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uBITX narrow IF filter mod for CW, anyone tried this?

Frank PA7F
 

Hi folks,

I wonder if anyone tried this mod and are you happy with the result.

As I am customizing my uBITX for CW only I was looking into the possibility to add 220 pF capacitors in parallel to the 100 pF capacitors across the ladder filter. This is Farhan's suggestion in thread #36820.

I also have a NEQRP Nescaf filter ready to be integrated into my uBITX.

73, Frank
PA7F

Alex - PA1FOX
 

Hi Frank,

Don't know if you are referring to a specific mod but I have very good results with a second 4MHz crystal filter parallel to the 12MHz SSB filter

My ubitx is made from scratch though, not based on the commercial kit.
(And I do use modified firmware)

73,

Alex PA1FOX

Frank PA7F
 

Hi Alex,

Thank you for your reply! I think you have a very nice set up there with 4 and 12 MHz IF's!

In order to narrow the 12 MHz IF for CW, Farhan suggested to add 220 pF capacitors across the existing 100 pF capacitors in the 12 MHz IF crystal filter. I had hoped somebody tried this. You described on this group how you narrowed down the 4 MHz IF bandpass in your uBITX by using 68 pF capacitors to achieve a 400 Hz bandwidth. So you reduced the value of those caps to narrow the IF.

I have integrated the NEQRP SCAF-filter (audio filter) in my uBITX, this works good, but not perfect as strong signals are leaking through the filter.

73, Frank
PA7F


QUOTE
12/21/17  
i have been using a straight key for sometime. i have found no glitches in the straight key. i used the iambic keyer just for a few quick QSOs. I am straight key guy, hence the keyer code will need some improvement.
the cw is NOT generated with a side tone injection. The CW is generated by the SI5351 by putting it directly on the tx frequency. For those wishing to have a narrower filter, all you hve to do is to add another 220pf capacitors in parallel to the 100pfs across the ladder filter.
UNQUOTE
 

Rag LB-Three-RE Stein-Roar
 

Hello

I saw one guy had a capacitor across to make 
Narrow for CW.

I also thinking about making a filterboard between cw/ssb.

I bougth a cw/ssb audio filter from sotabeam


Best Regards,
Stein-Roar Brobakken
LB3RE K3RAG 
Skype: lb3re.rag

12. mai 2018 kl. 12:04 skrev Alex - PA1FOX <pa1fox@...>:

Hi Frank,

Don't know if you are referring to a specific mod but I have very good results with a second 4MHz crystal filter parallel to the 12MHz SSB filter

My ubitx is made from scratch though, not based on the commercial kit.
(And I do use modified firmware)

73,

Alex PA1FOX

Mark Kesner
 

I used the same NESCAF filter and am very happy with it.  I have not tried it on contest conditions yet but for casual operation it has given a big improvement.  

Mark
KC4GIA 

ajparent1/kb1gmx
 

Increasing the caps as Farhan pointed out elsewhere would fine but then you should adjust the BFO
to put the signals in the passband.  That mod however makes SSB really poor both RX and TX.

While I've not done this on a bitx because to me its primary SSB radio.  I have made CW filters 
for tests using that technique.  A good example of this however is Kitsandparts 1W, the crystal
filter is optimized for CW and works well.

If one were really wishing to experiment replacing the caps with a smaller value and then putting
Varicap at each point results in an adjustable bandwidth filter [look up Jones Filter].  I've played
with this and it does work well.   It was applied in the Elecraft K2 receiver as a working example.

Allison