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Re: [Discuss-gnuradio] Cheap ADC at 35MHz

From: Marius Hauki
Subject: Re: [Discuss-gnuradio] Cheap ADC at 35MHz
Date: Wed, 17 Apr 2002 09:56:15 +0200
User-agent: Mozilla/5.0 (Windows; U; Windows NT 5.0; en-US; rv:0.9.4) Gecko/20011128 Netscape6/6.2.1

Hi Adrian,

does the chip have a separate input for the
sync tip sample input ? If it has, might it be possible
to hardwire this input to a fixed DC value via a decoupled
potmeter ?

Another idea :
Perhaps it would it be possible to AC couple the baseband input thru a
high pass filter. Then on the ADC side of the low pass filter, you could
insert a DC voltage that emulates the sync values. The filter must be dc blocked towards the outer circuit so it does not barbecue the outer circuits in the downconverter. The penalty with this arrangement is that you would loose some bandwidth in the lower part of the baseband. The band on the low side of the low pass filter will be a "dead band" dedicated to the DC input circuit. This DC input circuit could be a decuopled potmeter with a cap over the center and an inductor in series.
The cap is to avoid PSU noise to enter the ADC.
Anyway video is approx 5 MHz wide and I assume the sample rate is at least 10Ms / sec or more so there should be enough bw anyway. If the LPF is sharp it would limit the dead band. It should not be too sharp, becuase it will have large phase deviations close to the cut off. It should be possible to characterize the input complex frequency response by doing a
sweep on the input and measure the real and imaginary data. Then one could
implement a equalizer in software to linearize the response in amplitude and phase. To avoid interesting parts of the signal to appear at around zero and in the dead band, and thus be lost, the local
oscillator could be set slightly offset.

This is basically the same idea that is used in injecting a DC voltage on the coax to feed
a low noise microwave downconverter for satellite TV.

PS : posted the reply to the list since it might have general interest.

Best regards
Marius Hauki

Adrian Godwin wrote:

where did you get access to the driver ? Is this
available as open source ? Would it perhaps be possible to

bypass the DC restoration circuit if it is external to the
chips used  ?

The Bt848 is one of the best supported cheap framegrabbers. There's
a driver in the Linux kernel, and quite a bit of information around.
See http://www.metzlerbros.org/bttv.html

I didn't need the driver sources to capture those sample dumps,
but it would be necessary to change it in order to get continuous
samples as the vbi input is intended to grab only the data from
the lines in the vertical blanking interval (i.e. teletext)

The problem with the DC restoration circuit is that it's partially
insice the chip. Although some parts of the circuit (the AGC, really)
is routed outside, it appears that the 0 level is defined by sampling
the value of the data during the sync pulse. I'm not sure if it's
possible to avoid this.


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