*SoftRock Lite I-F Kits Available*

[WA1QHQ]

Stu et al

In going thru the archives on the softrock 40 group on yahoo I believe the question was answered by Tony Parks.

The Power SDR application or Rocky will take the entire audio spectrum below the maximum nyquist frequency set by the sound card +/- the center frequency set by the softrock crystal LO divided by four.

The reason for the offset is to avoid undesirable affects caused by
Ground loops between the softrock and the audio board.
DC offset caused by I/Q unbalance.
AC coupling that prevents DC response to the soundcard which would of course negate the first two items on the list unless the sound card is DC coupled.
LO leak-thru which I think can be nulled with I/Q balance, I could be wrong about this one.

At any rate there seem to be cures for most of these problems and they appear to be minimal in real world testing.

The phase noise issue of the digikey custom Xtal LO is of particular importance if the softrock is to be used for close in phase noise measurements as Frank WA1GFZ has already posted about. I have made critical phase noise comparisons of very good programmable clock generator chips vs Xtal reference oscillators where I multiplied both up to 10GHz to be able to actually see the phase noise above a very good spectrum analyzer's LO noise. The result was about a 20 to 30 db difference between the two. This is something that you would never notice for casual spectrum monitoring or SW receiving but would be a show stopper for narrowband phase noise measurements. I will investigate the digikey solution further.


Mark WA1QHQ

[KF1Z]

that makes the most sense...

If you don't want the "center spike" ( and the very small un-usable bandwidth at the lo freq) to show, you offset, and only use less than half of the bandwidth available to you.

Fact is, even in Softrock40 mode, power SDR does indeed display and demodulate signals above and below the LO freq of the softrock, or any other I/Q "mixer".

[AB2EZ]

Ok

You guys have convinced me.

Stu

[Opcom]

This sounds great, the only issue being the raggety PCs around here.

If I was going to the trouble, I'd want to be sure of a 96KHz capability on the sound card.

The people at the computer store do not know anything for example, if I pick up a used IBM NetVista (1GHz CPU, built in sound for $99), does it have the beans in the sound card?

So maybe I ought to try one and see how I like it. Just don't let the CRT based machinery see the heresy..

[Opcom]

BTW I have been looking for the schematics of this board. Can someone direct me to them please?

[KF1Z]

I had a netvista....
The onboard audio device was not great for SDR.

BUT, the only way you'll know for sure is to try it.
Mine was likely a different board/chipset, as it was a 2ghz pentium 4



The schematics for SR40 lite WERE always available in the files section of the softrock40 yahoo group.

If you can't find them, I can email them to you.

[KF1Z]

Ok

You guys have convinced me.

Stu

I don't believe you're convinced....I bet you just don't want to argue the point Stu!  :-)

Me either, I just wanted to point it out that there's no reason if you have a soundcard capable of 96khz (or 192khz) that you can't use the whole bandwidth available.

sometimes there are circumstances where one only WANTS to use a small portion of the available bandwidth...

Anyhow, even at twice the price the softrock lite was a few years ago, it is a dandy reciever stand alone (40meters and down or up with a preamp) or using it as an IF adapter.

[WA1QHQ]

A lot of guys are using external USB sound cards that are of higher performance than what you would typically find in an off the shelf PC.

Schematics and build instructions including BOMs can be found here...

http://www.wb5rvz.com/sdr/

Mark WA1QHQ

[AB2EZ]

Bruce
Et al.

Yes... I actually am convinced. After all of this discussion and throught, perhaps the easiest way to understand this is to look at the three (3) new attachments below.

You start out with the i.f. passband of the receiver (first attachment)... and this time I am showing positive and negative portions of the receiver's i.f. output spectrum.

You mix the receiver's i.f. output with a local oscillator (shown in the first attachment) to produce the spectrum shown on attachment 2 for each phase (I&Q)

However, through the magic of quadrature outputs and Hilbert transforms, you can (to some extent) null out the purple part of attachment 2... to produce what is shown in attachment 3 (mathematically, not as a real signal)

To the extent that you can null out the purple part, you end up with just the red part. Mathematically it occupies frequencies from -W to +W, where W is one half of the sampling frequency.

On the Softrock display, this appears as a spectrum whose full width is 2W... i.e. the sampling frequency.

Thanks for the mental exercise... and for straighting out my misunderstanding of what the Flex Radio algorithm is doing.

Stu