"K3 test w/o & w I7SWX 2T FSA3157 H-Mode Mixer”

[ik8tng]

The Elecraft K3 is one of the best performance commercial transceivers available to amateur radio operators.
While discussing with some friends it's receiver performances we decided to see if there were chances to improve it.  When a receiver has a dynamic range over 90dB it is not so easy to pull-out additional dBs.
One easier mod to experiment was the replacement of the 1st mixer.
 
This stage is located in a plugged PCB unit, avoiding the usual cut and solder when implementing mods in commercial equipment.

A volunteer K3 owner was found and we implemented the 1st mixer replacement using the I7SWX 2T H-Mode Mixer with FSA3157.A test was made with the original K3 configuration and with the I7SWX H-Mode Mixer replacement.
K3 original configuration: preRF (OFF) preIF (OFF)  BW (2,4kHz)
Rx Eq Flat

                                                 PRE-AMP OFF      
                           MDS (Sensitivity)      -126 dBm        

IMD MEASUREMENT WITH PRE-AMP OFF.
                            Two Tones Spacing     5 kHz      
              DR (2 tone Dynamic Range)    98.0 dB  
                  Third Order Intercept (IP3)    21.0 dBm  

K3 with I7SWX 2T H-Mode Mixer as 1st Mixer
                                                 PRE-AMP OFF      
                           MDS (Sensitivity)      -128 dBm        
                                 
IMD MEASUREMENT WITH PRE-AMP OFF.
                            Two Tones Spacing     5 kHz      
              DR (2 tone Dynamic Range)    101.0 dB  
                  Third Order Intercept (IP3)     23.5 dBm

The Audio Noise at loudspeaker output dropped of circa 6dB  

It is important to consider these numbers as relative and not absolute, although the equipment used for the test were not well calibrated, the mod presents a benefit of around 3dB on dynamic range, from 98 to 101dB at 5 kHz spacing, giving also a sensitivity increase of circa 2dB and an appreciable drop in audio noise.

Another interesting test was done with a Panoramic unit connected to the K3.
The antenna was connected to input 1 and the selection switched to input 2.
A screen copy of this test is reported in Files “K3 test w/o & w I7SWX 2T FSA3157 H-Mode Mixer”.
The bottom part is with K3 original configuration. No signals are visible and mainly front-end noise is seen.
The top part is with the I7SWX 2T H-Mode Mixer. You can see signals leaking through the antenna switch capacity and some noise reduction.

A limited number of K3 H-Mode Mixer kit is being made available to experiment this change.
For information you may contact Gian, I7SWX, at i7swxATyahoo.com.
The PCB view is reported in Files with the Panoramic file.

I hope you find this information interesting.

73

Sergio, IK8TNG
K3_test_wo_%26_w_I7SWX_2T_FSA3157_H-Mode_Mixer.pdf

[wayne burdick]

Hi Sergio,

Thanks for reporting these interesting results.

A few K3 users in extremely high-signal environments might benefit  
from a few extra dB of dynamic range, at the cost of a mixer add-on  
module like you described.

However, I'd be concerned about the possibility of additional mixer  
spurs. Switching mixers put out a large number of VHF/UHF spurs that  
can mix with harmonics of other signal sources (primarily the VFO) to  
produce HF-6 meter spurs. This is an inherent issue with high-level  
superhet receiver designs.

Our first mixer design trades about 1 dB of dynamic range and  
conversion loss for excellent rejection of mixer products above 60 MHz  
by integrating a balanced low-pass filter into the commutating path.  
Without this, there would be a "picket fence" of additional VHF/UHF  
products and thus many more spurs.

Please let me know if you do a comparison test of the two mixers in  
this regard. The best way to do it is to slowly sweep through each ham  
band in CW mode using a narrow crystal filter.

73,
Wayne
N6KR


On Sep 20, 2011, at 9:16 AM, ik8tng wrote:

> The *Elecraft K3 *is one of the best performance commercial  
> transceivers
> available to amateur radio operators.
> While discussing with some friends it's receiver performances we  
> decided to
> see if there were chances to improve it.  When a receiver has a  
> dynamic
> range over 90dB it is not so easy to pull-out additional dBs.
> One easier mod to experiment was the replacement of the 1st mixer....
______________________________________________________________
Elecraft mailing list
Home: http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:[hidden email]

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html

[ik8tng]

Hi Wayne,
Thanks very much for the prompt comments, very much appreciated. I have discussed your points with Gian, I7SWX.
Hi Sergio,

Thanks for reporting these interesting results.

A few K3 users in extremely high-signal environments might benefit  
from a few extra dB of dynamic range, at the cost of a mixer add-on  
module like you described.


This is one of the reasons why we tried the mod and also to see how much a good K3 could be improved, we are homebrewers… hi

However, I'd be concerned about the possibility of additional mixer  
spurs. Switching mixers put out a large number of VHF/UHF spurs that  
can mix with harmonics of other signal sources (primarily the VFO) to  
produce HF-6 meter spurs. This is an inherent issue with high-level  
superhet receiver designs.
Our first mixer design trades about 1 dB of dynamic range and  
conversion loss for excellent rejection of mixer products above 60 MHz  
by integrating a balanced low-pass filter into the commutating path.  
Without this, there would be a "picket fence" of additional VHF/UHF  
products and thus many more spurs.


This is a very interesting note. We have not thought about VHF-UHF additional products and spurs. The H-Mode Mixer has the mixing function done by the transformers and the FST/FSA Switches are not involved in the signal path. The 2T H-MM has an RF-LO isolation of >50dB, average 60/70dB, while the RF-IF isolation is >60dB. We believe that the Band Pass Filters in the K3 are good enough to attenuate strong signals outside their bandwidth.

Please let me know if you do a comparison test of the two mixers in  
this regard. The best way to do it is to slowly sweep through each ham  
band in CW mode using a narrow crystal filter.


We will see to manage some comparison with the two mixers. we have to see about the filter.
For some infos on the 2T H-Mode Mixer you may visit  http://martein.home.xs4all.nl/pa3ake/hmode/2t-hmode.html but probably you do know the site.
Thanks again for the comments.
Best 73
Sergio, IK8TNG


73,
Wayne
N6KR

[ik8tng]