Analog Devices – Experimental Engineering

These units are used to broadcast local audio, such as from a public address system or local microphone. They accomplish this by producing a modulated magnetic field that a hearing aid is capable of picking up.

Not many controls on this bit of equipment. A bi-colour LED for status indications, a microphone, external audio input, charging input & a power switch.

Popping the cover off reveals a small lead-acid battery, 2.1Ah at 12v. This is used when the loop is unplugged.

Here’s the main PCB, which takes care of the audio & battery charging. The inductive loop itself is just visible as the tape-covered wire bundle around the edge of the casing.

Here’s the input section of the main PCB. The microphone input is handled by a SSM2166 front-end preamplifier from Analog Devices.

This audio is then fed into a TDA2003 10W Mono Power Amplifier IC, which directly drives the induction coil as if it were a speaker. Any suitable receiving coil & amplifier can then receive the signal & change it back into audio.

Here is a quick look inside the FE-5060A Rubidium Frequency Standard. Above you can see the entire physics package, with the rubidium lamp housing on the right hand side. The ribbon cable running into the resonator cavity has the power & signal traces for the internal heater, temperature sensor & Helmholtz coil.

Here is the lamp end of the physics package, with the voltage regulator & RF driver for the lamp. The FETs soldered to the back of the housing are being used as heaters to maintain a constant temperature on the lamp in operation.
The temperature sensor can be seen between the two FETs, with a single copper wire running around the housing to connect to it.

Main frequency synth board. This contains the RS-232 interface & the AD9830A from Analog Devices. This IC is a direct digital synthesizer & waveform generator.

Tag: Analog Devices

Portable Hearing Induction Loop

FE-5680A Rubidium Frequency Standard – Initial Teardown