Telephone Eavesdropping

[EricaS]

Hello there

I'm convinced my landline telephone at work is bugged.
Its being used to transmit all room conversations when the phone is on-hook.
I've inspected the phone internally and compared it against many other identical phones. I've also used very sophisticated TSCM equipment to detect the presence of an unauthorised audio transmission. But nothing could be found.

After inspecting about seventy telephones I found that those phones which I suspect to be bugged at work all have the polarity around the ringer correctly fixed. The bugged phones have positive lead (red wire) connected to the positive terminal and negative lead (black wire) connected to the negative terminal. The clean phones have this connection reversed i.e. positive lead to negative terminal and negative lead to positive terminal. The ringer is a piezoelectric transducer.

Here are pics of the circuit:
https://bit.ly/1C1PoQh
https://bit.ly/1I0AYsd
Note the ringer is the black object seen in the pics.

I just recently came across an article describing how piezoelectric transducers can be used to transmit and eavesdrop.
https://www.reddit.com/r/badBIOS/comments/2e3yuv/badbios_transmits_ultrasound_via_piezo_can/

Can the ringer circuit on this telephone together with the piezoelectric transducer ringer be used to transmit room audio via RF?
P.S. I verified that the ringer can act as a microphone by connecting it to an amplifier.

 

[Tony Stewart]

Having designed VLF receivers for Doppler Navigation using the Navy' megawatt global transmitters, I have some experience in this matter.

The colour reversal of Red Black is more likely a rookie's design choice in deciding which colour to use for tip or ring from the line voltage from central office, as it is a negative battery or -48V. It makes no difference in fact and proves nothing in your assumption.

Since the half wavelength for 10kHz is a million times longer than 10GHz, the antenna efficiency is low , impedance is high and high voltage and a very long antenna is needed to broadcast unlike the mV signal generated by vibrations in your piezo buzzer.

 

[alec_t]

The bugged phones have positive lead (red wire) connected to the positive terminal and negative lead (black wire) connected to the negative terminal.

What makes you convinced those particular phones are bugged?

 

[EricaS]

Ok. Most people have said this.
Though the polarity does influence which direction the diaphragm of the ringer bends when a DC voltage is applied.

Can the ringer circuit somehow transmit RF?

 

[EricaS]

What makes you convinced those particular phones are bugged?

Déjà vu

 

[Tony Stewart]

Ok. Most people have said this.
Though the polarity does influence which direction the diaphragm of the ringer bends when a DC voltage is applied.

Can the ringer circuit somehow transmit RF?

Since the passive piezo is bipolar, only integrated oscillators are polar with Red black.

If 1mV modulation is applied to a high voltage modulated carrier, be it internal oscillator powered by 12V or ring voltage at 200V would have a modulation index such microscopic proportions not only from impedance ratios but modulation index, it would have no practical RF emanation levels even if inductively connected to a sophisticated spectrum Analyzer.

This is why RFID's can't and dont use VLF.

Even if you did have a high voltage Tx built into the phone with high modulation index modulating with 1mV of audio, with extra chipset, it could easily be jammed with any tiny reed relay buzzer and long wires breaking the coil contact current of 20mA with high E fields every 300Hz or reed buzzer rate up beyond the VLF, HF and up to the VHF bands for about $0.25.

 

[EricaS]

Since the passive piezo is bipolar, only integrated oscillators are polar with Red black.

If 1mV modulation is applied to a high voltage modulated carrier, be it internal oscillator powered by 12V or ring voltage at 200V would have a modulation index such microscopic proportions not only from impedance ratios but modulation index, it would have no practical RF emanation levels even if direct connected to a sophisticated spectrum Analyzer.

This is why RFID's dont use VLF.

I forgot to mention I'm not an engineer. So I have only a layman's idea of what you said.
So I assume you say "impossible"

 

[Tony Stewart]

Yes technically unfeasible without much additional circuitry, or "impossible"
The leak must be from some other source, as others have suggested.

 

[upand_at_them]

FYI, she has an identical thread on AAC that never amounted to anything:
https://forum.allaboutcircuits.com/threads/pizeo-buzzer-as-a-microphone-bug.109145/

 

[atferrari]

In fact, already four...and counting.

Funny "layman's" usage of specific acronyms. Come on...! Let him try in the next forum. Many still out there.

 

[DerStrom8]

As mentioned, this thread is identical to one over at AAC and the OP was deemed a troll. I am closing this thread until the moderating staff determines what to do with it.

 

[DerStrom8]

The moderation staff has elected to keep this thread closed. Paranoia in the workplace is one thing, but attempting to open, disassemble, or modify company property is not something ETO supports, and thus we will not host such discussions.