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.

PIC board working again

The PIC16F887A chips arrived yesterday.

I had to change some things as this chip is the A version. The broken chip is not an A version. After changing those settings the board worked again. Unfortunately this new chip does not solve the programming errors I sometimes get when using the Winpic programing software. Anyway I can continue working on the digital board.

I have changed the port for the switch. I try to use a port for either input or output. I don't want to mix input and output. In the previous configuration It was mixed. That may have destroyed the PIC.

Dependig on the switch setting I can change the center frequency or the frequency span. For each span setting a different sweep frequency must be used.
I use the PWM output to generate this sweep signal. I managed to make this frequency controllable. It now changes when the span setting changes.I am not satisfied yet. I can see the sweep signal make small jumps when a low sweep frequency is used. I want it to be smooth. I will adapt the software improve this behaviour.

Currently the software delays each output increase. this delay is adjustable. Effect is that the sample rate depends on this delay. So for low frequencies the lowpasfilter is not sufficient. I don't want switchable filters as it would require additional hardware. So I will try the approach used by a DDS. I will increase the output of the DAC starting at 0 to max using an adjustable stepsize. That way the sample rate will be fixed. Tonight I will try to implement this.

Dac Driver ready

This picture shows the pic 16F877 board. It worked till I tried to reprogram it. The PIC is dead since.

Software development continues on a 16F84 board. I put one of the DAC chips on that board. Then I wrote a driver for it. This was not hard to do.

The DAC uses a 3 wire serial bus. The protocol is simple. I uses 4 control bits, 10 databits and 2 don't care bits. Sending these bits to the Dac can be done fast. I made a sawtooth generator as test project . The resolution is 10 bits the period is 2Hz. If the PWM sawtooth generator won't met the expectations I can fall back to this one.

I have to wait for the arrival of the new 16F877 chips. Until then I will work on the analog board. That board will adjust the amplitude of the sweep signal. It will also add the center frequency offset and the centerfrequency regulation voltages.
I may put a linearization circuit on this board.

DAC chips arrived

The DAC chips LTC1799 have arrived. They will be used to set the center frequency.
Unfortunately my PIC16F877 has been destroyed. I don't know why. Some new ones have been ordered. In the mean time I will use a 16f84 to develop a driver for this chip.

My DAC chips are dual DAC chips. I am considering to use the 2nd DAC for the center frequency control loop. The original plan was use an external integrator in the control loop but I may as well do it in software. New plan: use one 10 bit DAC's to set the center frequency. A lookup table will be used. The actual center frequency will be measured and corrected by a control loop. De 2nd DAC will output the control signal.