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[Linrad] Re: RCF shift
- Subject: [Linrad] Re: RCF shift
- From: Dave Sublette <arrl.net; k4to@xxxxxxxxxxxxxxxx>
- Date: Sat, 11 Apr 2009 10:11:50 -0400
For my SDR-14, I have set my RCF shift to 1. That gives a base line of
+20 in the main spectrum window. I am still lacking enough gain and I
am working on a higher gain preamp to drive the direct input of the
SDR-14. I will keep you informed of my progress.
Leif Asbrink wrote:
> Hi Dave,
>> Is there an optimum RCF shift value when using an SDR-14?
> Yes. It may be critical, but it depends on what you want to do.
> The SDR-IQ is limited by the USB 1.0 link (as well as the SDR-14)
> The maximum dynamic range of your system may be determined by
> the USB link - but it is not necessarily so.
> The digital downconverter, the AD6620 inside the SDR-IQ uses
> 24 bit arithmitics, but with only 16 bits on the USB one has to
> decide whether one wants early saturation for signals within
> the visible passband or loss of weak signals due to rounding
> errors. Full performance is not available.
> For weak signal VHF operators the problem is small because we
> want to have a noise floor that is about 20 dB above the intrinsic
> noise floor of the SDR-IQ itself to preserve the last 0.1 dB of
> system noise figure. On HF bands it may be more difficult.
> In case there may be very strong signals outside the visible
> window one mighr reduce the RCF shift to make quantization noise
> smaller. One might even reduce the sampling speed drastically,
> down to as low as 4 kHz to ensure that all strong signals are
> outside the passband that is sent over the USB link.
> You may test your system by putting a dummy load (or nothing)
> in the SDR-IQ input. Then set rcf shift = 0 and measure
> your noise floor with the S-meter. Try stepping the RCF shift
> upwards and check the noise floor for each value of the shift.
> You should find that 1 unit of shift corresponds to exactly 6
> dB. At some point (shift=5 or so. Depends on your sampling speed)
> you will find that the noise floor drops by more than 6 dB.
> That is the point where the noise voltage is smaller than 1 bit.
> When you do the system check with preamp on/off it is wery important
> that you are well above this level because otherwise you might be
> led to believe that your system is OK while it actually is not.
> One thing is the RCF shift you need to evaluate your system performance.
> Another thing is the value to use while operating. You should use
> a RCF shift that places your noise floor at 20 dB in the main spectrum
> and that should be with a first FFT gain of 1000 (default)
> Leif / SM5BSZ
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