Listening to the ether

[carbonzit]

Sorry for the cryptic title; this is inspired by another thread here, about trying to listen to high-frequency signals normally out of auditory range. (For instance, bringing the HF components of a lightning strike down to the audio spectrum.)

So what if a guy were to build something like this:

**broken link removed**

Let's say the HF ranges of interest are somewhere in the 300 kHz-2 mHz region, with a few switchable notches on the oscillator. The low-pass filter would limit the signal to the audio range, rejecting the upper frequency from the mixer.

Not sure how the mixer would work. What if you just combined the two signals (wired together): wouldn't that produce the sum and difference frequencies? or do you need an active mixer stage here?

In any case, if anyone could come up with a quick first-pass circuit for this, that would be nice.

 

[dougy83]

Frequency shifting is usually done using a multiplier; this gives you a spectrum containing frequencies that are the sum & difference of those in the input spectrum & the mixing frequency. By using a simple summer, the beat frequency can be gleaned using an envelope detector.

Either way, you need to filter out the frequencies of interest before feeding them into the mixer or there will be all sorts of rubbish in the AF output.

 

[ronv]

Color me lazy, but I think I would just buy a cheap am radio and tune it to a spot with no stations.

 

[carbonzit]

Either way, you need to filter out the frequencies of interest before feeding them into the mixer or there will be all sorts of rubbish in the AF output.

OK; is this better?

**broken link removed**

(The bandpass filter would need to be adjustable to track the selected oscillator frequency.)

Color me lazy, but I think I would just buy a cheap am radio and tune it to a spot with no stations.

Heh; I'm lazy too, maybe just not that lazy!

 

[Dr_Doggy]

the Fselect would tune the bandpass filter, then you could run your lightning signal (Mhz am band) to something that divides the freq by say 1000 or 10k which would bring a signal to an audible level for the speaker, also generating different tones for different frequencies,

also isn't a bolt of lightning a 100ms HV DC current?

 

[carbonzit]

also isn't a bolt of lightning a 100ms HV DC current?

Yes (not sure about the exact duration, but I'll take yours as reasonable). But any DC discharge of that magnitude and duration generates tons and tons of harmonics, spikes, and just plain weird signals that go into the megahertz range. That's what one might want to "listen" to.

So I've decided, folks: I want to build this thing. Any suggestions on circuitry? any interest? surely someone out there could fire up their LT Spice or whatever and come up with a first-pass circuit.

 

[Reloadron]

Actually you may want to give this link a read. I also believe doggy's reference to a multiplier should have been mixer since a mixer is what was described.

Anyway, as humorous as ronv's suggestion was:

Color me lazy, but I think I would just buy a cheap am radio and tune it to a spot with no stations.

it was pretty much on target. What you want to build is a superheterodyne AM radio receiver. You mention a low end of 300 KHz and an upper end of 2 MHz. The AM standard US broadcast band is 540 KHz to 1.650 MHz. So if you want to build one you are really just building a simple superheterodyne AM radio receiver that will tune a little below and above the standard AM broadcast band. Links like this one will show up with a good Google of "AM Radio Superheterodyne Receiver Circuit".

This isn't really LT Spice material I don't think. All you would be doing is slightly expanding the bandwidth of a basic AM Broadcast Band superhet radio. The 455 KHz IF would be the same and you would modify the LO (Local Oscillator) a little. That is how I would approach it anyway.

What you choose to do with the audio out is your call. I guess you could feed it to a data logger and chart the data much like seismic activity would look. Don't know how you would correlate it to anything though?

Ron

 

[carbonzit]

OK, now we're getting somewhere.

Yes, I know this is looking suspiciously like a garden-variety AM receiver. But humor me: I really would like to try to build this in its present form. IF? Who needs IF? After all, I couldn't care less about selectivity here.

So regarding mixing the HF and LO (local oscillator) signals: can someone tell me if this

**broken link removed**

would work? If not, why not? As I see it, if you simply combine the two signals electrically, you'll get the sum and difference signals you want. ASS-U-MING, of course

• That the two signals are at approximately the same amplitude
  
• That combining them in this way will not excessively load one or the other of the sources

So far as what I'd want to do with the output, at first I'd just want to listen to see if there's anything aurally interesting in the signals.

 

[Brevor]

This type of thing has been done before, Google "Whistler receiver"

 

[carbonzit]

This type of thing has been done before, Google "Whistler receiver"

Thanks, but with all due respect, this isn't exactly what I'm looking for. Remember, I'm interested in frequency-shifting the original signal, down to the audio band.

