analog board

As explained before, the analog board scales the sweep signal and adds the offset voltage(for center frequency). See the picture in my previous post.

In order to be able to use 5V control signal I had to replace the 4016 IC's. Now I use 74HCT4066 chips. They are pin compatible.

The board works. I have one concern though. I had to add A DC component to the sweep signal. This was done in order to keep the voltages over the 4016 switches in range. A high pass filter removes that DC voltage. So far so good. However, while switching between ranges (MHz/div) it may take a few seconds for the filter to go to the new steady state. This will cause some unwanted visual effects.

analog board

4016 switches

I built and tested one of the programmable amplifiers. They consist of an opamp and a 4016 switch. It works but I found out that the 4016 don't like to switch negative voltages. The datasheet was not clear about this. So I added an offset. This solves the problem. Later I need to get rid of this offset by using a highpass filter.

Looking through several datasheet I found out that the 4016 can only be used when input or output is in the range of -0.5 V .. VCC. So I need to keep the amplification low. An opamp with fixed amplification will amplify the signal to the needed level.

I use 2 inverting programmable amplifiers. In this setup the voltages over the switches can be negative. To solve this I will use an inverter behind each programming amplifier. The TL-082 are dual opamps so I van use one of them as inverter. No extra chips are needed as I already planned to use 2 IC's.

So no fundamental changes are needed in my design. I just have change the gain distribution and add 2 inverters.

Analog board

I was on holiday so I did not do much work on the project.

Currently I am working on the analog board. This board must scale the sweep signal according to the span setting (MHz/V).

I use TL-084 opamps to amplify/attenuate the sweep signal. The microcontroller sets the amplification. I made an inverting amplifier that uses selectable feedback ressistors. Those resistors can be selected by a voltage controlled switch (4016).
The PIC controls these switches.

The selectable resistors are potentiometer trimmers so I can allign the gain.