Re: VK4PP uBitx LPF BANK add-on Board

Jerry Gaffke


The relays are not that bad, primary issue on the uBitx is how the board is routed. 

Look at the uBitx schematic, find KT1 near the 30m LPF of L11,12,13.
Assume the relays are in the relaxed state as drawn, for operation at 30m.

The 32dB number is for isolation from KT1 pin 14 to 16.
So when operating at 30mhz, we have 32dB of loss going from pin 14 to pin 16, 
through the 20m filter at L14,15,16, and then another 32dB of loss
going from KT1 pin 1 to 3 at the other side of the filters.  A total of 64dB down
to sneak around the KT1 filter by that path.  The third filter is not a factor for this
since it is an additional 64dB down.  

The roughly 40dB loss in going around the 30m LPF from KT1 pin 12 to 5
is the primary factor that the relays themselves contribute to blow-by,
that's acceptable unless the PA is putting out harmonics that are as strong as the fundamental.
That 40dB path disappears if we use 3 additional relays at the far side of the LPF's.
Blow-by when operating on 30m is the primary consideration,
operating at lower frequencies will have proportionally less capacitive coupling through the relays.

Someone proposed the scheme below a few weeks ago using a total of 4 DPDT relays
which I have named RA, RB, RC, RD.
I forget exactly who brought this up, it came as a web pointer to schematics for a board in production. 
There's blow-by when operating on 30m of 32+32=64dB through RB, RD, and the 20m LPF.
There is also a path of path across RA, RC, and the 40m LPF of 32+32=64dB.
And a path across the RB and RD relay sections of 40+40=80dB through that same 40m LPF.
With all three blow-by paths of 64,64, and 80 dB in parallel, we have a net of about 57dB of blow-by.
Not bad for just 4 relays, and the board routes sort out nicely.

               +----o RB                 RD o----+
               |    o---------[20m]---------o    |
               |                                 |
         o-----+                                 +--------o
---------o RA                                          RC o---------
         o-----+                                 +--------o
               |                                 |
               |                                 |

               |    o---------[40m]---------o    |
               +----o RB                 RD o----+

Jerry, KE7ER

On Fri, Aug 31, 2018 at 06:01 AM, Warren Allgyer wrote:
That 32 dB number is the killer for the uBitx relay scheme. It gets at least 3 dB worse with each relay added to the chain and all four relays are in the chain.

Join to automatically receive all group messages.