Unknow circuit theory
This circuit is part of a 13.56M RF reader, but how does it work and what is it used for? Does somebody understand?
This signal is ASK signal from C1 and it has 3.3V DC component, the ASK signal's carrier signal is 13.56MHz and modulation signal is 455KHz. In truth, I have't capture this ASK signal (because this AC voltage is too small about 10mv and DC voltage is too large), but in theory it does exist(because this signal is used for communication between RF TAG and RF reader in ISO15693 standard)
In my opinion, first it is a high pass filter which consist of C1, R1 and R2; second it looks like a integration circuit too, and the voltage at point 3 is 2.5V, that is all I know.
Attachment rar is multisim10 file
circuit.pdf
circuit.rar
It's a band-pass with about 500k center frequency and 200k pass band width. But due to the limited OP bandwidth, the real filter characteristic will be considerably different from ideal behaviour. The ISO 15693 subcarriers are fc/32 (424 kHz) and optionally fc/28 (484 kHz).
You need an OP with at least 10 MHz GBW to achieve the intended filter characteristic.
Hello FvM!
Thank you very much!
First, I ask a question how do you know it is a band-pass filter, is there some function to calulate this result? do you have some related data or design software can help me ?
Second, you say this is a pass-band filter, but the bandwidth is limited by OP bandwidth, you suggest me to choose an OP with at least 10MHz GBW. but the GBW = bandwith * Gain, So how can I calulate this circuit's gain? If I reduce the gain, for example if the gain is 1 and the bandwidth is 600k(500K center frequency add 100K upper side frequency), so the GBW = 1 * 600K = 600K, can I choise a OP with 1MHz GBW?
Hello FvM!
Now I find a way to simulate this circuit, and this is the result. This circuit does a band pass filter, and its center frequency is 300KHz, and its bandwidth is
about 100KHz, but I still can't understand how does it work? Do you have some relative data?
Attachment:
It is a single stage parallel biquad, I think. It will work a lot better if the opamp was a decent one for the required centre frequency - as FvM mentioned the MC33172 is far too slow which is why the centre frequency is not 455kHz.
Active Filters by Nuhertz Technologies
Keith.