K3 15khz 2nd IF?

[Brett gazdzinski-2]

Just wondering if anyone can clue me in on the 15 KHz second
IF frequency used on the K3.
I was wondering why 15KHz, instead of 10, or 50 KHz?

I know it gets harder to sample the higher you go, but what are the
advantages of 15 KHz over audio frequencies, and is not 20 or 50 KHz
possible? What about 10KHz?

Just what drives the choice of 15 KHz?

Brett
N2DTS

 


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[N2EY]

-----Original Message-----
From: Brett gazdzinski <[hidden email]>

>Just what drives the choice of 15 KHz?

Here's my semi-educated guess. Correct me if I'm wrong, folks!:

The reason for the conversion to a low last-IF is to feed the DSP
filter-decoder system.
The lower you go, the better, because you get more samples per Hz of
signal. (If you are
sampling a 15 kHz signal 150,000 times per second, that's 10,000
samples per Hz, but if you
were to sample a 150 kHz signal the same number of times per second you
only get 1000
samples per Hz.) All else being equal, more samples per Hz is better,
as is more bits per sample.
But increasing either means more processor power is needed.

You can't go much lower than 15 kHz without getting down into audio.
Plus you also
have roofing-filter issues at low IFs (if you tried to convert from the
first IF to, say, 5 kHz, the
oscillator is only 5 kHz from the filter passband, and the secondary
image is only 10 kHz away.)

Every design is a series of tradeoffs. 15 kHz is the optimum tradeoff
for all these issues given the
available parts and other design issues.

---

You are probably familiar with receivers of the 1950-60s era which used
a last IF in the 50 to 250
kHz range. From the Hallicrafters SX-88/S-76 to the Drake R4B and many
in between, this was
done because it allowed a reasonable number of practical LC circuits to
provide the selectivity.
Again, a tradeoff - lower IF was better for selectivity but made the
secondary-image problem
worse, while a higher IF meant more tuned circuits were needed.

The introduction of practical high frequency xtal filters ended that
design.

73 de Jim, N2EY
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[David Toepfer]

You can plug 1a 15kHz IF directly into soundcard and get 30kHz bandwidth of
software defined radio.  This would be good for DRM too, I immagine.

dt
.

--- Brett gazdzinski <[hidden email]> wrote:

> Just wondering if anyone can clue me in on the 15 KHz second
> IF frequency used on the K3.
> I was wondering why 15KHz, instead of 10, or 50 KHz?
>
> I know it gets harder to sample the higher you go, but what are the
> advantages of 15 KHz over audio frequencies, and is not 20 or 50 KHz
> possible? What about 10KHz?
>
> Just what drives the choice of 15 KHz?
>
> Brett
> N2DTS
>
>  
>
>
> _______________________________________________
> Elecraft mailing list
> Post to: [hidden email]
> You must be a subscriber to post to the list.
> Subscriber Info (Addr. Change, sub, unsub etc.):
>  http://mailman.qth.net/mailman/listinfo/elecraft   
>
> Help: http://mailman.qth.net/subscribers.htm
> Elecraft web page: http://www.elecraft.com
>

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[David Woolley (E.L)]

[hidden email] wrote:

> The lower you go, the better, because you get more samples per Hz of
> signal. (If you are sampling a 15 kHz signal 150,000 times per
> second, that's 10,000 samples per Hz, but if you were to sample a 150
> kHz signal the same number of times per second you only get 1000 > samples per Hz.) All else being equal, more samples per Hz is better, as
> is more bits per sample.

This doesn't make sense to me.  In a quadrature system, you only need to
sample at a sampling *rate* equal to the bandwidth (you meet the Nyquist
condition because the I and Q samples effectively double the rate).  In
practice you would want to sample based on the skirt bandwidth, not the
nose bandwidth.

I believe some DSP based receivers actually sample a band limited IF
directly, at the bandwidth dictated sampling rate.

Going low probably makes the whole process more reliable.
--
David Woolley
Emails are not formal business letters, whatever businesses may want.
RFC1855 says there should be an address here, but, in a world of spam,
that is no longer good advice, as archive address hiding may not work.
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