Radio Glen TBU

Posted on Jan 26, 2013

The heart(s) of the unit are the ETAL P3400 line modules available from Farnell. These modules mop-up all of the line interface functions like ring detection, physical hybrid, on-off hook handling etc. This means that the TBU as a whole would probably pass PSTN type approval testing if it was subjected to it. This sheet shows the differential amplifiers which take the signal from the RX side of the hybrid, and the differential amps which accept the mixer TX output for sending to the TX side of the hybrid. The TX signal is bandpass filtered with -3dB points of 200Hz and 3500Hz at -12dB/octave. This avoids sending unecessarily wide audio to the telephone line and makes cancellation of the TX crosstalk easier.

Click here to download the full size of the above Circuit.

RX refers to the signal being received from the far end of the phone line and sent to the studio mixer. TX refers to the signal going from the studio mixer to the phone line. Because of the imperfect nature of hybrids and phone lines, the RX signal typically has a roughly equal amount of the TX signal present due to crosstalk. This TX crosstalk is cancelled by subtracting the known TX signal from the RX signal. Measurements have shown that the TX crosstalk signal coming out of the RX side of the hybrid is also slightly phase shifted from the original TX signal. So an equivalent phase shift is added to the signal that is used to cancel the TX crosstalk signal. The cancellation amplitude and phase shift are adjustable on preset pots. To adjust these pots, the TBU has to be put into a call. The receiver of the phone that is dialled up may be simply left on a cushion to block the mouthpiece. A 1KHz sine is sent to the TX from the mixer and the amount of this sine on the RX is measured with an oscilloscope. The amplitude pot is adjusted first, to minimise the level of the crosstalk signal. Then the phase pot is adjusted to minimise the crosstalk signal. There is not much interaction between the two pots but it may be of benefit to readjust the amplitude pot and then the phase pot again. This adjustment should ideally be done when the TBU is connected to the actual line to be used in the field. In practice however, the...

Leave Comment

characters left:

New Circuits

Zero consumption Anti-theft High power Code Switch for E-Bike Batteries

This project fits more for custom made battery casing that can have more space to hide things inside. This switch with few alterations can be used for any other project as well but is designed based on e-bike needs! This project use a clever functionality of....
Ultrasonic distance finder circuit

The circuit described here uses ultrasonic oscillations and operates based on the propagation velocity of these oscillations in the air. Thus, we can easily determine the distance of two points if the time within which the wave travels this distance is....
Simple Metal detector circuit with CD4030

Sometimes the precious metals are hidden too deep and are not detected except with complicated devices. In many cases, however, small pieces of precious metal buried near the surface can be detected by relatively simple means. Everyone is very attractive to....
Fully automatic watering circuit for flower pots

Many times for various reasons we forget or can not water the plants that we have in our homes. And many humidity sensors units just notify us with a beeping sound or with a flashing light, that the pot needs watering. But what if we are away from home? This....
Simple deep searching metal detector circuit

The principle behind a metal detector is really very simple. Proof of this is the circuit that follows, in which it proves that the construction of a metal detector can be done in the minute, with few components that we find very easily everywhere. With the....
Sub-Harmonic Bass Converter for Electric Guitars

The sub-harmonic bass generator is a sound producing unit for guitars. The sound it produces looks very much like that of the bass guitar. The octave generator is sometimes quite unusual since it does not produce any of the usual ways of coloring the sound,....
Echo Depth Sounding Sonar for Boats

In the past the sounding (sea bottom measurement) was done with the "bullet", that is, with a heavy lead object that the seamen plucked into the sea hung from a calibrated rope. As soon as the "bullet" reached the bottom, the depth appeared directly from the....
100W HiFi Audio Amplifier Circuit

This Amplifier was designed to have the following specifications: Distortion less than 0.1% at full power of 100W even at 20KHz. Power has to be attributed to an extended bandwidth. The output transistors must be protected against short circuits. The power....
Plant-Pot Water Level Indicator Circuit

A series of LEDs serve to alert the gardener when plants need water. Using two conventional digital integrated LEDs and a series of LEDs, we make a very useful device for gardening. The device detects the amount of water in the pot and alerts the grower.....

Create NOW your QR MENU for your Restaurant / Hotel / Bar / Caffe,

Comprehensive ordering/paying features. fully personalized to suit your Business.

Update Anytime! Easy and Simple!

Popular Circuits

Voltage-monitor

If the battery voltage exceeds about 11 V, current flows through R1, D1, and R2. The voltage...

Bandswitched Grid Dip Meter

For checking resonances, tuned circuits, antennas, etc., this circuit covers the 2-to 20-MHz...

multi sound for guitars

Electric guitars use coils (guitarists call them pickups or elements) to convert the vibrations...

Schematic Diagram circuit andexplanation

This power inverter circuit will provide a very stable Square Wave Output Voltage. Frequency of...

Hot Tub Hack

This hack replaces the thermistor (temperature sensor) of a hot tub with an adjustable value that...