[Elecraft] Flex Radio - is it real?

[Jerry Hancock]

This is a good look at hybrid DDS and PLL.  Also written up in QEX.

http://www.qsl.net/ke5fx/synth.html

I've done a lot of work with the AD9854 DDS.  I works well, has low phase
noise and you can minimize the spurs through programming.  If you want a LO
that tunes 1GHZ continuously, it is a simple way to go.  Just about any LO
synthesizer has its quirks, like in the K2 where you get the pop every now
and then during tuning.  So I guess all these systems have tradeoffs.
Analog devices claims they can get over 90db out of this chip.  Not the
best, but.

Jerry KG6KGP

-----Original Message-----
From: elecraft-bounces at mailman.qth.net
[mailto:elecraft-bounces at mailman.qth.net]On Behalf Of John, KI6WX
Sent: Monday, May 31, 2004 6:43 AM
To: Lee Buller; Elecraft Reflector
Subject: Re: [Elecraft] Flex Radio - is it real?

Lee;
I'll comment on the DDS part of your question.  A DDS is a real good way to
generate frequencies with small tuning increments, but they also generate
significant spurious responses.  These spurious responses will limit the
receiver dynamic range unless some additional circuitry is used to reduce
their effect.

For instance, the DDS used in the KX1 will generate spurs at some
frequencies at levels of about -70 dBc.  At these frequencies, the third
order dynamic range will be limited to 70 dB.  While this is adequate
performance for a KX1, it is not acceptable for a K2.

The current tests done on amateur equipment by the ARRL are not designed to
catch these types of spurs.  The equipment vendor can even design the
equipment to avoid a problem from a DDS spur at the ARRL test frequencies.

The KX1 was designed using a DDS that has a 10 bit DAC.  There are DDS
designs with up to 14 bit DAC's, which should theoretically reduce the spurs
by 24 dB.  However, non-linearities in the DAC design keep the improvement
down to about 10 dB.

Someday, a DDS may be able to generate a clean oscillator that can be used
to directly drive a mixer in a high performance radio.  That day isn't here
yet, and probably won't be here for a decade or more.

-John
 KI6WX

----- Original Message -----
From: "Lee Buller" <k0wa at swbell.net>

> I stopped by the Flex Radio booth at Dayton this year and spoke to one of
the evangelists touting the new software defined radio using quadrature
sampling.  I wonder if any of you techies (I really am not a techie, just a
wannabe) have looked at this system?  The numbers seem to be very good on
the receiver and the hardware seems to be quite simple or maybe minimal.
The Quad Sampling method uses "direct conversion" using a DDS VFO going up
to 54 Mhz.  Now, I know this is not "mixing" but they doing a sampling of
the frequency and giving an output (Inphase - Quadphase) to be numerically
processed by software.
>
> I am just curious about what you guys think about this interesting piece
of technology?  I am really interested in the DDS VFO because you could use
one of those on old boat anchors that are crystal bound.  But, I am
impressed with the technology and the "numbers" they are giving for receive
figures.  Is this the future of HF?  I am not sure I want to have to haul a
computer along with me when I go portable or mobile.
>


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