The AM Forum

Thought this might be of interest to SDR aficionados.
http://pcovington.blogspot.com/2009/02/rffe1-why-you-may-or-may-not-need-it.html (http://pcovington.blogspot.com/2009/02/rffe1-why-you-may-or-may-not-need-it.html)

Philosophical discussion of design; comparison of QS1R with and without the RF front end. (RFFE1), with HDSDR "Mercury" and with "Persus."

Phil's last statement in the post;
 

Rick,
I agree with Phil. my RA6830 has an MDS of around -131 dBM. When I connect  the antenna on 10 meters the background noise increases. I suspect that is good enough unless it is very quiet. QSR1 as it is has about the same MDS. I like the idea of a buffer ahead of the big bucks A/D to protect it. HPSDR Atlas front end is coming and is supposed to have some selectable gain and attenuation gain. I don't know if it is necessary since the HPSDR 500 Hz. MDS is around -136 dBM. The Mercury saturates at -12 dBM when the attenuator is turned off. I measured the dynamic range at 110 dB. I have heard reports of local hams saturating the input but My transmitter only nails it once in a while.
I suppose Phil is trying to decide on his next creation. I wonder who will use the 170 MHz. A/D first. He told me the FPGAs don't have the horsepower so better units would be required. 
 

Of course that, to me, implies the cost of the front-end board will be about $300.

I sure hope my assumtion is incorrect!

Without a preamp, it's dead above 5mhz

Right now, I have a little home-brew wideband preamp, but no filtering.
Was hoping to aquire the "good" pre-amp  / filter / attenuator board.

But not sure ... depends how my guess at the price compares to reality.

you still made a good choice going with the QSR1. Consider a pair of 2N5109s in PP Norton biased at about 50 ma each to hold the same dynamic range.
I would set it for about 10 dB gain. That should be plenty of gain.
Dallas Lankford has a great article on Norton amps

In my relatively quiet RFI and radio environment my QS1R's done well, but as mentioned in earlier posts I've built an attenuator/pre-selector/pre-amp based on the DL2NBU low impedance (5 ohms) series tuned circuits.  Similar circuit showed up later in QSX and also copied by MFJ. My preamp has 20db gain but fairly high 4db noise figure.

Even more than preamplification especially on lower frequencies,  I think attenuation coupled with selectivity is required for many ham applications, e.g., 'field day.'  Even on 20, 17, 15, 12 and 10 attenuation and selectivity is paramount if other transmitters or harmonics thereof from Ham or BC xmitt'rs are present.

Phil's the first manufacturer I've heard of in a long time being honest to a potential customer base..... perhaps this could be interpreted as trying to 'discourage' potential buyers from buying a product.  He reminds me of Steve Wozniak looking for a Jobs  ;D   I suspect he's overwhelmed by the response to his survey where 70% or so wanted RFFE1's immediately.  Soldering those damn surface mount components takes a lot of time and steady hand on small piece work scale.  He's also concious of cost; believe it or not.  Even if relatively expensive, the RFFE1 coupled with the QS1R in the same 4" x 6" by 2" box makes excellent commercial sense.

Attached is pix of my kludged homebrew BC filter slotted just above the QS1R; this is where the RFFE will fit, along with pre-drilled and etched cover plates.  Cathy's also making great headway with the 3rd gen GUI SDRMaxIII.  The GUI will now have to have software buttons for the RFFE control so I suspect that's another reason Phil's being deliberate.

Not unless the mixer is deaf as a brick will you need much preamplification, probably none on the lower freq bands. Take a look at the average noise curves for HF. Over much of it, a 10 or even 20 dB NF is good enough.

Good point.
_{as usual}

What I really need is something to match my 'dumb as a brick' ears. Selective receiption might be a good start  ;D

For those of you who've never used a GUI or those interested in the latest in software defined radio here's a neat video by Cathy Moss, the developer of the as yet beta SDRMaxIII over Phil's (N8VB) QS1R board.

Run it well into the end; she eventually gets from BC freq's into SSB on 80 meters.  A lot of neat features are demonstrated.  BTW SDRMaxII is the current working GUI and works on surprisingly low end computers.  Both these GUI's have both waterfall and 'panadaptor' screens working simultaneously.

http://www.honda296.pwp.blueyonder.co.uk/SDRMAXIII-1.wmv
 (http://www.honda296.pwp.blueyonder.co.uk/SDRMAXIII-1.wmv)

Cathy's off to an alpha version.  Where'd the beta go?  ;D  But, what fun.

Reminder thread; In case you want to join the yahoo group of the QS1R and keep up.
http://groups.yahoo.com/group/qs1r/ (http://groups.yahoo.com/group/qs1r/)