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What's at 100 kHz ±

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jpanhalt

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I am building a LW receiver/amplifier like this: https://sidstation.loudet.org/hw-en.xhtml

My Max 275 filter (https://www.maximintegrated.com/en/products/analog/analog-filters/MAX275.html ) will arrive (I hope) Monday. I have added some small capacitors (8.2 to 22 pF) across the TL082 op-amps I am using as low-pass filtering. With just three stages ( about 60 dB gain) I am picking up something strong around 100 kHz. My target (center) frequency for this project is 250 kHz. Attached is a really crude LT Spice schematic that shows a close approximation of what I built. Some of the resistor values were changed to accommodate what I had on hand. The capacitor values are what I used.

What could it be? Location = NE Ohio about 40 miles West of Cleveland. My first assumption is it is some sort of oscillation that I will need to deal with. But just in case someone knows of a LW beacon in the area, I thought I would ask.

John

Edit: In real life, the coupling capacitors are not shorted. I did that to see the effects on the simulation.
 

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There is a navigation system called LORAN which is on 100kHz. Sounds like a rough noise when you listen to it.

JimB
 
Good morning Jim,

I remember LORAN C from my pre-GPS days. The Seneca Great Lakes station ( outside Rochester, NY) transmitted at 1 MW and is about 300 miles away. However, all USA stations apparently stopped transmitting in 2010.

On further investigation (I fixed my antenna and used the shorter of the two coils), I believe the frequency is closer to 53 kHz and comes from my Tek TDS210 scope. It is quite strong and directional (saturates my "receiver"). Here it is this morning:
52 kHz_Capture.PNG




Here it is after partially shielding it with the hat I wear ;) :
upload_2015-12-6_7-14-36.png


Can't wait for my tuning IC to come tomorrow.

On a slightly related subject, ICOM makes a couple of models that are called affordable when new. The portable ICOM IC-R6 (blocked for cell phone) is about $200 and the desk model IC-R75 is about $650. Do you or other hams here have any opinions about either model?

Regards, John
 
I have an IC-R72 which is the predecessor of the R75 I believe.
The R72 is a competent receiver. I use it as a general purpose bench receiver for listening to ... whatever needs listening to on my workbench.

It was very secondhand when it was given to me, it had spent a hard life on a North Sea fishing boat. Even now when the case is removed, there is the smell of stale cigarettes inside.
One day if I get the inspiration, I will replace all the push buttons on the front panel. The cigarette fumes make them a bit intermittent in operation.

JimB
 
UPDATE: The 'scope has a 47 uF/450V capacitor for the screen backlight power supply. It read about 1.5 Ω, but was very slightly puffed. I replaced it with a new cap (ESR = 0.53 Ω), wiggled the copper Brillo pad connecting the case to ground, and it works fine now. Noise on the signal channels is a little now than before, and I don't detect the interference at 53kHz.

John
 
Not walking distance. Google Maps is a good resource to get a more accurate distance from Cleveland, Ohio to Murfressburg, TN. It's about 512 miles.

John
 
So John, what is your interest in Solar activity? Is this to help you determine HF signal propagation on a given day based on solar activity?
I recall when I was stationed on a ship, we had this rack of gear called a CHIRP sounder, what it did was determine the best usable HF frequencies to use on a given day.
 
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