[hpsdr] Fw: A real newbie question

[Terry Fox]

I agree with Helmut on this.  The ADC inputs of the RP were designed for using with scope probes, and have a high input impedance.  Even with your Norton amp in front, and the splitter, there is probably some loss due to this situation.  Also, don’t forget that you are looking at a 14-bit ADC (and generic design) vs the QS1R SDR-specific (50 ohm) input circuitry.  I don’t remember offhand which “preamp” chip the QS1R uses in front of the ADC, but I believe Phil Covington did a better SDR design than the RP has.  So, its probably a little more sensitive.  IIRC the RP uses the LTC 6400, or 6405.  The QS1R SHOULD handily win any direct comparisons on receive.  At least unless you plan to use it/them for either a complete transceiver, or for diversity!  Also, the cost comparison is a winner for the RP!

I’m looking at an active preamp and Brainerd BPF in front of the RP, to help it out some on HF.  For LF, it’s probably not going to cut it.  Initial tests about a year ago showed some trash on 470kHz, and way too much garbage at 137kHz.

Still, the RP is a VERY interesting board to play with for around $250.  I’ve bought two now, and am looking at a third.  One will be to play with the bench test equipment stuff for it, and the others will be for SDR purposes.

I’m taking my wife to various doctors after a bad fall and some broken bones, so my time is rather limited at the moment.  But, I’ll let you know how my tests go as I get them done.  I DO plan to test under at least HDSDR, SDR#, GNU Radio, and PowerSDR, to be sure there aren’t many differences.
73, Terry, N4TLF


From: Helmut 
Sent: Wednesday, October 26, 2016 4:50 PM
To: 'Pete Smith N4ZR' ; hpsdr at openhpsdr.org 
Subject: Re: [hpsdr] A real newbie question

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Pete,

 

I guess you know that some modifications of the frontend are necessary to use the RP as an appropriate SDR. Impedance transformation from high to 50 ohms seems helpful as well as sufficient low-pass filtering for the 1. Nyquist zone. The preamp for frequencies > 7 MHz needs a redesign. When you have done this work calibration can be taken as Warren has described. To get best accuracy a high reference (signal source) level should be used to employ as much bits of the ADC as possible (just below clipping).

 

73, Helmut, DC6NY



cut to reduce size....N4TLF
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