Agilent Technologies 33120A Output Attenuator, Block 8 on block diagram page 129 Schematic on

Chapter 5 Theory of Operation

Output Attenuator

Output Attenuator

Block 8 on block diagram page 129; Schematic on page 138.

The Output Attenuator provides 0 to 30 dB of signal attenuation between the output amplifier section and the output BNC connector. Output signal levels are controlled by combining coarse amplitude control from the output attenuator section and pre-attenuator section with fine amplitude control from the Waveform DAC AMP_CTL signal.

Four switched output attenuator pads are combined to achieve the desired signal attenuation as shown in the table below. Relays K801 through K804 either bypass an attenuator pad or select that attenuator. K801 selects a 2 dB attenuator, K802 selects a 4 dB attenuator, K803 selects a 8 dB attenuator, and K804 selects a 16 dB attenuator. Relays are sequenced to provide signal attenuation in 6 dB steps. Intermediate amplitude levels are controlled by selecting 0 dB, 2 dB or 4 dB of signal attenuation through the pre-attenuator solid state switches in combination with reducing the output level of the waveform DAC itself. The AMP_CTL signal provides smoothly varying control of the Waveform DAC output level over a 0 dB to -2 dB range. This operation is described further in the Waveform DAC and System DAC discussions.

Output Attenuation	K801	K802	K803	K804
0 dB	set	set	set	set
6 dB	reset	reset	set	set
12 dB	set	reset	reset	set
18 dB	reset	set	set	reset
24 dB	set	set	reset	reset
30 dB	reset	reset	reset	reset

K801 through K804 are latching relays. Their set or reset state is selected by momentarily pulsing the appropriate coil of the relay. Relay coils are pulsed with 5 volts for 15 ms through relay drivers U301 and U302. The main controller, U102, writes data bytes to ASIC U103 which transmits this data to the relay drivers via the internal 3-wire serial data bus (SERCLK, SERDAT, and SERSTB) to accomplish the relay state changes.

A 30 MHz filter, composed of L801, C801, and C802, eliminates wideband noise from the function generator output. The output amplifier and output attenuators are protected from damage by clamps CR801 and CR802 and by fuse F801. The function generator is protected from accidental application of voltages <10 volts for short durations.

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