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K3 Sync AM receive freq. response

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Grant Youngman

K3 Sync AM receive freq. response

One of the things I’ve observed in sync AM receive, is the difference in AF response between LSB and USB (selected by the shift control).

The LSB side has significantly better bass response than the USB side. Likewise, USB has more highs and very little low end. It’s much like the response that occurs when the LSB/USB oscillator carrier frequencies are offset from their correct position relative to filter edges. Virtually anyone who has ever owned a KWM-2 or 75S-3x would understand what I’m talking about. The difference is apparent whether the sync detector is locked on a carrier or not.

Wonder if anyone can comment on this (Eric?). It occurs with either the 13KHZ or 6KHz filters.

Grant NQ5T

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Guy Olinger K2AV

Re: K3 Sync AM receive freq. response

The appearance is that the filter being used for RX does not have its
offset set for the center of actual measured filter band pass edges.

73, Guy K2AV

On Sunday, March 22, 2015, GRANT YOUNGMAN <[hidden email]> wrote:

> One of the things I’ve observed in sync AM receive, is the difference in
> AF response between LSB and USB (selected by the shift control).
>
> The LSB side has significantly better bass response than the USB side.
> Likewise, USB has more highs and very little low end. It’s much like the
> response that occurs when the LSB/USB oscillator carrier frequencies are
> offset from their correct position relative to filter edges. Virtually
> anyone who has ever owned a KWM-2 or 75S-3x would understand what I’m
> talking about. The difference is apparent whether the sync detector is
> locked on a carrier or not.
>
> Wonder if anyone can comment on this (Eric?). It occurs with either the
> 13KHZ or 6KHz filters.
>
> Grant NQ5T
>
>
> ______________________________________________________________
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Grant Youngman

Re: K3 Sync AM receive freq. response

That could be. But the DSP bandwidth (10 kHz or 5 kHz in sync mode) is sitting in the middle of a 13 kHz filter. There's a lot of room for slop before the filter edges should come into play ...

Grant NQ5T

Sent from my iPhone

> On Mar 22, 2015, at 3:35 PM, Guy Olinger K2AV <[hidden email]> wrote:
>
> The appearance is that the filter being used for RX does not have its offset set for the center of actual measured filter band pass edges.
>
> 73, Guy K2AV
>
>>
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Don Wilhelm-4

Re: K3 Sync AM receive freq. response

Grant,

Synchronous AM reception is rather like SSB, so the edges of the filter
are significant rather than the center. For non-synch AM, the center is
significant as you state.
The filter slope on the low frequency side will not be the same as on
the high frequency side.
Even though the 13kHz filter theoretically has zero offset, you may find
a better balance for the bass response by experimenting a bit with the
offset value for that filter.

73,
Don W3FPR

On 3/22/2015 4:50 PM, Grant Youngman wrote:
> That could be. But the DSP bandwidth (10 kHz or 5 kHz in sync mode) is sitting in the middle of a 13 kHz filter. There's a lot of room for slop before the filter edges should come into play ...
>
>

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Kenneth Christiansen

Re: K3 Sync AM receive freq. response

In reply to this post by Grant Youngman

Hi to the gang.

I also noticed this uneven AM response on the 160 meter AM net this morning on my K3 457.

73

Ken W0CZ w0cz at i29 dot net

Sent from my iPad

> On Mar 22, 2015, at 2:26 PM, GRANT YOUNGMAN <[hidden email]> wrote:
>
> One of the things I’ve observed in sync AM receive, is the difference in AF response between LSB and USB (selected by the shift control).
>
> The LSB side has significantly better bass response than the USB side. Likewise, USB has more highs and very little low end. It’s much like the response that occurs when the LSB/USB oscillator carrier frequencies are offset from their correct position relative to filter edges. Virtually anyone who has ever owned a KWM-2 or 75S-3x would understand what I’m talking about. The difference is apparent whether the sync detector is locked on a carrier or not.
>
> Wonder if anyone can comment on this (Eric?). It occurs with either the 13KHZ or 6KHz filters.
>
> Grant NQ5T
>
>
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
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Grant Youngman

Re: K3 Sync AM receive freq. response

In reply to this post by Don Wilhelm-4

That's something I plan to experiment with ...
but the 13 KHz filter is considerably wider than the 5 KHz DSP bandwidth in sync mode.

For what it's worth, the identical effect occurs with the 6 and 2.8 KHz filters also.

Grant NQ5T

Sent from my iPhone

> On Mar 22, 2015, at 8:25 PM, Don Wilhelm <[hidden email]> wrote:
>
> Grant,
>
> Synchronous AM reception is rather like SSB, so the edges of the filter are significant rather than the center. For non-synch AM, the center is significant as you state.
> The filter slope on the low frequency side will not be the same as on the high frequency side.
> Even though the 13kHz filter theoretically has zero offset, you may find a better balance for the bass response by experimenting a bit with the offset value for that filter.
>
> 73,
> Don W3FPR
>
>> On 3/22/2015 4:50 PM, Grant Youngman wrote:
>> That could be. But the DSP bandwidth (10 kHz or 5 kHz in sync mode) is sitting in the middle of a 13 kHz filter. There's a lot of room for slop before the filter edges should come into play ...
>

Joe Subich, W4TV-4

Re: K3 Sync AM receive freq. response

In reply to this post by Don Wilhelm-4

On 2015-03-22 9:25 PM, Don Wilhelm wrote:
> Even though the 13kHz filter theoretically has zero offset, you may
> find a better balance for the bass response by experimenting a bit
> with the offset value for that filter.

The 13 KHz filter has more than adequate bandwidth for Sync (USB/LSB)
AM. Even if it is off by as much as 2 KHz the bandwidth is enough to
pass the full 4.5 KHz recovered bandwidth. The upper/lower edge is
determined by the DSP characteristic, not the crystal filter bandwidth.

Looking at an audio spectrum display with "hold" it's fairly clear to
see there is roughly a 50 Hz difference in the lower frequency limit
between "LSB" and "USB" in Sync detection while by ear there is a
qualitative difference ("more open") high end in "USB". There is no
perceptible difference when moving the FM filter offset from limit
to limit. The difference persists even when forcing use of the 2.8
KHz crystal filter. It's purely an issue in the DSP function.

73,

... Joe, W4TV

On 2015-03-22 9:25 PM, Don Wilhelm wrote:
> Grant,
>
> Synchronous AM reception is rather like SSB, so the edges of the filter
> are significant rather than the center. For non-synch AM, the center is
> significant as you state.
> The filter slope on the low frequency side will not be the same as on
> the high frequency side.

>
> 73,
> Don W3FPR
>
> On 3/22/2015 4:50 PM, Grant Youngman wrote:
>> That could be. But the DSP bandwidth (10 kHz or 5 kHz in sync mode) is
>> sitting in the middle of a 13 kHz filter. There's a lot of room for
>> slop before the filter edges should come into play ...
>>
>>
>
> ______________________________________________________________
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>
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>