When to use the preamps on the KX2/KX3 vs. K3/K3S (Wayne Burdick)

[WB6DJI]

Wayne,
   Have a KX3 here and use the Rx I/Q Ports with external SDR
receiver and applications need I/Q outputs with an HF 3dB splitter with a DXE RTR-1A
DXE box with Protection from overload Rx protection


Noticed poor sensitivity due to the High NF when taking output from Rx I/Q at the
Rx I/Q has great amplitude & Phase Balance ( outstanding )


Noted that to increase sensitivity in my Go Box had to add
a Clifton Labs HF 11dB High IP2/High Ip3 LNA preamp to utilize the
Rx I/Q Ports on super weak Signals.


Found adding an External  Pre-Amp was necessary and with Pre-Amp On in the KX3 for Bands on
20meters and Higher was necessary to get enough sensitivity for my necessary Rx I/Q
applications and 3rd Party SDR Synced Receiver being driven by the KX3 and
WIN4K3 software


73 Mike





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[Bob McGraw - K4TAX]

One of the topics I've discussed with with noted engineers has to do
with use of a Preamp and use of the Attenuator and use of the RF Gain
control.   In general the optimum legacy receiver performance occurs
when the receiver noise floor is some 10 db below band noise.  Being
such,  how does one attain this?  If the no signal band noise is S-5 or
-97 dBm, and receiver noise floor is -133 dBm {i.e K3S}, this says one
should use a combination to total  26 dB Attenuation and RF Gain
reduction.   Certainly one would NOT want to have a Preamp engaged under
these conditions. Likewise, if the band noise is -121 dBm or about S-1
and the receiver noise is -138 dBm  {Preamp engaged} one might wish to
reduce the RF Gain a bit.

 From experience and my observing other hams operations, many seem to
think more gain is needed to hear or pull out weak signals from the
noise.   I find the proper use of the Preamp and specially the correct
amount of  Attenuation and RF Gain reduction, weak signals seem to pop
out of the noise.

73

Bob, K4TAX



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[K9MA]

The problem with simply reducing the gain is that the AGC may become
ineffective, forcing the operator to increase audio gain on weak
signals.  Then the strong signals can be loud enough to be
uncomfortable, or even cause hearing damage.  I like to keep the gain
high enough to get some AGC action on the weakest signals, and reduce
"RF Gain" when necessary to distinguish stronger signals from weaker
ones. "RF Gain" in modern receivers really just sets the maximum gain
the AGC will allow, so reducing it only attenuates weaker signals.

That said, turning off the preamp and/or enabling the attenuator should
be necessary only if strong signals are causing overload. Of course, if
the receiver has enough gain for a low enough noise floor and good AGC
operation on weak signals with the preamp off, there's no advantage in
using it.

73,
Scott K9MA

On 5/23/2018 08:07, Bob McGraw K4TAX wrote:

> One of the topics I've discussed with with noted engineers has to do
> with use of a Preamp and use of the Attenuator and use of the RF Gain
> control.   In general the optimum legacy receiver performance occurs
> when the receiver noise floor is some 10 db below band noise.  Being
> such,  how does one attain this?  If the no signal band noise is S-5
> or -97 dBm, and receiver noise floor is -133 dBm {i.e K3S}, this says
> one should use a combination to total  26 dB Attenuation and RF Gain
> reduction.   Certainly one would NOT want to have a Preamp engaged
> under these conditions. Likewise, if the band noise is -121 dBm or
> about S-1 and the receiver noise is -138 dBm  {Preamp engaged} one
> might wish to reduce the RF Gain a bit.
>
> From experience and my observing other hams operations, many seem to
> think more gain is needed to hear or pull out weak signals from the
> noise.   I find the proper use of the Preamp and specially the correct
> amount of  Attenuation and RF Gain reduction, weak signals seem to pop
> out of the noise.
>
> 73
>
> Bob, K4TAX
>
>
>
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--
Scott  K9MA

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[alorona]

Scott,

If the weakest signals are already activating the AGC, that's the exact condition that leads to the many 'my K3 is noisy' complaints that we see here. Plus, that tends to make all signals sound like they're the same strength, another complaint we've heard in the past.

In fact, you don't want weak signals activating AGC. You want strong signals, above a certain threshhold (AGC THR on the K3) to activate it.

This gives you a nice, quiet linear region where weak signals sound weak, and stronger signals sound stronger.


In addition, overload should not determine whether you turn off the preamp! That's way bad advice.

Here's the general principle: you should only use as much gain as you need. AF as well as RF. Less is usually better. Your ears will not be blown by reducing your receiver gain.

R,

Al  W6LX
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[Lyle Johnson]

In the K3/K3S/KX2/KX3, "RF Gain" adjusts receive path gain ahead of the
AGC detector while "AF Gain" adjusts the receive path gain AFTER the
AGC, if any, is applied.


73,

Lyle KK7P

> ... "RF Gain" in modern receivers really just sets the maximum gain
> the AGC will allow, so reducing it only attenuates weaker signals.

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[Don Wilhelm]

There are guidelines on my website www.w3fpr.com for how to set the
K3/K3S AGC Threshold and Slope for best results.
See the "Noisy K3" article.

73,
Don W3FPR

On 5/23/2018 4:37 PM, Al Lorona wrote:
> Scott,
>
> If the weakest signals are already activating the AGC, that's the exact condition that leads to the many 'my K3 is noisy' complaints that we see here. Plus, that tends to make all signals sound like they're the same strength, another complaint we've heard in the past.
>
> In fact, you don't want weak signals activating AGC. You want strong signals, above a certain threshhold (AGC THR on the K3) to activate it.
>
> This gives you a nice, quiet linear region where weak signals sound weak, and stronger signals sound stronger.
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[K9MA]

I make a deliberate choice to set the AGC threshold low, so I DO get AGC
action on weak signals. Perhaps my ears just don't have the dynamic
range they used to, but I find otherwise the strong signals are just
uncomfortably loud.  Yes, I have to ride the RF gain to distinguish
signals of different strengths, but I find that a fair trade-off.  Give
the average age of hams these days, I doubt I'm the only one with such
issues.  Of course, younger operators may want to treat their ears
gently, to preserve that good dynamic range.

Note that high acoustic background noise can further reduce the
difference between audible and painful.  The Radiosport headset is the
best remedy I've found for that, short of putting the amplifier in
another room.

73,
Scott K9MA


On 5/23/2018 15:37, Al Lorona wrote:

> Scott,
>
> If the weakest signals are already activating the AGC, that's the exact condition that leads to the many 'my K3 is noisy' complaints that we see here. Plus, that tends to make all signals sound like they're the same strength, another complaint we've heard in the past.
>
> In fact, you don't want weak signals activating AGC. You want strong signals, above a certain threshhold (AGC THR on the K3) to activate it.
>
> This gives you a nice, quiet linear region where weak signals sound weak, and stronger signals sound stronger.
>
>
> In addition, overload should not determine whether you turn off the preamp! That's way bad advice.
>
> Here's the general principle: you should only use as much gain as you need. AF as well as RF. Less is usually better. Your ears will not be blown by reducing your receiver gain.
>
> R,
>
> Al  W6LX

--
Scott  K9MA

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