[Discuss-gnuradio] Re: USRP at Ham Field Day

[John Ackermann N8UR]

John Ackermann N8UR wrote:

> The experience has made me curious to see just what the sensitivity of
> the system is with no filtering or preamp, so I'll hook a signal
> generator up to see what it can hear in the lab.  But from the field
> experience, I'm guessing that it was hearing signals in the 100uV range.

Got out the signal generator (HP 8920B service monitor) and I'm very
surprised by how sensitive the BasicRX board is.  Using hf_explorer_9a,
tuned to 7.120MHz and using the minimum 600Hz filter bandwidth, the
minimum discernable signal (by ear, with communications headphones) is
about 0.63uV (-111dBm).  6.3uV (-91dBm) brings the signal to -20 on
Chuck's FFT display.  89uV (-68dBm) brings it to 0 on the FFT display,
so there's a bit of nonlinearity somewhere.

A quick check on 10M (28.35MHz) shows just about the same sensitivity
(-92dBm for a -20dB reading on the FFT display).

So a front end with a bandpass filter, a single stage of
preamplification using a reasonably crunch-proof device, followed by
another bandpass filter, ought to provide very good receiver performance.

(I think that Chuck's program uses the 20dB programmable gain amplifier
on the USRP; I have no idea what the performance parameters of that
device are; it may be that turning it off and supplying more gain in the
front end might provide better performance, but at this point I don't
know if that would be the case.)  (And, by the way, I did notice several
fairly strong spurs that are present when no input is connected; my USRP
is running without an enclosure, so there are plenty of places where
those might come from.)

John