G?day
Along with several other topbanders who are
interested what Software Defined Radio can
currently offer in the way of weak signal/strong
signal performance, I have been waiting to see
how well a prototype of the Mercury receiver
board ? part of the High Performance Software
Defined Radio (HPSDR) project ? can work. Note
that Mercury is a fully digital receiver and
further details of HPSDR can be found at: see
<http://hpsdr.org/>http://hpsdr.org/
Below are the Mercury performance figures
measured on a prototype system by my friend Phil
Harman, VK6APH. Topbander Greg, ZL3IX, has a
similar prototype and measured similar
figures. As you can see, the figures are very promising!
Mercury prototype - Pre-amp off
MDS - 117dBm, IP3 +50dBm, Blocking Dynamic range 125dB, overload +8dBm
Mercury prototype - Pre-amp on
MDS -135dBm, IP3 +30dBm, Blocking Dynamic range 125dB, overload -10dBm
The IP3 at +50dBm is approximate since it is at
the limits of VK6APH?s test equipment.
The MDS was measured in a 500Hz bandwidth. Note
that the IP3 is INDEPENDENT of tone spacing.
Blocking Dynamic Range was measured at 100kHz and
5 kHz for 1dB gain compression with similar
results. The BDR is set by the overload point of
the ADC rather than being phase noise limited.
Unlike a conventional analogue radio, a Direct
Down Conversion (DDC) radio such as the Mercury
receiver will have a sudden overload point, as
the Analogue-to-Digital Converter (ADC) in it
reaches full scale. This overload point is the
limiting factor in terms of performance. In the
case of the ADC used in Mercury, the LT2208 is a
saturating ADC so increasing the input signal
beyond the overload point does not produce a
sudden catastrophic degradation in performance.
The phase noise of the 125MHz clock has been
estimated at -149dBc/Hz, which VK6APH thinks is
very good for a packaged crystal oscillator. Note
that the 125MHz oscillator was not phase locked
to a 10MHz reference for these tests.
The preamp used by VK6APH consisted of a
noiseless feedback power FET with 2dB noise
figure and 8dB of gain, driving a Norton BJT
stage with 12dB of gain. Greg ZL3IX used a two stage Norton BJT preamp.
Phil notes that the MDS with the pre-amp off is a
little high - most receivers give about -127dBm -
so a small pre-amp gain may be needed at all times to reach this level.
What with the coming of the Elecraft K3 and the
Flex-Radio SDR-5000, we live in interesting times.
Vy 73
Steve, VK6VZ
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