Hello Stefan,
I've played around with your code and also had a
look into the linux
kernel driver which contains some elonics code [1].
There the LNA gains have an offset of 3dB compared with rtl-sdr.
The absolute gain
is insignificant. The dongle will convert
an RF voltage to A/D converter bits and the number of mV per bit
will change if there is an offset on gain values. I can not see that
one is better than the other.
Those who know the internals of the chip might want to set the scale
offsets correctly because the different stages may actually have
gain or cause attenuation. Such a strategy is not unambigous however.
When the impedance level is changed we have to know whether dB refers
to a power ratio or a voltage ratio.
In the rtl-sdr header is also a constant defined but
not used, to switch
to 5dB gain steps. I'm wondering how this fit's with your observations.
I
have no idea.
I've read your blog entry and would like to
evaluate the gain values of
my device, but it is not clear to me what equipment is needed - I assume
at least a 144 MHz oscillator with defined level.
You just need a signal that is
stable in amplitude. Could be your local
FM station or any signal generator in the range 55 to 1000 MHz. Probably
higher, but I do not have generators above 1 GHz easy at hand so
I have not verified that.
Regarding your changes to the gain setting function,
fear they contain
lots of bugs, but I agree it would be nice to have a function which make
IF-gain setting easier.
Not bugs, but clumsy code. I tried to not change more than
necessary...
The elonics code also contains a more stuff to
configure the auto gain.
Maybe this is interesting for rtl-sdr too.
In general purpose SDR we do not want
any kind of auto gain.
Those who want to use the builtin decoders have a different
situation.
Regards
Leif / SM5BSZ
Regards
Stefan
[1]
https://github.com/tmair/DVB-Realtek-RTL2832U-2.2.2-10tuner-mod_kernel-3.0.…
Am 05.07.2012 13:27, schrieb Leif Asbrink:
I changed the numbers to get a reasonable
agreement with
the gain values I observed on the dongles I have.
I did not care to match the gain setting to the real gain
better than within 3 dB (the gain step size in Linrad.)
The reason is that I assume that the routines from osmocom
will be changed and that other softwares that support
rtl dongles will allow gain setting in a reasonable way and
that the performance will become similar to the current
Linrad performance. When that happens I will rewrite Linrad
to use the library as intended.
I see two alternatives, there could be lna gain as well as IF
gain as separate controls.
The other alternative would be a single gain control, but
in that case both lna and if gain have to be controlled.
The first few gain reduction steps have to be on the IF because
the most common problem is close range interference and the
improved dynamic range is useful. (In case the lna gets blocked
one should install a filter.)
Regards
Leif / SM5BSZ
On Thu, 05 Jul 2012 02:06:31 +0200
la(a)tfc-server.de wrote:
The gain
setting is clumsy
If it only was your gain setting function, but no, even your gain
values
for the lna are just plain wrong.
I just have to ask: Why? What made you "improve" them?