Laptop NFM emissions

[betwixt]

EricaS already has an R20 receiver to use, that isn't the issue. It's trying to localize the exact source of a carrier. A probe considerably smaller than a DVB-T stick is needed!

Brian.

 

[Borber]

ohh I missed he has a receiver. Other points of interest here are obvious.
Just to mention RTL-SDR receiver software has waterfall spectrum analyzer, different receiving modes-demodulators...

 

[EricaS]

A digital audio transmitter is quite a complicated device to build, especially in a small space and it would also require a power source and in-depth knowledge of the internal circuitry of the laptop to be able to interface to it.

When I disable the audio device in the control panel those "digital" signals are also stopped.

Just as a check, can you confirm the laptop doesn't have Bluetooth, if it has, try turning it off, the settings are probably in the control panel somewhere but not under the audio devices category. The other possible, but unlikely source is an SPDIF interface if the laptop has one. Some have a dedicated SPDIF socket, some hide it behind the headphone jack socket. If you look closely down the headphone socket hole in a dark room you would see a dim red light if it has one behind it.
Brian.

Bluetooth is switched off.
No dim red light.

The telephone I described earlier, is it possible that they feed RF (I would estimate between 115 and 150 MHz since this is the range on the laptop or maybe its another more appropriate frequency) down the telephone line and it finds its way to the ringer circuit? Thus emitting very low power RF - range not more than 2 m.

Im certain the telephone can also do this.
The telephone was used by them in the days before PCs had built-in microphones.

 

[betwixt]

When I disable the audio device in the control panel those "digital" signals are also stopped.

Don't read too much into that. Disabling the audio device also turns off lots of digital signals that cause the interference. The chances of anyone custom designing an IC for a laptop and replacing the normal one with it are almost zero. The laptop would have to be taken away and serious and extremely expensive surgery done on it to customize it and the original plans for the audio devices would have to be available in order to customize the design. The cost would be, at a guess, in the region of US $75K to $100K and even then it would only have a range of a few metres. It isn't impossible but it HIGHLY improbable that anyone would go to such lengths when much easier and cheaper spy methods work better. An analogy: it would be like someone sneaking an extra piston into your car engine without you noticing. It would take all the expertise of an engine designer, a foundry to make a different engine body, different gearbox, fuel system and probably several months of preparation.

Trying to get VHF signal down a phone line designed to handle no more than a few KHz woud be very difficult to do and very difficult to hide. As we have explained many times, if you can't see a high gain directional antenna (think *BIG* TV antenna) or a large dish pointing at you, with nothing obstructing the line of sight, the chances of the signals being picked up are negligible. If you want a simple way of eliminating any chance of RF being sent down the line, buy an ADSL 'broadband' filter and fit it in line with the phone cord. They are cheap and purpose designed to only allow low frequency signals through to the phone.

Brian.

 

[EricaS]

Trying to get VHF signal down a phone line designed to handle no more than a few KHz woud be very difficult to do and very difficult to hide.

The people who I suspect have access to everything including the telephone system. If its not impossible they can do it.

As we have explained many times, if you can't see a high gain directional antenna (think *BIG* TV antenna) or a large dish pointing at you, with nothing obstructing the line of sight, the chances of the signals being picked up are negligible.

So are you saying that the antenna I showed is not suitable to detect RF from the telephone?

Can VHF pass the capacitor and enter the ringer circuit?

 

[betwixt]

The phone is a simple ordinary POTS analog type so if in doubt, go to a local store and buy another one. It has standard ringer and dialler parts and the line interface looks like any other so it should be compatible with any ordinary domestic phone.

Yes, VHF can pass through a capacitor but the next parts in line are the rectifiers, they will not let it through. When ringing, a large (up to about 75V) AC voltage is applied across the line as well as the normal DC. The ringer will work equally well on AC or DC so the capacitor is there to block DC to stop it ringing all the time. The ringer therefore only works when the AC is also present but the circuit will only work on DC. In normal use the line voltage is blocked by the capacitor, when ringing voltage is put on the line, the capacitor lets it through to the rectifiers which convert it back to the DC the ringer circuit needs. So the ringer circuit has no power source whatsoever unless someone is calling you and you have not yet answered.

