[Elecraft] Shortwave filters for K3

[Don Wilhelm]

Steve,

The DSP audio bandwidth *can* be moved out to 5 kHz for AM - that is 
true, and it does not depend on the filter you have installed.
With the 6kHz filter at the IF, the bandwidth is constrained by the 6 
kHz roofing filter, so the realizable audio that can be demodulated is 3 
kHz - it makes no difference if the DSP opens the audio width up to 5 
kHz, the 6 kHz filter still restricts the audio to 3 kHz.
Now, install the 12 kHz roofing filter and the full 5 kHz audio width 
allowed by the DSP can be realized.

Study AM modulation for a bit and you should realize that the RF (and 
IF) must pass both sidebands, but the demodulated audio will be only 
half the width of the RF waveform.  The available audio high frequencies 
can be constrained by either restricting the RF (or IF) passband or by 
restricting the audio bandwidth - either one produces the same result.

With your "old shortwave receiver" controlling the IF at 6 kHz  (3 kHz 
of audio) when it begins to sound muffled?  I would think so because 
most of the filtering is done at the IF frequency.  Equate that IF width 
to the roofing filter width and the DSP bandwidth to the audio bandpass 
and you will be correct.

73,
Don W3FPR


Steve Ellington wrote:
> Don
> This doesn't quiet make sense to me. I have the 6khz filter in my K3. The BW 
> indicated on the K3 is 5Khz. If I add the 12 KHZ filter and the K3 still 
> says 5KHZ and that 5KHZ is the "audio" bandwidth, what good have I done? 
> Most any old SW receiver sounds a bit muffled at only 6khz bandwidth. 
> Opening them out to 10 or even 15 KHZ always makes them sound better. Yes I 
> know the BCB stations are limited to 5 Khz but just compare the K3 to a 
> wideband rx and note the difference.
> Steve Ellington
> N4LQ at carolina.rr.com
> ----- Original Message ----- 
> From: "Don Wilhelm" <w3fpr at embarqmail.com>
> To: <n4zi96 at yahoo.com>
> Cc: <elecraft at mailman.qth.net>
> Sent: Sunday, May 03, 2009 2:01 PM
> Subject: Re: [Elecraft] Shortwave filters for K3
>
>
>   
>> Bill,
>>
>> The 5 kHz width indicated for AM is the *audio* bandwidth, not the IF.
>> The IF bandwidth must be more than 2 times the audio bandwidth for an AM
>> signal - so your 12 kHz filter is good.
>>
>> Remember that the 6 kHz filter must be used for AM transmit.
>>
>> AFAIK, most broadcast stations limit their transmitted audio to 5 kHz,
>> so you are not likely to obtain much better fidelity than the K3 DSP
>> currently allows.
>>
>> 73,
>> Don W3FPR
>>
>> Bill Maddock wrote:
>>     
>>> Guys,
>>>
>>>   Just got done assembling my second K3 and this one has a 13 Khz
>>> filter where #1 had 6 Khz - I like to do shortwave listening - thought
>>> the wider filter would give better fidelity, but DSP only goes
>>> 5Khz wide is there a mode for this? I assumed with the 13khz roofer
>>> it would be opened up to 10 khz or more?
>>>
>>> Thanks and 73,
>>>
>>> Bill  N4ZI  K3 #1059 & K3 #2914
>>>
>>>
>>>       
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