Topband: Performance figures from a state-of-the art SDR receiver

[Steve Ireland]

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