[hpsdr] Mercury receiver performance figures

[Phil Harman]

Here are the Mercury performance figures measured on the prototype systems.  Greg, ZL3IX,
has a similar prototype and has measured similar figures.

Unlike a conventional analogue radio an DDC receiver will have a sudden overload point
as the ADC 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. 

Pre-amp off

MDS   - 117dBm,  IP3 +50dBm,  Blocking Dynamic range 125dB, overload +8dBm

Pre-amp on 

MDS   -135dBm,   IP3 +30dBm,  Blocking Dynamic range 125dB, overload -10dBm

The IP3 at +50dBm is approximate since its at the limit of my test equipment.

The MDS was measured in a 500Hz bandwidth. 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 DBR is set by the overload point of the ADC rather than being phase noise limited.

Greg estimates the phase noise of the 125MHz clock at -149dBc/Hz which is very 
good for a packaged crystal oscillator (NOTE: the 125MHz oscillator was not phase
locked to a 10MHz reference for these tests).

My pre-amp 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 used a two stage Norton BJT preamp. 

The MDS with the pre-amp off is a little high, most receivers give about -127dBm. In which
case we may need to use a little pre-amp gain at all times to reach this level. 

Very good figures for a fully digital receiver that should give excellent performance for
many users.

Thanks to Greg for assisting with these measurements.

73's Phil....VK6APH 
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