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# AFE5809EVM: AFE5809EVM Digital Demodulator

Part Number: AFE5809EVM

Hello,

I am trying to use AFE5809EVM Demodulator feature.

1.  If possible, Can you guys please help me to design filter for 40/32 = 1.25 MHz ?

2. If possible I would like to set filter cut-off freq to 350 KHz.

Please let me know if you need any further information.

Thank you.

• In reply to Sahana Krishnan:

Hi Sahana,

I am using 0x47B(1147 decimal) as a value for N. my Fs = 20 MHz. Which brings my mixer frequency to ~350KHz. The problem I am having is with the Decimation.

I want to use 32 decimation factor. and 200KHz as LPF cut-off frequency,

THanks

• In reply to Vatsal Naik:

Hi Vatsal,

(1) You had previously asked a question about only getting In-Phase information from down conversion. This can be done by setting the value of the register with address '0x0A'. This is a 16-bit register with bit 15 as the MSB and bit 0 as the LSB. Bit 14 of the register (MODULATE_BYPASS bit) controls the channel output mode which can be selected as IQ modulated or single channel output.
When bit 14 = 1, only the I channel data appears on the LVDS line
When bit 14 = 0, both I and Q channel data appear on one LVDS line

(2) The decimation factor and cut-off frequency can be set by generating appropriate coefficients that can be used to program the RAM. Section 8.6.2.1.3 (Programming the Coefficient RAM) in the AFE5809 data gives some details on this. The coefficients are symmetric so there will need to be eight 14-bit coefficients in 2's complement format. I am still looking into how to generate these coefficients.

I hope this helps!

Sincerely,
Sahana
• In reply to Sahana Krishnan:

I also forgot to mention for (2) in my previous reply: There is an ‘fir1’ function in Matlab used to design filters. It takes in the filter order and corner frequency as parameters. I am still looking into how we can utilize this function in order to generate the coefficients.
• In reply to Sahana Krishnan:

Hi Sahana,

1. Thanks for the information.

2. yes, I got the filter coefficients! The question I have is, Does a number of coefficients depend on Decimation factor? In the datasheet, It is given that coefficient should be 8 x decimation factor. So what happens if I use less or In the datasheet, It is given that coefficient should be 8 x decimation factor. So what happens if I use less or more number of coefficients?

I am usign 32 decimation factor so, 256 coefficients are enough, but I can increase coefficient for some better filter!

Regards,

Vatsal

• In reply to Vatsal Naik:

Hi Vatsal,

The device has only been tested as per the specifications in the datasheet. So, for a decimation factor M, it expects 8M coefficients to be stored in the coefficient RAM. We have not verified what will happen if less or more information needs to be stored in this space!

Sincerely,
Sahana