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/diy/ - Do It Yourself


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1537384 No.1537384 [Reply] [Original]

Why do crystal radios need to be 'tuned' to a second station for amplification? Why does it have to filter it if it's only using it as a power source? What prevents them from just using a full-wave rectifier and a capacitor?

>> No.1537400

>>1537384
This is /diy/, there is no 'them'. What prevents you?

>> No.1537402

>>1537400
What prevents the radios from using a full-wave rectifier and a capacitor?

>> No.1537407
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1537407

>>1537402

go ask in the ham radio thread; i bet those guys can tell you. I'd guess that the original ones were so crude with the cat's whisker diode that getting one to work was hard enough, and getting four to conduct was either too much of a pain, impossible, or didn't help. My understanding is that they do use capacitors in some crystal radios, but it seems to be part of the tuner, and not just an audio smoother

>> No.1537413

>>1537407
I don't mean for audio smoothing. You collect radio transmissions (without tuning), filter it to get a relatively constant voltage, then use it to amplify the extracted low-voltage signal of the desired channel.
Also, if you're using a germanium diode, why use coils and not a potentiometer?

>> No.1537416

>>1537413
>why use coils and not a potentiometer?

because an inductor and a capacitor makes a better tuned circuit than a resistor and a capacitor? Like I said, go to the ham thread, or the ohm thread, and you'll find some knowledgeable guys, but I'd refine my questions first, and not ask one like this one.

>> No.1537421

>>1537407
With regards to the cats whisker on xtal diode, the ones using blue razors as makeshift trench radios on the battle front were even more fun, however you usually were lucky enough to have that one asshole somewhere within an area that knew how to do it.

>> No.1537427
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1537427

>>1537416
Wouldn't a bandpass filter with two resistors and two capacitors be good enough?
>>1537407
Yeah, but I mean for modern use.
Why did they make the old ones tuneable? Why not just make one radio for each frequency and swap out the earphones, if those were the expensive part?

>> No.1537529

>>1537427
Because its cheaper to make one.

Each diode has voltage drop and when you are dealing with stuff all voltage in the first place....

Also search Q factor.

>> No.1537571

>>1537384
>Why do crystal radios need to be 'tuned' to a second station for amplification?
Crystal radios work fine tuned to one station.
>Why does it have to filter it if it's only using it as a power source?
The filter is to select the station you want to listen to and block the others.
>What prevents them from just using a full-wave rectifier and a capacitor?
Voltage drop across the diodes.

Do you understand how crystal radios work?

>> No.1537577

>>1537529
It'd be much smaller with semiconductors though.
>>1537571
Yes, I do.

You're misunderstanding. If you want to pick up a distant station, then un-amplified it will be very weak. If you use signal A as a power source to amplify signal B, you can pick up on more distant stations. Why would you need to do any kind of filtering on the 'power source' signal? That's also why I'm wondering about the full-wave rectifier.

>> No.1537644

>>1537577
It would be simple enough to just build one and demonstrate your improved version.
Be certain to post your astounding results.

>> No.1538427
File: 19 KB, 500x344, crystalradioschematic.jpg [View same] [iqdb] [saucenao] [google]
1538427

>>1537384
(not my drawing BTW, yanked from the internet)
>The Arduino generation can't into even a basic crystal AM radio!
This is why everyone needs to learn basic analog electronics BEFORE fucking around with microcontrollers.

The coil and variable capacitor form a tuned circuit so you can select a particular frequency (i.e. station).
The diode (detector) turns the tuned RF AC signal into a variable RF DC signal.
The capacitor between the cathode of the detector and ground low-pass filters the RF component of the signal, so all you get is the amplitude variations (i.e. the '''modulation''' of the carrier wave).
Now on the output all you get is the audio signal from the radio station.

If you google 'crystal radio schematic' you'll see some that don't have the filter capacitor on the cathode of the detector. That's because they're counting on the inherent capacitance of the '''earpiece''' (which is a piezoelectric crystal-type earphone or headphone) to filter the RF component of the signal.

>Why do crystal radios need to be 'tuned' to a second station for amplification?
I'm not sure what you mean here. In a crystal AM radio receiver there is no '''amplification''' of anything, it's running entirely on the strength of the carrier wave. That's why you need either a crystal earphone or a high-impedance (~2000 ohm) headphone for it to work at all; there's so little curent available that something like an 8-ohm speaker would give you nothing.

>What prevents them from just using a full-wave rectifier and a capacitor?
A '''full-wave rectifier''' would have double the voltage drop because of twice as many diode junctions, when the signal is so small to start with. That's why you always see these using something like a 1N34A diode, which is germanium instead of silicon, and has a typical drop across the junction of 300mV (or less).

>> No.1538445

>>1538427
No, I know how it works, but there's also a variation outlined on the wikipedia page where you use a high-strength local station to amplify a remote station. My question is why it can't use the carrier wave as a whole, without first filtering it.

>> No.1538462

>>1538445
Do you understand how Amplitude Modulation works? Describe it to me in your own words so I know whether you understand it or not. No copypasta from the internet.

>> No.1538471
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1538471

>>1538462
If you have an audio signal of amplitude N, you generate a sine wave that varies between +N and -N. So if your frequency is denoted by f and your audio signal is denoted by f(x), you're transmitting f(x) sin fx/2pi.
Then the receiver calculates the envelope, for instance by throwing away everything below 0 and computing a moving average.
Why does the filtering make it easier to utilize as power source?

>> No.1538483
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1538483

>>1538471
>Why does the filtering make it easier to utilize as power source?
Why does filtering stepped-down AC line voltage make it easier to utilize as a power source?

I get now the (strange, but comprehensible) idea you're talking about, now.

What you're describing, theoretically, is 'harvesting power' from a strong local AM signal.
The filter in this case is just there to accumulate charge, which (theoretically) could be used to power an RF amplifier for another AM signal you're tuned to.
The amount of power available would be very small though. At best I'd think you could use it to power a single FET used as an RF amplifier in front of the diode detector.

I'd also think that the best way to implement a scheme like this would be to *not* tune a single AM signal to rectify and use as a power source, but to use *all available* AM signals. You might even use a separate antenna to 'harvest' all the available AM carriers for this purpose.

However this sounds like a more complicated idea than it has to be, if all you're trying to do is receive a relatively weak AM radio station. A better, simpler idea is to simply use a longer longwire antenna, placed as high up as you possibly can. Also be sure to use a good solid earth ground.

>> No.1538516

>>1538483
>I'd also think that the best way to implement a scheme like this would be to *not* tune a single AM signal to rectify and use as a power source, but to use *all available* AM signals. You might even use a separate antenna to 'harvest' all the available AM carriers for this purpose.
Yeah, so that's why I'm wondering why you tune it to a specific station and not just use anything you can get your hands on.

>> No.1538523

>>1538471
>using f for a constant and a function

oh shit wtf are you doing

>> No.1540875

>>1537384
i dont know