AM Synchronous Demodulation / Detection

- overview of synchronous demodulation or detection of amplitude modulation, AM, signals with details of methods, operation and advantages.

Synchronous forms of demodulation have inherent advantages over other forms of demodulation, although the additional levels of complexity mean that they are not always used.

Synchronous AM demodulation is generally reserved for higher performance radio receivers, although many integrated circuit technology means that it can incorporated into a chip with relative ease.

What is AM synchronous demodulation?

the simplest form of detection for an amplitude modulated signal utilises a simple diode rectifier. To achieve improved performance a form of demodulation known as synchronous demodulation can be used.

When looking at the synchronous demodulation of an AM signal, it is first useful to look at the spectrum of an amplitude modulated signal. It can be seen that it comprises a carrier with the two sidebands carrying he audio or other information spreading out either side. These two sidebands are reflections of each other.

Diagram of the spectrum of an amplitude modulated signal showing the sidebands extending above and below the carrier
Spectrum of an amplitude modulated, AM signal

The system uses an oscillator signal to mix with the incoming signal to convert it down to the baseband signal. If the local oscillator signal has exactly the same frequency as the carrier within the AM signal, this will appear as a DC component at the output - the DC level will depend on the phase between the carrier and the local oscillator. The sidebands of the AM signal will appear relative to zero frequency, i.e. as the original audio or other modulating signal.

Diagram showing the basic concept behind synchronous demodulation for an AM signal
Synchronous demodulation

Advantages and disadvantages of AM synchronous demodulation

There is a balance to be made between utilising a simple diode detector and a synchronous detector. It is not always viable to incorporate an AM synchronous demodulator into a new design. Other formats may be more suitable.

The advantages and disadvantages of a synchronous AM detector compared to a simple diode detector are tabulated below:

Advantages & Disadvantages of AM Synchronous Demodulators
Advantages Disadvantages
  1. Increased linearity - lower levels of distortion.
  2. Considerably less affected by selective fading experienced on the medium and short wave bands.
  3. Improved sensitivity.
  4. Improved signal to noise ratio.
  1. Considerable additional complexity, although this is not such an important consideration if the synchronous detector can be included in an IC.

Methods of achieving AM synchronous demodulation

There are several ways in which synchronous AM detection can be achieved. They all have their own properties, but all follow the basic principle whereby a signal is injected into a mixer at the same frequency as the carrier to re-constitute the audio or other data.

  • Filter method:   The most obvious method to create a local oscillator signal on the same frequency as the carrier is to use a fixed frequency filter that is tuned to remove only the carrier frequency. This can be phase shifted, 90°: and entered into the mixer. The phase shift will ensure the DC component at the output of the mixer is minimised.

    The drawback for this method is that the carrier has to be positioned exactly on the frequency of the filter for it to work. This means that the tuning has to be exact.
  • Phase locked loop:   Phase locked loops provide a convenient method of extracting the carrier. The phase locked loop will lock on to the carrier of the AM signal and the VCO output can be fed into the mixer with a 90°: phase shift as before.
  • Hard limiting amplifier:   Possible the cheapest and easiest option to implement is a hard limiting amplifier approach.

    This circuit operates using the principle that if the signal is hard limited, the output of the amplifier will not have any amplitude variations on it - hence it will only allow through the carrier without modulation. This is exactly what is required for the mixing process.

    Diagram showing how a high gain amplifier can be used in a circuit to recreate the carrier only and inject this into a mixer to provide synchronous detection for an AM signal
    High gain limiting amplifier synchronous detector

AM synchronous detection performance

A synchronous detector is more expensive to make than an ordinary diode detector when discrete components are used, although with integrated circuits being found in many receivers today there is little or no noticeable cost associated with its use as the circuitry is often included as part of an overall receiver IC.

Synchronous detectors are used because they have several advantages over ordinary diode detectors. Firstly the level of distortion is less. This can be an advantage if a better level of quality is required but for many communications receivers this might not be a problem. Instead the main advantages lie in their ability to improve reception under adverse conditions, especially when selective fading occurs or when signal levels are low.

Under conditions when the carrier level is reduced by selective fading, the receiver is able to re-insert its own signal on the carrier frequency ensuring that the effects of selective fading are removed. As a result the effects of selective fading can be removed to greatly enhance reception.

The other advantage is an improved signal to noise ratio at low signal levels. As the demodulator is what is termed a coherent modulator it only sees the components of noise that are in phase with the local oscillator. Consequently the noise level is reduced and the signal to noise ratio is improved.

Unfortunately synchronous detectors are only used in a limited number of receivers because of their increased complexity. Where they are used a noticeable improvement in receiver performance is seen and when choosing a receiver that will be used for short wave broadcast reception it is worth considering whether a synchronous detector is one of the facilities that is required.

By Ian Poole

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