Re: [Lowfer] SDR RX Suggestions

 Hello Mike ...
And thank you for the e-mail and your suggestion.
I am currently using an old TS-590s with Argo that out performs the RSPdx
With the set up as is, I have copied EAR, SJ, WM and SIW, in fact, I can hear them all the time.
Was just looking for something better to snag KE, TIM and 22A.
For listening I use a LF Engineering Active antenna and a 1/4 wave insulated 160 meter vertical.  
Believe it or not the 160 meter vertical works the best which I wouldn't have expected.
73 de Brian, KG8CO


    On Sunday, January 31, 2021, 05:59:54 PM EST, N8OOU <n8oou at meekfarm.us> wrote:  
 
 Brian;