Re: System Gain

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

Hi Fletch,

> On a slightly different subject, can you point me to any notes on
> calibrating the Linrad with the IQ?  I have arrives at a fairly
> comfortable set of operating parameters for use with the IQ thanks to
> a note from Leif regarding RCF parameters so now I need to cal
> things.  Any suggestions?

The input is 30 MHz wide (I think)

This means that short pulses that are big enough to reach
the saturation level and turn on the red LED will become
small pulses when the bandwidth is limited to whatever
you selected. This may be perfectly OK, but only if your
pulse source does not emit white noise between the pulses.

A filter of some kind that filters out one or two MHz
will allow a much stronger pulse without turning on the LED
and that will improve the pulse S/N.

You may also use a square-wave generator with limited rise 
time and set the SDR-IQ to perhaps 3 MHz.

This page contains some information that might be helpful:
http://www.sm5bsz.com/linuxdsp/flat/ampcal.htm

With the SDR-IQ there is no calibration for I vs Q balance.
It is already perfect:-)

Actually the calibration for the frequency response should
be superfluous, the entire filter chain is digital and it
is only my laziness that makes the calibration necessary.
The frequency response of the RCF filter is known to Linrad
and the contributions from the CIC2 and CIC5 filters could
be evaluated to make the calibration needless. The user would 
just have to specify what frequency response he likes to 
have on screen.

Now, with current Linrad versions, calibration IS necessary
and I think other things are more interesting than doing
something about it.

73

Leif / SM5BSZ



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