F.Y.I. an introduction to the term "Homodyne".

  • ZoneAdmin
  • ZoneAdmin's Avatar Topic Author
  • Offline
  • Administrator
  • Administrator
  • Posts: 60
  • Karma: 1
  • Thank you received: 8

ZoneAdmin created the topic: F.Y.I. an introduction to the term "Homodyne".

Shared with me by N9VV, I thought I should post it here for your review and discussion if you so desire.

A friend wrote to me and said:

"The KX3 and KX2 are Homodyne SDR's, requiring mixer calibration every 6
months to maintain carrier/IF and opposite-side-band suppression. A
spectrum analyzer is required for this procedure."
That reminded me of the Dan Tayloe Quadrature Sampling Detector or
Mixer (circa: 1999). That inexpensive design became popular in so many
of the inexpensive SDR rigs of that era (like the Softrock) and was
commercialized in the Flex 1500, 3000, and 5000 rigs.

The comment that a "Homodyne" designs require re-calibration is
absolutely true. There were various work-around schemes to keep the QSD
in phase. One of them was the famous VE3NEA "Rocky" receiver software
that included a careful measurement and computation to keep I and Q
signals in Phase as you changed frequencies over a narrow range:
<URL: www.dxatlas.com/Rocky/Advanced.asp
> (picture attached). I don't know if Elecraft recommends
re-calibration of their KX3/KX2 or on what schedule?.
The DDC scheme using lightning fast FPGA and the "CORDIC" algorithm were
introduced to Ham Radio by Phil Covington N8VB in 2007. He created the
QS1R Receiver using the LTC-2208 ADC and Cyclone-III FPGA (pix
attached). That same architecture was Commercialized by TAPR and Apache
to create their Transceivers. Flex followed suit and discontinued their
QSD gear in 2012 introducing the current 6xxx lineup using DDC

The DDC architecture is self-calibrating in the sense that the FPGA
operates at nano-second (billionths of a second) speeds and employs
mathematical operations (32bit float point) that keep the unit in
precise alignment (I/Q balance), at all times, on all frequencies. There
is never an "imbalance", "center spike", or "carrier suppression" issue
in a properly designed DDC Trx.

The greatest disadvantage to the DDC design is that it requires a
skilled, specialized, FPGA Engineer who can "think like a state
machine". While DSP chips in the back end of a Homodyne can usually be
programmed by any good "C" programmer using an Arduino or small ARM CPU.
Please do not take this as any shot at Elecraft or Elecraft bashing. I
have always yearned for, and drooled over, the Elecraft gear. I built
their K2 #169 in 1997 and loved it. They have outstanding CW
performance. Various DX Certificate hunters claim it is the best
DXpedition rig ever invented. There are tens of thousands of very happy
customers all over the world.
It is fun to dig out the facts and separate it from the hype that you
hear on the air <LOL>

thanks for reading,
your comments please,
73 Ken
The following user(s) said Thank You: KD8TUT

Please Log in to join the conversation.

Time to create page: 0.401 seconds