Agilent Technologies 8922

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

datais “Differentially Encoded”. This means that the modulation (either + or - 90 degrees)

isdetermined by examining the current data input (1 or 0) and deciding if it is the same or

differentthan the previous data bit. If the current data bit is different than the previous bit,

thecarrier 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

causethe 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

previous6 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.

Thisinformation 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

waveformthat can be used to directly drive the A27 DAC/Upconverterassembly to get the

correct analog waveform.

Thediagnostics program checks the A5 Premod Filter and NSM assembly by making sure

theinternal loop can lock to an external 270.833 kHz signal. Since the HP/Agilent 8922B

and HP/Agilent8922G clock signals are generated internally in other modules, it may be