So are you saying that the antenna I showed is not suitable to detect RF from the telephone?

I'm not sure which one you are referring to. Please tell me the post number it is in so I can take a look.

Brian.

 

[EricaS]

Yes, VHF can pass through a capacitor but the next parts in line are the rectifiers, they will not let it through.

I know audio frequencies cannot get pass the capacitor. At what minimum frequency does it start to pass the capacitor?

 

[betwixt]

Audio frequencies CAN get past the capacitor, so can RF. It's purpose is to hold back the DC on the line so it doesn't continuously power the ringer. What it does is let the AC through, the AC is produced at the outside switch if you have a direct line or the switchboard if your line passes through one and is only present when the phone is asked to ring. The issue is that the ringer circuit will not work on AC (it did in the days of mechanical bells which is why it remains as a legacy) so the separated AC has to be converted back to an independent DC supply again. That process involves components that will not pass frequecies above about 1KHz and will not allow the 'reverse' flow of sounds picked up by the ringer to get back to the line.

Without the AC ringing voltage on the line, no voltages beyond the capcitor exist and the whole ringer circuit, the IC and piezo included have no power source.

Brian.

 

[EricaS]

Without the AC ringing voltage on the line, no voltages beyond the capcitor exist and the whole ringer circuit, the IC and piezo included have no power source.

Would a small amount of AC current, sufficient to power the ringer circuit only, get pass the capacitor?
Maybe they're not sending RF down the line, just sufficient AC to power the ringer circuit?

 

[betwixt]

Technically that might be possible but there would be a problem with it:

The ringing AC is low frequency, typically 25Hz but has to reach a certain voltage threshold before it is passed to the ringer circuit. Typically the threshold is around 6V RMS although I don't believe I've ever seen a written specification for it. The normal 75V is so much bigger that the lower threshold is relatively unimportant. The threshold exists so that any interference on the line doesn't accidentally make the ringer operate and is quite intentionally built in to the phone for that purpose. Without it you might hear occasional crackles or beeps if for example there was a thunderstorm nearby or big electrical machnery was switched on and a spike of interference was picked up on the lines.

The problem would be that the 6V AC would have to be present before ANY voltage reached the ringer so to provide it power to work from it would need to be more than 6V. The level of normal audio on the line is very low, only a few hundredths of a volt so it would be completely drowned out by the ringing voltage. Also bear in mind that the ringing voltage is generated back at the external switch (line exchange) or switchboard and they are designed to kill the voltage immediately if the handset is lifted so the ringers supply would be cut off during a call. If it didn't, all you would hear is very loud buzzing in the earpiece.

Think of it like standing in front of a running jet engine and shouting to someone behind it. You know your voice is coming out at the back end almost as loud as it enters but as a proportion of the total sound it is completely inaudible. (wear protective clothing if you try this :shock

Brian.

Brian.

 

[EricaS]

The problem would be that the 6V AC would have to be present before ANY voltage reached the ringer so to provide it power to work from it would need to be more than 6V.

Do you think its possible that the standard ringer IC was replaced with something that could generate RF?

As far as I can tell with the naked eye, the ringer ICs are identical for the bugged and unbugged phones. Perhaps they relabelled them to look identical.

 

[davenn]

I know audio frequencies cannot get pass the capacitor. At what minimum frequency does it start to pass the capacitor?

this is a totally incorrect statement
if that were true, there wouldn't be a bit of audio equipment ion the planet that would work

How about learning some real electronics and give up on these paranoid delusions

Dave

 

[vfone]

How about learning some real electronics and give up on these paranoid delusions

Dave, you cannot do anything in this regards.
The user tggzzz already pointed before to these paranoid delusions:

https://www.edaboard.com/threads/337161/#post1439012

After 14 (fourteen) years on this forum I got deleted a post because showing that we are fooled all together.