[Elecraft] Kx3 RxIQ

Mon Feb 16 16:24:01 EST 2015

Chris,
      The IF out on the K3 is straight IF as it is going to be used in 
the radio. That is correct. It is not useable as it is for DSP purposes 
without a little work. The absolute simplest is to order a LP-Pan for 
the K3. It takes the IF out and gives you baseband I&Q analog streams to 
be used for your purposes.

     Let me get a little technical. You must have a way to discern not 
only amplitude but phase information. By obtaining this information you 
can describe all of the signal information present in the streams.  
Doing a A/D on two streams of analog data that has a known phase shift 
between them allows for this. Quadrature is the easiest, IMHO, so that 
is why that is what is used.

     You could use the IF out and split it into two analog streams, 
shift one 90 degrees, and then do an A/D on them. However, getting a 
good A/D converters that operates at least 2X the K3's IF frequency is 
going to be expensive, and you probably want to over sample at 3-4 times 
the IF frequency. Going to baseband allows you to do it all with a 
$50-%100 sound card, simple minded economics and engineering. And it 
turns out that sound cards can do a superior job.

     The real trick in making a good SDR is making sure you don't 
overload the A/D converter and while making sure you receiver has enough 
sensitivity to not be internally limited by noise. Good sound cards have 
24 bit A/D converters that should allow over 100 db dynamic range and 
most have pretty good internal noise figures. So, if you are really 
careful about gain distribution in the analog portions of your radio, 
you can end up with quite a radio. Elecraft is a prime example of this 
in both the K3 and the KX3.

     Hopefully, I have given you enough to think about to figure out 
what you want to do. Of course, there are number of ways to go, Ask a 
room full of engineers for an answer and I'd be shocked if you didn't 
get a room full of answers. The reason is that there are lots of 
compromises and which ones you make are how you view things. Very best 
of luck in where you want to go.


73,
Barry
K3NDM


On 2/16/2015 12:49 PM, Chris Hallinan wrote:
> Thanks Barry, that explanation helps alot.  So I'm assuming that the
> IF output of the K3 is a very traditional IF as in other radios, ie
> it's not Quadrature, etc.  Would that be correct?
>
> If so, another approach would be to feed into an A/D, and then do the
> rest in software, ie gnuradio.  I wouldn't be surprised to find out
> that gnuradio already supports similar configurations, including down
> converting (for certain), splitting and creating the quadrature output
> (unknown).  Sounds like a fun project.  Wish I had time to pursue it!!
> ;)
>
> If I'm reading the block diagram correctly, it looks like an 8.125 MHz IF.
>
> -Chris
>
> On Mon, Feb 16, 2015 at 11:03 AM, Barry LaZar <k3ndm at comcast.net> wrote:
>> Chris,
>>      If you are going after the K3 IF output, it is a single audio signal at
>> the IF frequency. You will need to downconvert it to baseband and split it
>> into two streams, one of which will need to be shifted by 90 degrees. It is
>> these signals that go to a good sound card for playing. It will be the only
>> way it will work. The P3 does everything it needs for spectrum display. The
>> KX3 already gives baseband audio streams that are separated by 90 degrees
>> meaning all you need to do is feed the streams to your sound card and
>> operate on it with software.
>>
>> 73,
>> Barry
>> K3NDM
>>
>>
>>
>> On 2/16/2015 9:34 AM, Chris Hallinan wrote:
>>> I'm quite interested in experimenting in this area.  I've been playing
>>> around with SDR via gnuradio with a cheap TV tuner dongle.  Can
>>> someone please explain the characteristics of the signal coming from
>>> the IF Out port on the K3?  Is it basically in the audio domain? Is it
>>> this connector that would plug into the P3?  I'd like to experiment
>>> with viewing the signal spectrum using something similar to gnuradio.
>>>
>>> Brian, how did you connect your KX3 to the iPad?
>>>
>>> Thanks,
>>>
>>> Chris
>>>
>>>
>>> On Mon, Feb 16, 2015 at 8:54 AM, Brian Waterworth
>>> <brian.waterworth at gmail.com> wrote:
>>>> Yes you can.
>>>>
>>>> I used to do this with an iPad (a tablet computer) running iSDR and a
>>>> piglet connected into the ACC1 jack.  I used Hamlog to control the KX3
>>>> and
>>>> iSDR to view the RX I/O.  I decided to buy the PX3 (i.e., a specialized
>>>> computer) instead, though, as its power consumption seems better than the
>>>> iPad.  Plus, flip flopping back and forth between iSDR and Hamlog wasn't
>>>> helping me be productive.  You can never have too many screens :-)
>>>>
>>>> Now I use the PX3 as my panadapter instead of iSDR and can still use
>>>> another computer to control the KX3 (PX3 has a pass through ACC1 jack).
>>>>
>>>> On the horizon...I am excited about the Raspberry Pi 2 (on back order) as
>>>> this platform can support a number of Ham oriented programs I like to use
>>>> (FLDigi, WSJT-X).  Current draw for field work is about an amp-hour and I
>>>> can use my iPad to VNC into the R-Pi.  For field day or more coordinated
>>>> events, I would just bring a monitor and connect that to the R-PI.
>>>>
>>>> regards,
>>>> Brian
>>>> VE3IBW
>>>>
>>>> On Mon, Feb 16, 2015 at 8:34 AM, Suite, Wayne <wsuite at dfwairport.com>
>>>> wrote:
>>>>
>>>>> Can I use this output to a separate computer as a 2nd receiver while
>>>>> using
>>>>> another computer to run HRD etc?
>>>>>
>>>>> KD5SPX
>>>>> Wayne
>>>>>
>>>>>
>>>>>
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