Advanced Electronics for Mobile Telephony
Duplex Filter
The duplex filter makes it possible to transmit and receive simultaneously with one antenna. Before the duplex filter, it was only possible to either transmit or receive.
A duplex filter consists of two bandpass filters which pass the transmit signal and the receive signal and suppress frequencies outside the band. This ensures that no power from the transmitter enters the sensitive receiver.
Duplex filters are often elaborate cavity resonators made of metal. They are large and heavy. Only with advancing electronics it was possible to create smaller filters as they are used in todays mobile phones.
DTMF
Until the 1960s (in Europe until the 1980s), telephone numbers were entered using a rotary dial. These controlled the good old dialing relays of the switching centers. Bell Systems developed an alternative in the early sixties. Each digit was to be described by two tones. One of four low tones and one of four high tones. Thus, 16 digits could be described. The advantage of this kind of dialing was that advanced electronic switches could be used and the dialing speed was dramatically increased.
| Frequenz (Hz) | 1209 | 1336 | 1477 | 1633 |
| 697 | 1 | 2 | 3 | A |
| 770 | 4 | 5 | 6 | B |
| 852 | 7 | 8 | 0 | C |
| 941 | * | 0 | # | D |
The digit 1 was therefore described by two sine tones (697 and 1209 Hz). These tones were soon generated with ICs (e.g. from Fairchild). They could be identified by so-called filter banks on the receiving side. These dual tones (DTMF = Dual Tone Multi Frequency) can still be heard today in landline telephones and are familiar to every user. For practical reasons, they led to the familiar key layout and also to the additional characters * and #. The „keys A B C D“ were practically never used and were missing in the keyboards.
Light Emitting Diodes (LED)
Until the 1960s, there was little variety to display information. The most common means were small light bulbs or an analog display, for example, to show the reception strength of a transmitter.
Once again, an innovation came from the field of semiconductor electronics. Let us consider once again a semiconductor diode. When a diode is operated in forward mode, electrons move from the n-type doping to the „holes“ of the p-type doping. They lose energy during this „jump“. This energy is not converted into heat as in normal electrical resistors, but directly into light. The frequency of the light corresponds to the emitted energy. In „normal diodes“ this light is in the non-visible range. But soon diodes were developed where the emitted light is visible. These are so-called Light Emitting Diodes LED). The best known were the deep red emitting diodes. The first red LED where created in 1962 in small quantities. It was taking another century until Fairchild could create LED using a planar process and in the mid seventies LED were cheap enough to be mass produced for 8 segment displays e.g. for pocket calculators and upcoming digital watches.
Memory
Storage of information was a complex thing in electronics. If you don’t want to use mechanical or magnetic memories, you need circuits of transistors to store a piece of information. For one bit 6 transistors are necessary. Memory became especially important for the development of computers, so semiconductor companies like Fairchild and Texas Instruments quickly developed integrated circuits in which information could be read and written. These were called Static Random Access Memory.
Intel was founded in 1968 to build memory. Intel’s first product was a 64 bit memory.
Logic
Logical elements are the basic blocks e.g. for control systems. Examples are AND and NAND devices. Logic elements are realized by transistors and are available as IC’s from Fairchild or Texas Instrument. As an example serve the logical AND.
| AND | 0 | 1 |
| 0 | 0 | 0 |
| 1 | 0 | 1 |
The AND has two inputs and one output. Inputs and outputs are always off or on respectively a 0 or a 1. With the AND the output is always 0 unless both inputs are 1. There are all possible versions. It is also possible make a not AND (NAND) from an AND.
| NAND | 0 | 1 |
| 0 | 1 | 1 |
| 1 | 1 | 0 |
Logic devices could be selected and purchased from a large catalog from Texas Instruments, for example.
Phased Locked Loop
An important new digital logic was the frequency divider. As the name suggests, it divides incoming frequencies. For example, if a frequency divider is set to 100, it counts the incoming pulses/waves until it gets to 100 and then outputs a pulse. A corresponding low frequency sine wave can then easily be generated from these low frequency pulses.
If the frequency divider is set to 99 instead of 100, the VCO is set to a frequency of 99 MHz. Via the frequency divider it is thus possible to set multiples of the oscillator frequency. Such a circuit can be used to set different channel frequencies. The crystal oscillator should correspond to the frequency spacing of the channels.