joesugar

Papilio Digital Audio Input?

5 posts in this topic

Hi guys,

 

Anyone here ever do any work getting digital audio data into a Papilio?

 

I'm working to create a waveform generator for amateur radio use using the Papilio One.  The idea is to accept data from another computer in I/Q format and upconvert it in the FPGA to produce a signal in the 3 - 30 MHz range.  Think a lower-frequency version of the GNU Radio USRP.

 

I'm at the point where I need an interface to read in the I/Q data and am leaning toward creating a block that will impersonate the I2S interface of an audio codec.  I'd be satisfied with a bandwidth of around 50 kHz (for now :)) and I like how I2S explicitly identifies the right/left channels.  However, I'm open to other ideas and thought I'd see if anyone here has done something similar before committing myself.

 

Thanks for any suggestions,

Joseph

 

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Hi,

 

an audio codec can do it but it's AC coupled with uncontrolled phase response.

 

The Analog grove wing is very cheap and has an adequate ADC but it doesn't extend into negative voltages. So if your IQ source is for example +/- 1 V fullscale, you'll need to offset the signal. One solution could be to load the generator with 50 ohms into vRef/2, so that the generator 50 ohms and the load 50 ohms form a 1:2 voltage divider and offset (assuming your source is 50 ohms).

 

In this frequency range it isn't hard to find suitable eval boards, e.g. Analog devices or TI, with dual- or quad channel devices.

They often come with convenient on-board opamp circuitry. For higher bandwidths, going down to DC will be the main challenge.

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>> The Analog grove wing

 

PS if you (or someone else) considers this ADC for IQ sampling, buy two:

 

- The control logic simplifies : During the bit sequence of reading channel n, I have to program already the channel for read operation n+1. It's no rocket science but it can cause some confusion

- The effective sampling rate doubles

- Using one ADC for I and Q will sample both signals at slightly different time instants. This causes weird effects at high frequencies, and digital correction (polyphase filtering) isn't completely trivial.

- They are cheap anyway

 

Needless to say, there are better ADCs. There is always a better ADC :)

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