[aprssig] Building a new digi (RX performance?)

Scott Miller scott at opentrac.org
Thu Apr 2 01:21:44 EDT 2009

I've tested the T2-301 with weak signals and I've been very happy with 
the performance.  I'd have to check my notes, but I'm almost certain I 
had it down to at least -116 dBm.  I'll check at the office tomorrow, 
and I'll do the same test on my TH-D7A for comparison.

Keith mentioned the T2-135, and honestly the 135 (at least prior to PCB 
v1.2) has had the most trouble with receive sensitivity.  The new v1.2 
board has improved filtering and so far seems to be a lot more 
consistent.  Overall performance is improved, but more importantly, so 
far I haven't run into any poorly performing units.  I thought at first 
that it might be variation in the 2211 chip causing some boards to have 
weak RX, but the variability doesn't show up in the other T2 versions.

Also, the newest firmware version improve RX performance in all T2 
models.  When everything is already adjusted right it should just be a 
minor improvement, but it seems to make a significant improvement in 
some marginal setups.  The change involved tweaking the sample rate and 
the clock recovery logic.


Robert Bruninga wrote:
>> The T2-301 will actually do 9 or 10 watts 
>> if you crank it up all the way.  Assuming 
>> you're not using it on commercial frequencies, 
> I'm sure the radio specs are very good on receive sensitivity
> too, but what really matters is how well the combination of
> radio and TNC can decode received packets.  We have done some
> testing over the years and have seen as high as 20 dB or more
> decoding problems (usually due to other people's transmitted
> signals formed by just plug-n-playing a TNC to a xmtr and not
> adjusting levels..
> If you listen to 144.39 in the USA, I'd say 80% of the
> transmitted signals are an embarassement.
> I use the TH-D7 as a bench mark, since so many of them are out
> there, and they are an all-in one factory integrated receiver
> and decoding system.  I have found that they can decode signals
> down to one bar on the LCD display.  We measured that minimum
> signal as -117 dBm 
> I wonder if you have done any testing of your combination as to
> on-air receive and packet decode performance?
> Although decode performance on a tracker isnt usually that
> important, it is when it is placed in digipeater service.  ...
> Just curious.
> Thanks
> Bob, WB4APR
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