[linrad] Re: Linrad gain programming (hardware side) / Linrad-TX

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

Hi Zaba,

> Linrad is actually a quite remarkable system regarding gain control 
> as it seems
> to be able to cope with most situations without need to change the hardware
> configuration either with attenuators or any AGC-action. At least in 
> my private
> 50 MHz experiments with a simple I/Q-direct receiver I have never come across
> a signal level situation where any change in the basic gain layout was needed.
Good! 

> Just in case more dynamic range is needed in Linrad hardware or other 
> I/Q-systems
> I would like to point to a new product by National Semiconductor that 
> will allow a
> precise adjustment in gain in two or more channels, if needed. The 
> LMP 8100 allows
> 24 dB gain regulation with 0.03% precision. Close gain tracking 
> between channels is
> beneficial in (multiple) I/Q-systems as there will be no need for 
> re-calibration when
> jumping between gain settings. Its noise performance is very similar 
> to fixed gain
> amplifiers and its slew rate is excellent with a 33 MHz unity gain bandwidth.
> 
> http://www.national.com/pf/LM/LMP8100.html   (with a link to .PDF-specs)
Actually I do not think this is needed. There should be no reason to 
change the  gain in the baseband, the amplifiers after the I/Q mixers
should always be optimised for the best possible dynamic range.
In case more dynamic range is needed, the variation should be applied 
earlier in the signal path where accurate phase/amplitude is not critical.

With the WSE converters, one could allow for an attenuator (and/or amplifier
bypass) at 144 MHz for the RX144 but for the RXHFA there is already an
attenuator/bypass that allows -20 to +10 dB gain in 5 dB steps at the
input. Should even more be needed, an attenuator at the antenna input
would be the proper solution.

73

Leif / SM5BSZ

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