[Elecraft] SDR Filter improvement

[Walter Underwood]

Two things about terminology. First, IQ is generally in-phase and quadrature.

Also, what you are describing as direct conversion is commonly called direct sampling.
The K4 is a direct sampling receiver. The KX2 and KX3 are direct conversion receivers,
converting RF directly to baseband without an IF. The KX2 and KX3 are also SDRs, but
with sampling at baseband. The filtering, demodulation, and modulation is done in software.

This article has a nice bit of history about direct conversion receivers.

https://www.n5dux.com/ham/files/pdf/Direct%20Conversion%20Receivers%20History%20-%20W7ZOI.pdf <https://www.n5dux.com/ham/files/pdf/Direct%20Conversion%20Receivers%20History%20-%20W7ZOI.pdf>

wunder
K6WRU
Walter Underwood
CM87wj
http://observer.wunderwood.org/ (my blog)

> On Jul 3, 2021, at 5:48 AM, Geoffrey Feldman <geoffreyf at comcast.net> wrote:
> 
> My career has been in software/hardware systems including some with signal
> processing.  Digital filters ultimately embody mathematical equations which
> operate on a matrix of samples represented as complex numbers.  IQ stands
> for incident and quadrature, sine and cosine, real and imaginary or x and y.
> All are the same concept but different nomenclature as represented in
> memory.  Connect to the IQ out of your Elecraft rig you are getting that
> stream as left and right base band audio.  A sound card and a bit of
> software will convert that base band left and right to the numerical
> representations.  From there you can implement whatever is actually possible
> in this technology as a software experiment on your own PC.  Create your own
> SDR Radio using the Elecraft hardware and experiment with filters, create
> your own pan adapter or whatever.   If successful you will have a prototype
> to share with Elecraft.
> 
> 
> 
> What is more likely is that you will more fully appreciate the value you
> receive from Elecraft and all the hard work they did.   It's not easy to
> improve on.   Signal Processing ALWAYS involves some fairly hairy math and
> hairier software to implement it correctly as a real time experience.  The
> filter you want may be one that actually can be implemented.
> 
> 
> 
> Direct Conversion SDR is a great technology to reduce cost and/or size in
> relationship to a similarly good receiver.  However, it has limitations
> which relate to artifacts from the sampling process and dividing a
> continuous stream of audio into "bins" or snapshots in the stream.  Analog
> filters in the IF stage is still a factor in design and you will see hybrid
> SDR as an alternative to direct conversion designs.  Similarly even in
> direct conversion, there are some RF pre-filters.  This last bit, is
> argumentative and wise persons will differ.  It's the same discussion as
> tubes/transistors or vinyl/cd's  etc. in the audiophile world.  A lot comes
> down to perception and how we personally discriminate what we hear, want to
> hear, don't want to hear.   Additionally there is processing (digital or
> analog) of received audio, changing frequency response as an aid to this
> perception process.  The Heil PRAS is a great example of that sort of
> processing.  He has a terrific demo of that.
> 
> 
> 
> Finally people with traditional hardware training find themselves at the
> deep end of the pool with all this talk of matrices of complex numbers.
> People with software training find themselves at the deep end of the pool
> with the pragmatics of actually getting RF into their world of 1's and 0's.
> It's fun to watch them trying to get along in a state of panic while in the
> deep end of the pool.  Again, hats off to Elecraft for helping them all to
> swim and catch great products.
> 
> 
> 
> Geoff
> 
> W1GCF
> 
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