[TenTec] 2nd Try: SDR Technology ?

Carl Moreschi n4py3 at earthlink.net
Sat Dec 27 12:22:18 EST 2014

Let me take a shot at this.

There are basically two types of SDR panadapter receivers.

1) Direct converison receivers.  These use the same method as the old 
Tentec Century 21 and convert RF directly to audio and then digitize it. 
  The audio image is removed by the phasing method like the very old 
original SSB transmitters.  The disadvantage to these radios is you 
might hear the oscillator running and this might appear on the display. 
  Normally they run the oscillator with a 10 khz offset to get it away 
form the signal of interest.  The other disadvantage of this type of 
receiver is it will use your sound card.  The stock sound card in most 
computers only allows 48 khz of spectrum.  You can get a more expensive 
sound card to put in your computer that can increase the spectrum to 192 
khz.  Examples of this type are all the Softrock receives.  All the Flex 
radios prior to the 6000 series use this method.

2) Direct sampling receivers.  These radios convert the RF signal at the 
antenna directly to digital.  They do not use your computer sound card. 
  They are normally very sensitive.  They usually can display many mhz 
of spectrum.  Examples of these receivers are the Tapr Hermes, Flex 6000 
series, Elad FDM-S2, Microtelecom Perseus.

Clearly the direct sampling receivers are better but they also cost a 
lot more. And if the direct conversion radios are done correctly, they 
can also work very well.

Carl Moreschi N4PY
58 Hogwood Rd
Louisburg, NC 27549

On 12/27/2014 5:35 AM, Rick - DJ0IP / NJ0IP wrote:
> First pass brought a lot of suggestions for SDR models, and I have an exact plan about how I want to attach the receiver to my Eagle, and manage it with N4PY software.  I have a short list of 2 receivers.  I may possibly be able to borrow a Flex 1500 for a couple of months and use it as a receiver.
> However, my fundamental question did not get answered, in fact not even really addressed.
> "What technology differences do we compare when evaluating SDR receivers?"
> (Let's stick with the low cost - under $300 -  for this thread please.)
> Basically all of the SDR radios are sensitive enough.
> I guess, at least for the low cost models, the selectivity is determined by the sound card.
> I want to compare radios that have built in sound cards because I don't want lots of wires running to my laptop.
> But what do I look at?
> Do we compare sound card chips in this case?
> If so, what specs of the sound card chip are important?
> What else?
>> From what I have been able to find on the web, all of these low cost models have a close in BDR3 ranging from about 65 to 75dB.  Not great but acceptable for daily use, at least in SSB.  Better receivers go for $500 or more.  No surprises there.
> For my initial project, I will just use some simple stuff, but I'm curious to learn more about this technology.
> It won't be too much longer before the theory we all learned is school is not even used anymore.
> BTW, this is one of the best pages I have found so far on this topic:
> http://www.rtl-sdr.com/roundup-software-defined-radios/
> 73 - Rick, DJ0IP
> (Nr. Frankfurt am Main)
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