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Transmitter testing
For transmitters that are designed for the transmission of speech or music, two frequencies within the audio band can be injected into the normal input of the transmitter. The output of the transmitter can be examined with a spectrum analyzer to look for intermodulation products. This kind of end-to-end testing tests all parts of the transmitter for non-linearity: from the audio stage, through the mixing and IF amplifier, to the final RF power amplifier. Likewise, a transmitter used for passing data can be injected with two frequencies within the baseband of the data stream. In some cases, there is no accessible input to a transmitter. Radar transmitters, for instance, do not take an input; the circuitry generating the radar signal is internal to the transmitter. In such cases the tones must be injected at some internal point of the device, or else the amplifiers and other stages must be tested as separate components.[13] A dummy load may be connected to the output of the transmitter to prevent it actually broadcasting, and a directional coupler, possibly together with an attenuator, used to provide a feed to the spectrum analyser.[14]
The spacing in frequency between the two tones is of some significance in transmitter testing. The spacing determines whether intermodulation products are going to be in-band or out-of-band. That is, whether or not they occur within the band that the transmitter is designed to operate. In-band intermodulation is problematic because it interferes with the operation of the transmitter. However, out-of-band intermodulation can be an even greater problem. In most countries the telecommunications authority licenses the operator to use specific frequencies. Out-of-band signals are required to be virtually suppressed altogether. However, the greater frequency difference between the wanted and unwanted signal makes out-of-band intermodulation products relatively easy to remove with filters.[15]
Just as two tones provide a more realistic test than a single tone, multi-tone testing can be used to even better simulate the behaviour of a real signal. The idea is to spread the tones over the bandwidth of the real signal with a similar frequency power density. For accurate results, it is important that the phase of the tones relative to each other is considered. It is usually undesirable that the tones are in a synchronised phase relationship as this can give misleading results. For this reason, it is often endeavoured to generate tones with random phases in multi-tone testing.[16]
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en.wikipedia.org
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For transmitters that are designed for the transmission of speech or music, two frequencies within the audio band can be injected into the normal input of the transmitter. The output of the transmitter can be examined with a spectrum analyzer to look for intermodulation products. This kind of end-to-end testing tests all parts of the transmitter for non-linearity: from the audio stage, through the mixing and IF amplifier, to the final RF power amplifier. Likewise, a transmitter used for passing data can be injected with two frequencies within the baseband of the data stream. In some cases, there is no accessible input to a transmitter. Radar transmitters, for instance, do not take an input; the circuitry generating the radar signal is internal to the transmitter. In such cases the tones must be injected at some internal point of the device, or else the amplifiers and other stages must be tested as separate components.[13] A dummy load may be connected to the output of the transmitter to prevent it actually broadcasting, and a directional coupler, possibly together with an attenuator, used to provide a feed to the spectrum analyser.[14]
The spacing in frequency between the two tones is of some significance in transmitter testing. The spacing determines whether intermodulation products are going to be in-band or out-of-band. That is, whether or not they occur within the band that the transmitter is designed to operate. In-band intermodulation is problematic because it interferes with the operation of the transmitter. However, out-of-band intermodulation can be an even greater problem. In most countries the telecommunications authority licenses the operator to use specific frequencies. Out-of-band signals are required to be virtually suppressed altogether. However, the greater frequency difference between the wanted and unwanted signal makes out-of-band intermodulation products relatively easy to remove with filters.[15]
Just as two tones provide a more realistic test than a single tone, multi-tone testing can be used to even better simulate the behaviour of a real signal. The idea is to spread the tones over the bandwidth of the real signal with a similar frequency power density. For accurate results, it is important that the phase of the tones relative to each other is considered. It is usually undesirable that the tones are in a synchronised phase relationship as this can give misleading results. For this reason, it is often endeavoured to generate tones with random phases in multi-tone testing.[16]
Link to full article
Two-tone testing - Wikipedia
en.wikipedia.org