125Khz clock for RFID

[Guest]

I am modifying 125Khz RFID reader for Proximity card,
The original design of the reader circuit is quite simple, it use CMOS 4069 inverter IC to demodulated the signal to digital.
For the clock 125Khz, it is derived from 8Mhz crystal , and divide down to 125Khz by 74HC393 .

I have success decode the read data using 16F628A.
In order to save the crystal and 74HC393, I like to use 16F628A to generate the 125khz clock as well.
I use hardware PWM to generate 125khz, 50% duty , a nice square wave is generate at pin_b3.

However, when this PWM clock signal is feed to MIC4429 to drive antenna, the RFID detection range reduce by half, compare by using crystal divide clock source.
Also, there is more noice appear on 4069 when using the 16F628A PWM clock source.

Any special requirement for RFID clock? It shall be very straight forward as I think, but the two different clock source signal is given 2 different result.
They look almost identical under osciloscope. I have check grounding and other like decoupling cap, but not helping at all.

The 16F628 is using 4Mhz internal ocsilator. Will this be a problem?

Pls help, I am almost cracy about this problem.
I will post schematic if needed, or let me know if you need more info.

Thank you.

[Humberto]

[Victor]

Sorry, previous post was made by me. (Forgot to login when made post)

I am using the correct version of 4069.

Instead, it is work, but when I use PWM as 125Khz clock source, there is unknown noise on 4069, and cause reduce of range.

Is there any different of 125Khz (PWM 50% ), and 8Mhz clock divide from 7hHC393?

When scope them, it look identical , both is same TTL level, but it give 2 different result.

Any thing need to take precaution? Please give me some hints.
Thank you.

[Ttelmah]

The 74HC outputs, are specified to pull 'down' to 0.33v, for loads up to 7.8mA. The PIC outputs, only specify 0.6v at the same current. However the input circuitry of the driver you are using, should not be affected by this. Do you have a 'F628', or a 'LF628'?. The latter has significantly slower transition times on the outputs (nearly double), which might have an effect.

Best Wishes

[Victor]

I am using 16F628A, INTRC_IO.

One question, is the 125Khz clock for RFID must be a 50.00% duty cycle?
I am worry the generate PWM is slight phase shift , or not perfect 50% duty, and cause distorion, later pickup by the antenna itself, and cause internal oscilation.

I am almost give up about this, worst case I will use 8Mhz crystal + 74HC393 to generate the 125Khz, which is tested no problem.

I taught I can save the 2 components in the designs, but seem like I am wrong in this case.

Thank you.

[Tom-H-PIC]

Hello
I'm vary interested in this is there a schematic reference for this circuit on the web some place?
Thanks
Tom