[linrad] transverters and homebrew receive chains

[w3sz <comcast.net; w3sz@xxxxxxxxxxxxxxxx>]

Hi, Josh,

Just a brief additional note with some more info. I wrote last night's  
post very late and well past my bedtime, so it was short on practical  
detail to help you get started, which is I think what you want.  I  
apologize if this is too basic or not what you wanted.  If that is the  
case, please let me know.

I would again recommend as I did in my previous email that you start with  
a preamp [tower-mounted] for the band you want [144 MHz, for example],  
followed by a TUF-1 Mixer [for example], an LO at 116 MHz [for example,  
for 144 MHz RF input, of course], and then the SoftRock 7.0 as a 'get  
started' system.  The TUF-1 and the 116 MHz LO will of course convert the  
144 MHz input signal to 28 MHz, suitable for input into the SoftRock 7.0.

I use SSB Electronics Preamps for Terrestrial work on all bands thru 1296,  
and DEM for 2304 and 3456, and then DB6NT for 10 GHz and 24 GHz.  So I can  
speak to the effectiveness of those particular devices.  THEY WORK!
http://www.ssbusa.com/gaasfet.html
http://www.downeastmicrowave.com/
http://www.ssbusa.com/mku23.html
http://www.ssbusa.com/mku24.html
The SSB units have noise figures of 0.8-0.9 dB which is plenty good for  
terrestrial use.
The DEM units are <0.7-0.8 dB.
The DB6NT units are 0.7 dB on 10G and 2.0 dB on 24G.

I have used cavity-input preamps for 144 MHz EME; first KA0RYT design, and  
more recently [past 3 years] LNA Technology
http://www.lnatechnology.com/
These units are spec'd to have NF < 0.2 dB. You don't need that on 144 MHz  
for terrestrial use.

The SSB Electronics Preamps have helical filter inputs and I did not have  
any problems with the image frequency during the several years that I used  
homebrew schemes such as this for both terrestrial and EME reception  
[before I got the WSE boxes].  From 2000-2004 a simple homebrew receiver  
of this type [different IFs and LO's due to my being limited by the parts  
I had on hand] was my 144 MHz EME receiver and it worked great, better  
than any commercial receiver I'd used, including my FT1000MP or Elecraft  
K2 with commercial transverters.  That is because it had an excellent  
preamp with cavity filter in front of it, and Linrad behind it.  While  
this did not use a SoftRock, the point is made that you can do very well  
with a SoftRock and what I am recommending, because the 'rear end' that I  
used was nothing as nice as the SoftRock, and yet I got excellent  
results!  A schematic of this old system [which used conventional mixers,  
not I/Q mixers] is at:
http://www.nitehawk.com/w3sz/w3szdspnew.htm#figure2 .  You actually want  
figure 3, but this URL centers the schematic in my browser window here.   
If not, use
http://www.nitehawk.com/w3sz/w3szdspnew.htm#figure3 .

With most of the iterations I used I did not need additional amplification  
[beyond that provided by the preamp on the tower] before the TUF-1, but  
with some I did need such amplification to optimize overall system  
performance.  If you know the noise figures and gains for the individual  
compnents in your system, you can calculate the expected system  
performance for these values, and you can play with these values and see  
what effect it has on overall system performance.  Balancing system gain  
can pay off nicely with improved performance.

As a non-RF guy I need simple formulas, and so I remember that the overall  
system Noise Temperature is equal to the sum of [the noise temperature of  
each stage divided by the products of the gains of the preceeding  
stages].   So T = T1 + T2/G1 + T3/[G1*G2] + T4/[G1*G2*G3] +  etc.  For  
Noise Factor this becomes F = F1 + [F2-1]/G1 + [F3-1]/[G1*G2]  
+ [F4-1]/[G1*G2*G3] +  etc.  Noise Figure is just 10 * log[F].  Its also  
important to know whether you are sacrificing useful dynamic range with  
your choices.  At this point the calculations get beyond what I want to do  
manually.

Fortunately, the late/great HP had developed an excellent tool called  
AppCAD that has in its main menu a 'noise calc' function that lets you  
input the parameters for your system's various gain [loss] stages and play  
with the variables to see what effect this has on your overall system  
Gain/Noise Figure/Noise Temperature/MDS/IIP3/OIP3 etc.

You can download it from:
http://www.hp.woodshot.com/

It is highly recommended and lots of fun.

I hope that helps!

Please let us know how you make out!

73,

Roger
W3SZ
--
Roger Rehr
W3SZ
http://www.nitehawk.com/w3sz/

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