Agilent Attenuator Controller

Welcome!

It so happened that we got two attenuators Agilent 8494H and 8496H. They lay idle for a long time because there was no control controller. Of course, four simple switches would cope with this task, but I wanted an implementation with a display showing the current attenuation value and software control. The project was made in EasyEDA and Atmel Studio.

The controller is built on an Atmega 328 located on a double-sided printed circuit board. The circuit is powered from the USB port (5 V). To switch the solenoids in the attenuator, a voltage of 24 V is required, which is obtained from the DC-DC boost circuit (5 V – 24 V). This voltage is applied for a very short time, approximately 50 ms.

Fig. 1.1 – Electrical diagram of the controller

To control the attenuator stages, a binary code (from 0 to 7) is supplied from the microcontroller to the three inputs of the decoder (CD4555B), then their eight outputs are connected to Darlington transistors (ULN2803), which pull the ground to the corresponding wire.

There are a total of 9 wires in the cable, 24 V is supplied to red, and ground is supplied to the rest, depending on the position of the attenuator.

Fig.3.1 - Wiring the cable to the DB-9 connector — Fig.3.1 – Wiring the cable to the DB-9 connector

Since the device is used to automate measurements, it includes software control via a virtual COM port.

When the controller is turned on, there is a maximum attenuation of 11 dB and 110 dB. The encoder is used to increase and decrease the attenuation, the encoder button is used to select the attenuator.