Proxmark3 community

loren

Contributor

Registered: 2012-11-30

Posts: 6

125kHz communication issues ... HELP!

I'm having 125kHz communication issues:

- Some vehicles can "hear" me emulating a tag without any error and some vehicles cannot "hear" me
- Antenna orientation is critical and touchy
- I am able to recieve data from the vehicle (i.e. reader - BPLM) without difficulty

I have built numerous antennas and played with tuning and have acheived antennas > 25V @ ~125kHz.

I am wondering about Proxmark's ability to perform load modualtion. In /doc/system.txt under "Analog Transmit Path" of the proxmark source it says:

"The tx strength, given good antenna tuning, is determined by the series
resistors. Choose the ratios to stay within the rated current of the
buffers, and to achieve the desired power ratios by enabling or disabling
nOEs for the desired modulation index. It is useful to populate one of the
resistors as a high value (~10k) for the simulated tag modes; this allows
us to look at the incident carrier without loading the reader very much."

Can anyone help me understand how Proxmark's load modulation works or how to improve load modulation?

vivat

Contributor

Registered: 2010-10-26

Posts: 332

Re: 125kHz communication issues ... HELP!

You can try this:
1. Enable/disable OutputEnables at FPGA source code, like in the module lo_edge_detect:
assign pwr_oe1 = 1'b0;
assign pwr_oe2 = tag_modulation;
assign pwr_oe3 = tag_modulation;
assign pwr_oe4 = tag_modulation;
2. Replace electrolytic capacitors to tantalum capacitors, which will give you nearly + 10% voltage
3. Try building your own antennas using different cables

PS
Can you post photos of your antennas with size?

loren

Contributor

Registered: 2012-11-30

Posts: 6

Re: 125kHz communication issues ... HELP!

Hi Vivat, Thank you for the reply! Many great suggests.

1) My FPGA source is left unchanged and remains the default:

assign pwr_oe1 = 1'b0;
assign pwr_oe2 = tag_modulation;
assign pwr_oe3 = tag_modulation;
assign pwr_oe4 = tag_modulation;

I'm not entirely sure how the pwr_oeX pins change the modulation index, but changing them didn't seem to impact me results.

2. I'm using ceramic tuning capacitors as tantalum capacitors seem to be only available in the > 100nF values

3. I have used many different antennas but never built my own, what you you propose I build? does wire gauge play a significant role in load modulation? .

I have found the medium size 125kHz from Son Micro to yield the best results so far and is shown in the picture below. 

http://www.sonmicro.com/en/index.php?option=com_content&view=article&id=49&Itemid=66

# LF antenna: 34.64 V @   125.00 kHz
# LF antenna: 22.02 V @   134.00 kHz
# LF optimal: 34.64 V @   125.00 kHz
# HF antenna:  0.10 V @    13.56 MHz
# Your HF antenna is unusable.