[Laser] discrete vs op amp sensitivity

[glennt at charter.net]

Hi all!

Note that there are some outboard soundcards with performance that exceeds even the Delta 44/66 inboard cards. For example, the Roland EDIROL box interfaces with the computer via a FireWire port and provides 24 bit sampling as good as or better than the Deltas. The laser crowd may not need it for low bandwidth operations, but the EDIROL also provides the option of a 196 KHz sample rate, as well as the 96 KHz, 48 KHz and 44 Khz rates on the Deltas.

I suspect that the "another mailing list" Art references is the SDR-1000 group. One might have access to the laser baseband with AM/FM/SSB/CW/digital modes by using the SDR-1000 (open source) software and an optical receiver that provides both I and Q from baseband. I and Q should be easy to generate via two receivers and some means of doing an optical quarter wave shift between them. Alternatively, one might just use the SDR-1000 hardware to transceiver on the laser baseband, though I don't know how good the SDR mixer is at low audio frequencies...

73 de Glenn wb6w

To: <laser at mailman.qth.net>
Subject: [Laser] discrete vs op amp sensitivity

Hi all,

I followed the recent discussion about the relative performance of the discrete type of front end (vs op amp based) very closely. I'm not sure we ever resolved whether front ends like the PGP front ends provide better performance than the conventional op amp front end.

But, I did want to let the group know that I ran across so me additional information on this topic in another mailing list and that I'm following that discussion as well. It seems the BAD thing about a conventional op amp front end is that the resistor that determines the bandwidth/gain also contributes noise because this resistor is large and because it is in the gain determining path of the circuit. This is a problem that the PGP front end doesn't have, whether it tips the scale in favor of a PGP type discrete FET front end or not is still a big question.

The same group has also been evaluating sound card performance and the discussion has been very enlightening. It seems not all sound cards are created equal or even remotely equal. Many have high noise levels that might render them incapable of receiving a very weak signal. The group has also measures the MDS of many types of sound cards and found that some high end 24 bit (vs 16 bit) sound cards allow much weaker signals to be received.

I don't know how many are actually using a computer sound card for a laser receiver....but for those that do, be aware that a bargain basement sound card can definitely degrade the ability to decode weak signals. Cost is not an absolute measure of performance, the very expensive Sound Blaster Audigy 24 bit sound card performs no better than many bargain basement 16 bit sound cards. For all out performance, nothing equals the performance of the Delta 44 and Delta 66 (both 24 bit) sound cards.

I am following the ongoing discussion of both of these topics and will report to the group as more information becomes available.

Regards,

Art