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Comment by ncmncm | Hacker News Reader

If you believe AC cannot be, at the same time, steady current, you are in for many more surprises. If you think your cell phone antenna picks up signals by some sort of "heating effect", you are in for many more surprises.

In fact all varying E-M fields induce electrical currents where they are absorbed. The effects of those electrical currents vary in myriad ways, depending both on the currents and on where they are absorbed. In living matter the currents are carried by motion of ions in solution. Not coincidentally, life is conducted largely by motion of ions in solution.

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How is alternating current steady when varies in magnitude? There is no current flowing on a zero crossing, there is a reason AC and radio use RMS current, are you sure you know what your talking about?

Incidentally the ADC's that sample the input dissipate the electrical energy captured by the antenna as heat however little it may be as is the case with all electrical circuits that sink electrical energy. So yes your cellphone does use a "heating effect" because where else does the energy go? Do you think it charges the battery or emits light with the captured energy?

All energy, no matter where absorbed, ends up as heat. So, the heat is meaningless to talk about unless you are making popcorn.

It is not the heat, at the end, that makes your phone work. You may read up on how antennas work, if you care to have any actual, you know, facts. I promise it won't say one word about heat.

No the ADC use resistors to measure voltage, resistors turn electric energy into thermal energy, they literally use how much time it takes to dissipate electricity as heat to measure voltage.

I know someone who needs to read up on antennas and digital receivers and its not me.

> they literally use how much time it takes to dissipate electricity as heat to measure voltage.

I never heard that, and looking at the list in https://en.wikipedia.org/wiki/Analog-to-digital_converter#Ty... I can't find any that work this way.

> The Wilkinson ADC is based on the comparison of an input voltage with that produced by a charging capacitor. The capacitor is allowed to charge until a comparator determines it matches the input voltage. Then, the capacitor is discharged linearly. The time required to discharge the capacitor is proportional to the amplitude of the input voltage. While the capacitor is discharging, pulses from a high-frequency oscillator clock are counted by a register. The number of clock pulses recorded in the register is also proportional to the input voltage.

The capacitor is discharged using a resistor of known value.

Also take it down to the comparator level which basically all ADC use the op-amps use transistors and resistors and are themselves dissipating the input power in order to measure voltage.

The word transistor is a contraction of transfer resistance, they are electronically variable resistors and as everyone knows dissipate power as heat during function.

After reading your comment a few times, I think now I understand the misunderstanding. There are two interpretations:

A) The main method that use a ADC to work is heating a resistance and then measuring the temperature or the time to cool down or something like that. [1]

B) As a side effect of the internal working, the ADC produce heat. The heat is not important for the operation, but it's an inevitable side effect.

My opinion: A) No, B) Yes

Can you both please answer A) and B) with "Yes" or "No"?

[1] Note that something similar is used to measure pressure https://en.wikipedia.org/wiki/Pirani_gauge .

Right, none has ever worked that way.

Radio receivers do not depend on an analog to digital converter, or even a resistor, to extract a signal from an antenna. The essential component is a diode.

(That is not to say that, having detected, amplified, and demodulated an antenna signal, the result won't finally drive an A/D converter; you can drive anything you want to after all that. Even a speaker. But operation of the speaker, too, does not depend in any way on heat.)

Digital receivers which are in cellphones are most certainly based on A/D convertors which is what you originally referenced. All captured antenna energy is dissipated as heat through resistance in order to extract useful information and convert it into the digital domain.

Even in a analog receiver the power amplifiers used to drive a speaker or whatever are going to dissipate the signals power as heat in order to modulate the amplifiers output power (out power will always be less than power supply power).

If you are talking about a crystal radio, then yes the input power drives a speaker and is turned into sound instead (along with some power lost heat in the circuits resistance).

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