[hpsdr] MW DXpedition Ozy/Janus/Atlas Comments

[dx at guyatkins.com]

Hi Phil, Ahti, and Ramakrishnan,

Thank you all for your comments. Yes, I did have JP1, JP4, JP3 and JP6 set
to "open" before using the balanced input. The panadapter trace has been
nice and flat in both the balanced and unbalanced input modes...that is one
thing I like very much; no "humps" at the ends of the passband.

Although I had to switch to the unbalanced input for the remainder of the
DXpedition at least, I'm pleased with the receiver response using the
unbalanced method. The sensitivity and ability of the radio to recover weak
audio is now equal to other communications gear in use here. 

I have also been pleased with the low noise floor using the unbalanced
Janus input, compared to the stock SDR-1000 with the Presonus Firebox I
used earlier. With a 50-ohm terminator, after calibration using an XG-1, I
am seeing a -145 to -147 dBm noise floor on some frequency ranges (4 to 5
MHz seems to give the best NF). If I recall correctly, the *balanced* input
method gave a very similar NF.

My balanced cable is very short, has a high density outer shield, and the
IQ pairs go through ferrite core RFI suppression for good measure (although
I didn't calculate any supression or response in advance; I just wrapped a
few turns through a binocular core). To be really effective, I'd probably
need more impedance and the audio frequencies of IQ. I will have to give
the balanced method a try again once I return home; it does seem to need
about 10 dB more gain as Ahti has mentioned.

Well, I'd better leave this internet cafe and return back to the DX
site...it is time for the trans-Atlantic MW DX to start rising above the
band noise! We get a few hours sleep after the trans-Atlantic catches fade
down, and then the Asian and Pacific DX starts to roll in. By the way, we
have been using numerous Beverage antennas, large terminated loops, and two
prototype Wellbrook phased arrays.

73,

Guy

Original Message:
-----------------
From: Ahti Aintila oh2rz.sdr at gmail.com
Date: Sat, 22 Sep 2007 20:00:17 +0300
To: phil at pharman.org, vu3rdd at gmail.com, dx at guyatkins.com,
hpsdr at lists.hpsdr.org
Subject: Re: [hpsdr] MW DXpedition Ozy/Janus/Atlas Comments


Hi Phil, Ramakrishnan and Guy,

First of all, thanks to Phil for the great design success of the Janus
board, to Guy the welcome report from the field and valuable private
comments from Ramakrishnan when estimating the noise levels.

My measurements of Janus with balanced inputs and bypassed INA163 are
not necessarily correct due to some common mode interference of the
power supply I have in my shack. In any case they indicate too low
gain before the Janus ADC, perhaps some 10 dB of more gain would be
needed for upper HF bands, quiet environment and low noise/weak signal
ionospheric conditions.

Based on the specified 123 dB dynamic (blocking) range of AK5394A, we
may calculate 20 ENOB for the ADC. That translates roughly to 20 dB
noise figure as referred to 50 ohm input and zero gain before the ADC.
Is that about right, Phil? Or did I miss something?

I have to agree with Guy that at least in my test arrangement and
layout I got very high spurious level that disturbed the MW listening.

Guy's question:
"Ahti or anyone else-- if you have suggestions on improving the signal
levels with the *balanced* connection, I would love to hear your ideas"

What to do? Perhaps slightly improved QSD (including less phase noise
of the DDS output), more preamplifier gain, less switching noise of
power supplies and careful shielding of the balanced I/Q lines. Hard
work in any case!

73, Ahti OH2RZ



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