System diagram

A project log for Handheld 3GHz Spectrum Analyzer

Includes RF detector up to 6GHz and a datalogger

roelhroelh 08/16/2014 at 13:430 Comments

There were some more things that I had do to comply with the prize regulations:

- make the entry official So I did.

- add a system diagram. I added this to the 'details' section today.

- have at least four project logs

The system diagram was made with Excel. You can easily draw boxes around your cells, and your boxes will align nicely. It is also easy to change the size of your boxes to make the text fit. It also lets you use arrows. At the end, I make a screenshot (gridlines off) and let Paint convert it to a graphical format.

In this project log I will explain some sections that have not been discussed in the 'details' section.

The RF power detector chip has 2 identical channels. The input signal is split in two branches: The first branch passes a high pass filter and then goes to the first detector channel. The second branch passes a low pass filter and goes to the second detector channel.

The filters are not very steep. The outputs of the filters will have an amplitude ratio that is dependent on the input frequency, so the input frequency can be recovered by taking the difference of the two outputs of the logarithmic RF detector. Of course, this principle does only work if only a single frequency is present (or if other frequencies are much lower in amplitude).

The firmware uses the output difference to calculate the frequency. It uses a table with calibration points, that is determined for each device individually, and stored in a separate flash section of the controller.

This frequency measurement is also supposed to work for short RF pulses. Therefore, the outputs of the RF detector are routed to the two different ADC's, and the ADC's will sample the signal at exactly the same moment.

The RF detector is not very sensitive, it needs at least -60 dBm, whereas the spectrum analyzer will be able to detect signals as low as -110dB.

The on/off button is a momentary switch, that will set the electronic power switch in the ON position when pressed.

When you want to switch the device off, you press the on/off button again. This will NOT directly cause the power switch to switch off. But the firmware will notice that you pressed the button, it will wait until you release the button, and will then close the logfile and switch the power off. The firmware can also switch the power off if the battery voltage becomes too low.

The electronic power switch is placed after the step-up converter. By placing it at this position, a voltage drop over the power switch will give us less efficiency loss then if it was placed before the step-up converter. Note that when the power is off, the transistor of the step-up converter is also off, and the step-up converter does not consume any current because its control circuits are powered by one of the linear regulators.