Re: [linrad] A simple front end

["w3sz" <w3sz@xxxxxxxxxxxxxxx]

Hello, All!

Please forgive me if my comments are offbase or just plain dumb.  This is not an area
where I am an expert, but the setup I am going to describe worked well for me on
Linrad from VLF thru 144 MHz EME.  Leif is on vacation this week [I think] or he
would be giving the correct answers.  Here are my thoughts based on Ron's very
interesting query and the excellent responses posted so far.

1.  You don't need a phase shifter to get a 40 KHz bandwidth if you use an mAudio
Delta44 soundcard, or another sound card with 96 KHz sampling.  I suspect you know
this, but I mention it just in case.  This is the route I chose, as I wanted to use
Linrad from VLF to VHF [and have done so] and I felt that maintaining an accurate 90
degree phase shift over that range would be a bit difficult ;)

2.  If you don't need the 90 degree phase shift, it becomes very easy.  You could do
direct conversion.  I chose to do dual conversion so that I could put a bandpass
filter at the first IF, but that is not absolutely necessary.  I have used various
mixers:  TUF-1, TUF-1H, TAK-893 [I think I have that number right; it is from
memory], SBL-1 and all have worked.  The Minicircuits site lists the differences.  I
used AD797 OP amp for the final baseband amplifier, as that has the lowest noise of
any easily available amps I could find.  Details of what I did are now a couple of
years old.  If you haven't found them on my website, let me know and I can point you
there [ http://www.qsl.net/w3sz/FAQ.htm#5a ].  At various times I used 1st IF's of
10.7 and 40 MHz, as I had a variety of filters I wanted to try, with varying
bandwidths.  You could use an ERA-5 for the first IF amp; I used a discrete
transistor.  Leif has pointed out that the gain distribution of the system is very
important.  I re-discovered this with his help and really improved the performance of
my system by setting the gains properly.  He has posted all of this to the list I
think.

I bought 2 R2PRO kits when they first came out at Dayton about 15 months ago,
thinking I would try them with Linrad.  I have not built them as I was not sure how I
was going to get 90 KHz bandwidth out of them.  My simple understanding was that it
would be very difficult to get a 90 KHz-wide signal to feed into the soundcard and
maintain the proper phase relationships across the 90 KHz bandwidth.  I suspect there
is an easy way to do it that I was too dense to see, and look forward to hearing from
Pierre ON5GN how to do it.  Then I will have another project to build.  Note that it
is not the RF bandwidth we are speaking of here.  It is the audio phase-shift network
and diplexer that I thought would not have the necessary bandwidth.

My recommendation would be, for 20 MHz CW;  

either direct conversion, which I haven't tried, or dual conversion with:


RF input to 20 MHz bandpass filter [3 coils and 2 capacitors] to

RF Preamp.  Homebrew or MAR or ERA or something like a Cougar or Q-Bit amplifier to

TUF-1 or TUF-1H or TUF-3 or TAK-893 depending on how much LO drive you can muster.
LO could be either 24.7 or 3.3 MHz for 10.7 MHZ first IF.  to

10.7 MHZ, 50 KHz bandwidth surplus filter for 1st IF filter to

homebrew discrete amp or MAR or ERA type device for 1st IF amp to

TUF-1H 2nd IF mixer with
10.7 MHZ LO [exact LO depends upon your 1st IF filter width] to

AD-797 Baseband amp to

96 KHZ sampling rate soundcard.

I know this is very crude and simple but it worked very well for me and the parts
were available in my limited junkbox and at hamfests [ and eBay too].

A direct conversion receiver has fewer parts, but I was more confident that this
design was relatively 'fool-proof' or at least 'fool-resistant', which was what I
needed ;)

Hope that helps some.  Comments welcome.
I apologize if this is wrong or just too obvious to be of any help.

I also enjoyed very much playing with the ESS Time-Machine, as Pierre mentioned. 
There is no quicker way to very quickly be playing with Linrad ;)

73,

W3SZ
Roger Rehr


----- Original Message Follows -----
> I am keen to experiment with the "very easy way to get 40 kHz of 
> radio spectrum into a PC" that Leif described in Fig 2 of the Jan/Feb 
> 2003 QEX. My interest is only the cw end of 20 meters
> For an RF stage I plan to use a Mini-Circuits ERA-5 amplifier with a 
> band-pass filter ahead of it. Instead of the quarter wave of coax the 
> SPH-16 voltage controlled phase shifter looks like the way to go 
> unless I can find a cheaper solution. I looked for low noise audio 
> ICs that were not surface mount and the best I can find is the LM833. 
> Initially I will use a signal generator for the LO but a crystal osc 
> would make it a self-contained front end that could be packaged in 
> quite a small box.
> I would like to share work of this kind with others, either via the 
> list or direct.
> -- 
> 73
> Ron
> W6WO


73,
Roger Rehr
W3SZ
FN20ah
http://www.qsl.net/w3sz




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