I'm trying to make a 60 kHz WWVB (time broadcast) radio receiver.
I built two op-amp active filter circuits with lots of gain, tuned them to 60 kHz, connected them in series, connected a loop antenna to the input and looked at the result with my oscilloscope.
With the antenna connected, there was a strong 60 kHz signal but no modulation was visible at all, no matter how much I tried to fine-tune the filter center frequency.
But then Sunday night (Monday morning), I woke up a 4 AM, couldn't sleep so I figured I'd fuss around with tuning the filters again.
Surprise! There was the WWVB signal, as crisp and clean as you could want. Every second, the signal dropped in amplitude, then raised again in long and short pulses. Steady and solid.
Monday evening, around 8 PM, there was a strong unmodulated 60 khz output again but zero pulse modulation was visible.
On Tuesday morning around 4 AM, just like 24 hours earlier, the pulse modulation was crisp and sharp.
Both 4 AM signals seemed to be a fair amount lower in peak amplitude than the evening signals, but the pulse modulation was crisp and sharp. The evening signals showed more overall power but no pulse was visible.
I could understand that the pulsed signal would vary over the time of day, but why would there be a strong unmodulated signal at 60 kHz?
Is that more likely man-made noise, or natural noise?
I built two op-amp active filter circuits with lots of gain, tuned them to 60 kHz, connected them in series, connected a loop antenna to the input and looked at the result with my oscilloscope.
With the antenna connected, there was a strong 60 kHz signal but no modulation was visible at all, no matter how much I tried to fine-tune the filter center frequency.
But then Sunday night (Monday morning), I woke up a 4 AM, couldn't sleep so I figured I'd fuss around with tuning the filters again.
Surprise! There was the WWVB signal, as crisp and clean as you could want. Every second, the signal dropped in amplitude, then raised again in long and short pulses. Steady and solid.
Monday evening, around 8 PM, there was a strong unmodulated 60 khz output again but zero pulse modulation was visible.
On Tuesday morning around 4 AM, just like 24 hours earlier, the pulse modulation was crisp and sharp.
Both 4 AM signals seemed to be a fair amount lower in peak amplitude than the evening signals, but the pulse modulation was crisp and sharp. The evening signals showed more overall power but no pulse was visible.
I could understand that the pulsed signal would vary over the time of day, but why would there be a strong unmodulated signal at 60 kHz?
Is that more likely man-made noise, or natural noise?
