[Elecraft] Receiving AM on the K3 - Odd behavior

[Harry Yingst via Elecraft]

Not confusing them just providing details so others can replicate.







      From: Edward R Cole <kl7uw at acsalaska.net>
 To: Elecraft at mailman.qth.net 
 Sent: Tuesday, January 27, 2015 12:20 PM
 Subject: Re: [Elecraft] Receiving AM on the K3 - Odd behavior
   
Harry,

Others may have replied off the list, so what I say my be redundant:

You are confusing RF bandwidth with audio bandwidth.

For AM 6-KHz RF bandwidth is proper for best quality demodulation and 
either AM or FM filters work as bandwidth is controlled by DSP ckts 
and not by the roofing filters.  Audio output of the K3 is limited to 
4-KHz so you will note that you cannot open the bandwidth higher than 
4-KHz in SSB (FM is fixed and you will see "N/A" if you try to adjust 
the "width" knob).  BTW I see 5-KHz displayed at max width in AM on 
my K3 (still, I believe audio is limited to 4-KHz)(I am using the 
13-KHz roofing filter for AM).

Note: I do not have the P3 so what it displays may be counter to what 
I have said (my comments are for the K3).

73, Ed - KL7UW

From: Harry Yingst via Elecraft <elecraft at mailman.qth.net>
To: Elecraft Reflector <elecraft at mailman.qth.net>
Subject: [Elecraft] Receiving AM on the K3 - Odd behavior
Message-ID:
        <631043647.573239.1422324055017.JavaMail.yahoo at mail.yahoo.com>
Content-Type: text/plain; charset=UTF-8

I noticed something odd when receiving AM.
I have the AM and the FM filter installed and both enabled for AM Receiving.
The manual states:In AM mode, the I.F. bandwidth required for good 
fidelity is about twice the AF bandwidth.?This is why a 6 kHz or 
wider crystal filter is needed to effectively use the 3 kHz 
NORM?setting. If a 15 kHz FM filter is installed, it can be used in 
AM mode to provide good?fidelity at even higher AF bandwidth settings.?

Watching on The P3 as I adjust the Filter width between 1.3 kHz and 3 
kHz it tracks properly but?between 3 kHz and 5 kHz it's really only 
varying between 3 kHz and 4 kHz (with 4 being the max)?3 = 3 , 3.5 = 
3.25, 3.5 = 4, 4.5 = 3.75, 5 = 4
I confirmed this by loading up Spectrogram and looking at the audio 
output and it tracks the same way.
To further test I went in to SSB mode set FC to 2 kHz and BW to 4 kHz 
then moved the shift ?to 2.2 kHz and the output moved to 4.2 kHz and 
the P3 tracked it as well as Spectracom.
I also looked at the Spectracom display in FM Mode with the squelch 
open ?and it appears that the audio?stages could easily reproduce out 
to 6 kHz with little roll off and ?even displayed out to a bit over 18 kHz.

Have others noticed this?


73, Ed - KL7UW
http://www.kl7uw.com
    "Kits made by KL7UW"
Dubus Mag business:
    dubususa at gmail.com

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