Message-ID: <004901c1eb9f$a4915580$09dc9384@ma.ultranet.com>
From: "John Andrews" <w1tag@charter.net>
To: rsgb_lf_group@blacksheep.org
References: <7D653C9C42F5D411A27C00508BF8803DCB9A7E@mail.dstl.gov.uk>
Subject: LF: Re: DDS Amplitude control
Date: Wed, 24 Apr 2002 10:41:22 -0400
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Andy,

> Not being able to control the amplitude of the AD9850/51 DDS chip output
> waveform makes it difficult to generate decent signalling waveforms.
> However, this is possible by changing the current into the Rset, pin 12,
> which sets the full scale amplitude of the D/A output.  On my DDS board
this
> current is set by an innocuous 3.9k resistor.  Replacing this with a FET
> acting as a variable resistor allows a DC level to control the amplitude
for
> non critical applications - such as bandwidth limiting a signal by
envelope
> shaping.

Note also that this makes a good place to key the output on and off without
resorting to software. I have a FET in series with the original 3.9k
resistor, and keying that eliminated a "backwave" problem when I keyed
subsequent stages.

John Andrews, W1TAG