This receiver, for example, just amplifies the RF and feeds it to a soundcard. Kinda-sorta what I'm trying to do, but not really.

By the way, I have no doubt that I'm not the first person to come up with this idea. In any case, I'm after circuits. Give me circuits!

And I really would like to know if my degenerate signal mixer up there will work or not.

 

[Reloadron]

IF? Who needs IF?

You have IF,

The IF (Intermediate Frequency) is what is shown as your Mixed Signal Out. Taken from the link I posted:

3. A superhetrodyne receiver works on the principle the receiver has a local oscillator called a variable frequency oscillator or V.F.O.

This is a bit like having a little transmitter located within the receiver. Now if we still have our T.R.F. stages but then mix the received signal with our v.f.o. we get two other signals. (V.F.O. + R.F) and (V.F.O. - R.F).

In a very basic receiver like this you have the Local Oscillator and you have the RF (Radio Frequency) signals. They are mixed in such a way the constant difference is 455 KHz and that 455 KHz is the IF. This shows as Mixed Signal Out in your drawing. Much easier to amplify a single frequency than the entire tuned RF spectrum. Eventually we demodulate the signal and the end result is the audio.

Ron

 

[carbonzit]

Well, yes, except that in this case my IF is AF. In other words, I want to set up things such that

RF - LO = AF​

(while rejecting the sum signal, RF + LO)

so I'm not really using this as IF in the conventional sense. Keep in mind that IF is normally an extremely narrow-band signal, used as you pointed out so you can have one or more stages of amplification with a narrow bandwidth.

Since I'm not demodulating the signal, I just want the difference signal. I'm not rejecting the carrier, as in a superhet receiver; the "carrier" (RF) is the signal here.

In other words, if I tuned my receiver to a normal AM signal, I'd hear the carrier as a constant tone somewhere in the AF range (and probably the modulated signal on top of the carrier as well).

 

[Reloadron]

OK, you want to listen to lightning? How about just building a plain and simple crystal receiver as found on these pages. Note the placement of the old 1N34 germanium diode. That is the detector and demodulates the received signal. You have the old forever 365 pF variable cap and a coil forming your tuning. Look Ma, no batteries. I built dozens of variations of the basic circuit as a kid. Rest assured it will "hear" lightening as well as strong local AM broadcast stations.

Then note the additions to the basic circuit for audio amplification. That is the simplest AM receiver you will find.

Ron

P.S. When the storm is upon you shunt the antenna to ground and pray.
The shunting won't do much but sounds good to say.

 

[dougy83]

OK; is this better?

Yes. The quality of the output signal will still depend on the quality of the initial bandpass filter.

would work? If not, why not? As I see it, if you simply combine the two signals electrically, you'll get the sum and difference signals you want.

It will sum the signals equally only if the source impedances are the same; this is why summers normally employ a couple of resistors to set the "gain" of each source.

see

 

[carbonzit]

It will sum the signals equally only if the source impedances are the same; this is why summers normally employ a couple of resistors to set the "gain" of each source.

[/quote]

So I take it you like this better?

**broken link removed**

Basically a dumbass impedance matcher, right?

Don't see as how I need a doubly-balanced mixer or any such here. I just want to get the beat frequency between the two signals.

 

[dougy83]

So I take it you like this better?

**broken link removed**

Yep. .

 

[carbonzit]

Hokay. So I have my mixer. Can probably come up with the LO part. I'm just missing two pieces, the bandpass filter and lowpass filter.

Anyone want to take a crack at a bandpass filter for this?

Specs: adjustable to a few ranges in the neighborhood of 300 kHz - 3 mHz or so. 3 ranges should do it.
Bandwidth large enough to encompass the fat middle of the AF range (let's say 100 Hz-5 kHz or so).
High selectivity not needed.

 

[ronv]

How will you keep out all the radio stations that are in that range?

 

[carbonzit]

How will you keep out all the radio stations that are in that range?

Welllll ... I guess I'd have to be able to skip around between them. I'd need to know what the average (or widest) bandwidth of their signal is, then set my LO to try to avoid those spots where there's a radio transmitter operating. Might need a more selective bandpass filter, too.

Is there such a thing as a sweepable (continuously-adjustable) bandpass filter over a certain range that has fairly high selectivity? or is this getting into exotic, non-DIY territory?

 

[ronv]

I guess you could use a resonant circuit like reloadron posted in #13.