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FM Broadcasting and Signal Detection

    
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The UABC80 a typical triple diode triode valve on a B9A base

Frequency Modulation uses changes in the radio carrier frequency to encode the sound. FM is widely used in VHF broadcasting where it is able to provide a medium for high quality audio transmissions. Its resistance to fading and interference is a great advantage over conventional Amplitude Modulation that is used on Long, Medium and Short wave transmissions. Much interference comes from electrical impulses which appear in the form of amplitude spikes and it is quite easy to make FM receivers insensitive to these variations unlike AM receivers.

With AM, a 100% modulation limit exists. With FM, increasing the modulation volume justs increases the deviation of the carrier. Signals with a large deviation are able to support higher quality transmissions, but they occupy an ever greater bandwidth. For VHF radio broadcasting a deviation of + and - 75 kHz is used and this gives a frequency range up to 15 kHz for the audio signal.

In order to be able to receive FM, a receiver must be sensitive to the frequency variations of the incoming signals. The receiver is made insensitive to any amplitude variations by having a high gain IF amplifier. Here the signals are amplified to such a degree that the amplifier operates as a limiter. That is all signals appear as a constant maximum amplitude and in this way any amplitude variations present on the received signal are removed but at the cost of extra gain. This was a major factor with domestic valve receivers.

In order to be able to convert the frequency variations into voltage variations, the demodulator must be frequency dependent. The ideal response is a perfectly linear voltage to frequency characteristic. There are a number of circuits that can be used to demodulate FM. Each type has its own merits.

The simplest FM demodulation is known as slope detection. It simply uses a circuit that is tuned to a frequency slightly offset from the carrier and a diode detector. As the frequency of the signal moves up and down according to the modulation, so the signal moves up and down the skirt of the tuned circuit frequency response. This causes the amplitude of the output signal to vary and reconstitute the programme. The circuit is not efficient and is also unlikely to be linear and this defeats purpose of moving to FM in the first place.

A typical Ratio Detector Circuit

Valve circuits for FM demodulation were either the Foster-Seeley discriminator or the ratio detector. Of these the ratio detector was the most popular as it offers a better level of amplitude modulation rejection within the detector itself. This enables it to be used with fewer stages of IF amplification than the Foster-Seeley. For top of range hi-fi designs the Foster-Seeley would be used but for normal domestic receivers the ratio detector was the circuit of choice. In both circuits a phase sensitive element is used to extract the wanted signal.

The transformer is a specialist device and thus quite expensive. The tertiary winding (L3) is very tightly coupled to the primary (L1) to ensure phase synchronisation. The secondary (L2) is loosly coupled to the primary and generates a voltage 90° out of phase. The primary winding forms the anode load for the final IF stage.

The Ratio detector requires matched diodes with separate cathodes. The valve manufacturers responded by making triple diode triode valves such as the EABC80, PABC80, UABC80 & 6LD12. The double diode cathode was strapped internally to the triode cathode and the single diode had a separate cathode connection. In use one diode of the pair would be used for AM detection, the second diode of the pair and the single diode would form the ratio detector and the triode provided the first audio amplifier stage.

A Practical unbalanced Ratio Detector Circuit

The triple diode triode valves have the triode and two diodes sharing a common cathode and so this cathode needs to be connected to ground for three separate circuits to use it. In the unbalanced ratio detector circuit above it can be seen that one cathode is grounded. The top diode in the circuit would be the single diode within the valve. Actual circuits from the mid 1950s were based on this type of circuit configuration.

Improving the ratio detector

Once VHF FM broadcasting became established it was realised that multipath reception was a more significant problem that at first expected. Rather than moving to the expense of better saturated amplifiers or limiters with the extra costs involved it became clear that by improving the matching (balance) of the halves of the ratio detetor a marked reduction in this 'tissue-paper' distortion was effected. The circuit above shows that the inclusion of a 5k Ohm preset potentiometer can solve the problem.

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