[Linrad] Re: Progress

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

Hi Dave,

> I have added 10 dB of amplification to the direct input of the SDR-14 
> for a total of 25 dB now. When I disconnect the 144 mhz signal from the 
> input to the SDR-14, the main spectrum base line now drops 20 dB, I 
> believe this tells me that I have enough gain ahead of the SDR-14 to 
> have proper operation.
Hmmm, not quite. What happens when you switch off the mast mounted
preamp? (The 10 dB amplifier could be a noise generator.) You can
not be sure on the whole system until you verify that the noise you
hear is dominated by the antenna and first amplifier.

There is of course no guarantee that all is ok by this test, but
as long as you trust the preamp to have a low noise figure, you
can be syre.

> I experimented with RCF shift and found that a value of 4 gives a base 
> line of +20 dB in the main spectrum display. If I read everything 
> correctly, I believe this is the desired level.
On the linrad home page:
http://www.sm5bsz.com/linuxdsp/linrad.htm
there is a section: "Dynamic range considerations"
with a link to
http://www.sm5bsz.com/linuxdsp/install/dlevel.htm

Since MAP65-IQ expects 16 bit data you have to run the backwards
FFT (from fft1 to timf2) with 16 bit arithmetics.
Rounding happens at the fft1 output and you have to set the
signal level for the noise floor in each bin of fft1 to be well
above the quantization noise that the rounding from float to integer
leads to. Press 'A' and read the value after "Floor" it is given
in bits at the fft1 output. Never allow floor to go below 3.
(That corresponds to a level of about 13 dB in the main spectrum.)

Under normal circumstances, place "Floor" between 6 and 9.
(It is in units of bits in linear scale) The noise floor in the 
main spectrum will then be around 20 dB.

Note that the SDR-IQ is a 16 bit device. The output shift it does
is a rounding and you have to be sure that the associated quantization
noise is very small.

When you find 20 dB noise floor with an output shift of
4 all is ok on the input side of fft1 (the SDR-IQ output)
ONLY if the first FFT amplitude is 1000 (or more)

In case you increase the output shift by 2 units and then
compensate by making the first FFT gain 4000 you will still
see the noise floor at 20 dB and a good "Floor" value at
the output of fft1, but you would have lost 2 bits at the
input and then the quantization noise caused by the output
shift in the SDR-IQ would no longer be negligible.

There is one more place where one has to worry about rounding
errors when using MAP65 and that is at the output of the
backwards fft1, the timf2 function. The noise floor in timf2 should
be about 20 dB above 1 bit. That is the red line in the horizontal
bar in the high resolution graph. There is nothing wrong in setting 
the noise floor higher as long as timf2 does not saturate.
(Check by pressing 'A' that there is a margin.)

The links above give more details.



> The sensitivity of the LINRAD now appears to be very close to the same 
> as the K2 receiver that I use to copy by ear. I can see signals on the 
> waterfall as weak as I can hear them in the K2.
Yes. I expect you to find that the antenna mounted preamp now
raises the noise floor by at least 15 dB:-)

> However, the audio demod from the baseband through the speaker is at 
> least 10 dB less sensitive than the K2. In other words, I can copy 
> signals on the K2 by ear and can see them, but they are not audible in 
> the speaker of the computer, nor is there a visual indication in the 
> baseband window. I am sure that the BFO tuning is identical on both 
> because on stronger signals, the CW tone is the same. I use about 450 hz.
> 
> I think maybe this is due to not having a narrow enough filter setup in 
> the LINRAD parameters. I am still struggling to understand all of the 
> various FFT's and multiplying factors.
OK. You have little vertical yellow bars in the baseband graph to set 
the filter bandwidth. There is also "First mixer bandwidth reduction"
that sets the total span of the baseband as a fraction of the input
bandwidth. 

> But, progress is progress and I am pleased to have gotten this far.
Good:-)

I hope I will read from you soon that you can now copy better with 
Linrad than with the K3;-)
(But you will need earphones to use Coh modes.)

73

Leif / SM5BSZ


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