[Linrad] Re: RCF shift

[Leif Asbrink <sm5bsz.com; leif@xxxxxxxxxxxxxxxx>]

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)

  73

 Leif / SM5BSZ

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