[Lowfer] re Re: WWVB clock on 60 kHz ( LPF usage)

[Michael Sapp]

FB Garry:  OK on all.  A decace ago when I was VHF/UHF contesting at W3KWH 
Dave K1WHS suggested I do a power level sweep on all the antennas to get a 
better handle on the kind of overload levels the receivers were seeing. That 
was instructional and lead to the construction of 1/4 wave spaced dual 1/4 
wave stubs to suppress amplifier harmonics, which cured 6M 3rd harmonic 
overload energy on the 6M amplifier output to solve the 2M station receive 
overload. Likewise a pair of 1/4 wave stubs for 432.6 MHz on the 2M 
amplifier output cured the third harmonic receiver overload at 432 receive. 
The killer was being 1 mile or so from an L-band ATC radar that would dump 
10mw into the 1296 loop yagi when pointed at the radar site. After the 2nd 
trip to the repair shop for front-end failures in a TS-2000X we had to find 
a fix.  You could just about charge a battery with a diode connected to the 
1296 antenna (lol) pointed at the radar site.

Point being that similar overloads can occur at LF/MF and the daylight power 
levels for AM BC are much different then night for a local environment. So 
it pays to do an RF level check for both mid-day and evening times as many 
AM stations run low power at night.  I have found that my 5th order 500 kHz 
LPF works OK at night, but I need the 9th order 300 kHz LPF in the day to 
suppress enough AM station and detectable AM spur emissions to get maximum 
LF rx system performance.   I think I copied every 15 minute sequence on 
K3SIW on 1750m all day since 1230 UTC using the 300 kHz LPF, but that would 
never happen with the 5th order 500 kHz LPF, it just does not have enough 
cut-off attenuation to handle the offending AM BC signals in my local RF 
environment.

You might find that a high order LPF in the 200~300 kHz cutoff range may 
work better than your current AM band reject filter for improving LF system 
performance.  I stayed with the Butterworth LPF design for it's excellent 
return loss given that I use a high level DBM that wants to see a good 50 
ohm I/O match. That has worked well for me, but not having a PhD in physics 
I cant claim any expert knowledge in the subject area...

73 Mike wa3tts


----- Original Message ----- 
From: "Garry" <k3siw at sbcglobal.net>
To: <lowfer at mailman.qth.net>
Sent: Monday, February 06, 2017 5:24 PM
Subject: Re: [Lowfer] WWVB clock on 60 kHz


> Mike, I just got home and uploaded some SIW wspr-15 decodes. I see you're 
> decoding the signal too. I have an HP435B power meter with a high 
> sensitivity probe (good down to -70 dBm). Will try that on the signal 
> going to my sdr-iq with and without the band-reject filter. As far as what 
> spectrum laboratory and spectravue show at LF there isn't any significant 
> difference in the noise floor, but the spurious garbage seems to be less 
> and that's a good thing.
>
> I have several RTL2832 dongles running here at the moment and for $20 they 
> do an incredible job.
>
> On PC1 I listen to the local NOAA weather channel at 162.425 MHz. That SDR 
> is preceded by a selectable 100 MHz upconverter so I can tune in things 
> down to almost 0 Hz if desired. I also have no problem copying my local 
> 1296 MHz beacon directly with it. Even with the miniature clock crystal 
> it's quite stable. But since the units are so cheap one of these days I 
> may tear one apart and apply a GPS-referenced clock.
>
> On PC2 I'm currently monitoring aircraft traffic via ADSB packets at 1090 
> MHz and RTL1090 + adsbScope2.7 software. That PC also produces weather 
> maps from the NOAA -15 and -19 satellites using SDR#, WXtoImg, and 
> Orbitran software. Running all that plus spectravue, spectrum laboratory, 
> argo, firefox, and thunderbird still only loads the CPU about 15%.
>
> And when radio gets boring I have RTL2832-type dongles that decode NTSC 
> and ATSC signals so I can watch TV too, hi.
>
> I'd like more A/D bits than my ancient SDR-IQs provide too. I know the 
> Perseus does 14 bit conversions. Wonder if by now there's stuff out there 
> that is up to 16 bit resolution (or more!) and what it costs.
>
> -- 
> 73, Garry, K3SIW, EN52ta, Elgin, IL
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