[hpsdr] CASMIR design v0.4 plus introducing COPERNICUS!

[CT1IZU]

----- Original Message -----
From: <pvharman at arach.net.au>
To: "Lyle Johnson" <kk7p at wavecable.com>
Cc: <hpsdr at hpsdr.org>
Sent: Monday, June 12, 2006 2:05 AM
Subject: Re: [hpsdr] CASMIR design v0.4 plus introducing COPERNICUS!


> ***** High Performance Software Defined Radio Discussion List *****
>
> I think we should avoid relays if possible. The latest switches should
have a
> high enough IP so as to not be the limiting factor on the receiver
> performance.   If we do have to use relays then bleed a little current
through
> the contacts to help keep them clean over the long term. The CDG2000 does
this
> along with a number of commercial designs.
>
> In my SDR I use the bandpass filters from the Pic-A-Star design. Each is a
ham
> band wide filter with good shape factor and low insertion loss on the high
> bands. The inductors were hard to find about 18 months ago so I expect we
will
> need to find alternatives. There are also lots of 1% silver mica
capacitors
> that I'm not sure have an SMD alternative. It was a lot of effort to hand
> build the board but the performance is excellent.


Some late comments on this topic.

The Star approach to bandpass filters  is elegant. It uses FST3126 as
switches instead of relays.  2 poles as in/out switch, one as control, AND
ONE TO GROUND THE FILTER IF OFF LINE.

The inductors use standard ready wound TOKO 10K coils.As Phil notes some of
these are possibly now obsolete, a better solution may be the Loadstone
Pacific L45-6 and L41-6 formers provided people are prepared to wind their
own.

G3SBI tested all these inductors in a three section ladder band pass
confirguration with a load Q of 8 for IP3.
TOKO 10K  +37dBm      coilQ 70
L45-6        +47dBm                  125
L41-6        +55dBm                   70
Standard Micrometals/Amidon T50-6 was +50dBm at Q of 125.

I really don't see the nessesity of silver mica in this low level
application. Murata offer a big range of SMD caps in NP0 and N750 up to 100V
working. I have used similar Phillips leaded types in 5W LPF`s and not
noticed any difference

Shel CT1IZU