handheld RF Detector - Page 1
Forum section: electronics / rf-microwave

author: Thedon
author_rank: Contributor
timestamp: September 18, 2020, 06:17:10 am
content: Hello All,I have been wanting to share this project for a while the previous version was completed about 5 to 6 years ago and the current version has just been completed. The detector is able to detect Rf signals from 750MHz up to 3GHz with power levels from -60dBm up to +5dBm, I wasn’t willing to test at higher levels even though the datasheet for the Rf amp states 13dBm max input for fear of destroying the Rf amp which I have done in the past. The range of the unit is approximately up to 20m depending on frequency being detected and the room environment. I haven’t tested in an outside environment. The following is a breakdown of the blocks within the detector;1. The Rf amp (Gali-39) and T filter which is designed for a low frequency cut-off of 788Mhz. I designed the T filter as a high pass filter for detecting mobile phones.2. MAX4003 Rf detector.3. LM358 acting as the signal conditioning circuit between the MAX4003 and the Arduino Nano. The MAX4003 produces a pulsed output so to convert to DC the LM358 acts as a peak detector and a non-inverting amp to produce a larger range DC output.4. Arduino Nano and 128x160 LCD, previous version PIC16f876 and 16x2 LCD.5. Piezo buzzer and amplifier that produces an audible frequency dependent on strength of signal detected.6. Power supply includes a MCP73831/2 for the LI-ON battery management and a Pololu U3V12F5 +5V DC to DC converter.The PCB is designed to fit within a 25x71x110mm enclosure I chose this common aluminium enclosure to improve shielding. The PCB was designed using Dip Trace apologies for the low-quality pictures if anyone is interested, I can supply the actual files.Top layerBottom layerWriting the Arduino code was fairly straight forward except displaying the battery charge level which I’m not entirely satisfied, it seems the TFT library is limited as to what you can do with shapes, I’ll admit I’m not the best when it comes to writing code.I’m not satisfied with the level of accuracy as can be seen from the excel plots there is an error of ± 5dBm I was hoping to get at least an accuracy of ± 1dBm and at 3GHz it’s atrocious I didn’t think it was worth plotting.Considering I’m no RF PCB expert and my previous experience has been below 1GHz maybe the Rf gurus can provide suggestions on how to improve the design. The obvious one would be to use 4-layer PCB for accurate microstrip widths (can’t use Dip Trace for free for 4-layer) and probably component layout. I realise that I will never get the level of accuracy that a Boonton and or HP power meter would provide but at least better than what I have now.There are two design possibilities as to what I could do to improve the accuracy they are;1. Replace the Rf amp, MAX4003 and the LM358 with the AD8318 and AD7887 configuration which has already been done by somebody else.https://www.dl2sba.com/index.php/elektronik/messtechnik/rf-powermeterhttps://www.mikroe.com/rf-meter-click-click2. Replace the components listed above with the Ascel electronics AE204014 power module which is available as a separate component.Both options will involve significant redesign but I think are achievable the Ascel option will involve extra software to calibrate the unit for higher frequencies and will probably require the 1ppm TCXO time base for the required accuracy. I was wondering because I have never designed a 4-layer PCB what difference would a 4-layer PCB make? Specifically, impedances of the Rf stage? Could the design be simulated using Sonnet software or something similar? Or is this a question that is difficult to answer definitely?

author: electrolust
author_rank: Unknown
timestamp: September 24, 2020, 11:12:29 pm
content: nice job. thanks for sharing.

author: Thedon
author_rank: Contributor
timestamp: September 25, 2020, 10:09:18 am
content: No worries thanks. I'm curious to find out what happens to the impedance over it's frequency band so willing to supply a working unit for experimenting with a VNA. Of course any discussion regarding 4 layer board design is welcome.