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.
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:
Here it is after partially shielding it with the hat I wear :
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?
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.
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.
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.