Agilent Technologies 8922 manual 14-10

Block Diagram Theory of Operation

Block Diagram 2

Compared to common modulation formats like AM, FM, and phase modulation, the 0.3 GMSK format is more complex and requires special equipment (like the HP/Agilent 8922) to generate and analyze signals. A brief explanation is included here as an overview of the format of 0.3 GMSK.

The 0.3 GMSK format was chosen because it is very efficient in terms of the amount of information that can be transmitted in a given amount of frequency spectrum. To understand 0.3 GMSK, it is necessary to first understand MSK (Minimum Shift Keying). MSK is phase modulation where the carrier is shifted + or - 90 degrees as each data bit is received. This instantaneous phase shift causes “splatter” in the frequency domain and appears as noise spikes on a spectrum analyzer. This is not a good system for digital communications because it would cause noise in adjacent communication channels. To eliminate this noise, the digital signals are first low-pass filtered to eliminate the instantaneous phase shifts. The filter cut-off frequency chosen was 0.3 times the data rate 270.833 kHz = 81.25 kHz. The shape of the filter chosen was Gaussian, which explains where the “G” in “0.3 GMSK” was derived. The effect of the 0.3 Gaussian filter is to smooth out the sharp digital transitions and causes a more continuous phase modulation that has low spectral splatter.

To further reduce the frequency splattering in the frequency spectrum, the digital input data is “Differentially Encoded”. This means that the modulation (either + or - 90 degrees) is determined by examining the current data input (1 or 0) and deciding if it is the same or different than the previous data bit. If the current data bit is different than the previous bit, the carrier is modulated -90 degrees; if the current data is the same as the previous bit, the carrier is modulated +90 degrees. For example, a series of data…01010101.…, would

cause the carrier to be continuously modulated -90 degrees each clock period. Similarly, a series of all 1’s or all 0’s would cause the carrier to be continuously modulated +90 degrees each clock period. This can be seen by viewing the HP/Agilent 8922 output with constant 1 or 0 data input. With modulation turned on, the carrier is “offset” +67.7 kHz. This is caused because the carrier is modulated at +90 degrees times 270.833 kHz = 67.7 kHz. This also explains the common misunderstanding about why the carrier seems “offset” when no data is being applied.

Because of ISI (Inter Symbol Interference) caused by the low-pass filtering, the effects of previous data bits can be seen on the RF output. To generate this complex signal, the A5 Premod Filter and NSM uses a shift register to hold the current data bit, as well as the previous 6 data bits. These seven bits are used along with a look-up ROM to find the exact phase output that the HP/Agilent 8922 should generate, given the effects of ISI and 0.3 Gaussian filtering.

This information is given digitally to the NSM (Numerical Synthesis Machine) chip. This IC is a digital synthesizer that converts the digital input data into a digitally coded analog waveform that can be used to directly drive the A27 DAC/Upconverter assembly to get the correct analog waveform.

The diagnostics program checks the A5 Premod Filter and NSM assembly by making sure the internal loop can lock to an external 270.833 kHz signal. Since the HP/Agilent 8922B and HP/Agilent 8922G clock signals are generated internally in other modules, it may be

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