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UHF RFID VCO problems

Karsten Pilested
Associate II
Posted on February 09, 2017 at 17:34

In the UHF RFID reader ST25RU3993 I am having a little bit of a problem with startup of the VCO.

In VCO control register 11h I can change the internal oscillator bias current. What do I gain by changing it from the default ?

I can also manually select the VCO range segment, I will like to do that to speed up the change of frequency, but how do I figure out the best VCO segment, is it possible to calculate it ?

In section 2.3.1 in the data sheet it says that: ' Details on using the 1800MHz VCO are described in a dedicated application note'.

Where can I find this app. note?

#uhf-rfid-reader-st25ru3993
1 ACCEPTED SOLUTION

Accepted Solutions
Bart Herse
Senior II
Posted on December 13, 2017 at 09:40

Hello Karsten,

the internal bias oscillator should be kept at the default.

The best VCO segment can be determinded by activating the mvco bit in register 0x11.

This will initiate a measurement of the VCO controll voltage.

The result is stored in the register 0x2C.

Note register 0x2c is a paged register, which means in order that this register displays the information corresponding to the VCO control voltage measurement it should be configured in register 0x29 accordingly (r2Cpage[1:0] = 01).

Once the result is available the mvco bit in register 0x11 should be cleared again.

The result can range from 0 to 7 and a good result, meaning that the correct segment is selected, should be from 2 to 5.

If outside this range the next above or below segment should be selected.

A manual selection of the VCO segment is done again in register 0x11.

Please note that the manual VCO selection mode should be enabled first.

This mode is entered by sending the direct command 0xA5.

The entire VCO selection process can be automated as well by activating the automatic VCO range selection mode - send direct command 0xA4.

The draw back of the automatic VCO range/segment search is that it is taking more timeto complete compared to a typical manual segement search and selection.

You will find that while there are in total 16 VCO segments available typically only a few 5,6, etc.. will be usefull for current UHF RFID frequencies, depending in which country/regulation your reader is going to operate.

Cheers,

Bernhard

View solution in original post

3 REPLIES 3
Rak Rak
Associate
Posted on November 15, 2017 at 12:26

Hi,

By seeing your question, we guess you are using a development board. Can you share the details how can we quickly make a prototype board using ST25RU3993 ? It will be great help to us.

Thanks & Best Regards

Rakesh

Karsten Pilested
Associate II
Posted on November 16, 2017 at 20:27

Hi Rakesh,

I do have a demo board from the time the chip was made by AMS, if you send me your email address to

mailto:karstenpil@gmail.com

  can send you the schematic and board layout for it 
Bart Herse
Senior II
Posted on December 13, 2017 at 09:40

Hello Karsten,

the internal bias oscillator should be kept at the default.

The best VCO segment can be determinded by activating the mvco bit in register 0x11.

This will initiate a measurement of the VCO controll voltage.

The result is stored in the register 0x2C.

Note register 0x2c is a paged register, which means in order that this register displays the information corresponding to the VCO control voltage measurement it should be configured in register 0x29 accordingly (r2Cpage[1:0] = 01).

Once the result is available the mvco bit in register 0x11 should be cleared again.

The result can range from 0 to 7 and a good result, meaning that the correct segment is selected, should be from 2 to 5.

If outside this range the next above or below segment should be selected.

A manual selection of the VCO segment is done again in register 0x11.

Please note that the manual VCO selection mode should be enabled first.

This mode is entered by sending the direct command 0xA5.

The entire VCO selection process can be automated as well by activating the automatic VCO range selection mode - send direct command 0xA4.

The draw back of the automatic VCO range/segment search is that it is taking more timeto complete compared to a typical manual segement search and selection.

You will find that while there are in total 16 VCO segments available typically only a few 5,6, etc.. will be usefull for current UHF RFID frequencies, depending in which country/regulation your reader is going to operate.

Cheers,

Bernhard