some pictures

I made some pictures to show the progress.


The red swich selects the function of the rotary encoder. Up selects the center frequency, Down selects the span. Pressing white button icreases the tuning speed.



Software updates implemented today:
-make the encoder more responsive.
-invert the sweep signal.
-reverse the function of the white button.
-increase the sweep frequency.

I have connected the analog board to the digital board. This caused an interface problem. The output of the PWM output is filtered. This filter needs to see a high impedance. The input impedance of the analog boar was too low. I added an emitter follower to take care of that problem. It works but it clips the output signal as the signal out of the PWM can be lower than 0.6 Volt. I think I can fix it by conneting the emitter resistor to -12 V instead of 0V.

Software

I spend a lot of time refactoring the software. It was getting a bit messy.
After cleaning up I added some functionality.

Now the frequency counter works. It can measure the center frequency. It is suppossed to work even when a sweep is in progress. This has to be tested.
I also made a regulator for the center frequency. It compares the actual frequency with the setpoint. I case of an error the software integrator is increased or decreased. This integrator value is send to a DAC. It works but I have to test it with the real tuner to be sure.

In the meantime I found an enclosure for the analyzer. I also drilled some holes for the controls.

restart

I took a building break. I wanted to finish my CB to 40 meter transverter.
Now this project has finished so I will continue building the spectrum analyzer.

I made the connection between the analog board and the microcontroller. Now I can control the sweep amplitude and the DC offset. I also added a switch to speed up tuning. When pressed the tuning speed is 5 MHz per click instead of 1 MHz per click.

Next task is to improve the software. I want to prevent impossible situations. For example: center frequency is 10 MHz and the span is 20 MHz/ division. That would suggest negative frequencies on the left of the screen.

After that I will make a frequency counter in software. Then I can display the actual center frequency. I could use this counter to adjust the center frequency as described in my previous logs.

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.