[hpsdr] Dither on Mercury-EU

[Helmut]

Hi Hans,

 

of course I’m familiar with calculating noise figures in cascaded arrays,
but I didn’t investigate the noise figures of the single stages. You can
‘bypass’ the preamp by  adding  the 20 dB pad in front of the preamp and
you’ll have similar  readouts  concerning SNR reduction.  It seems that
these readouts are ok, because they correspond with my MDS, blocking and IM
tests I made with external test equipment. 

 

73, Helmut_DC6NY

Von: h-hartfuss at t-online.de [mailto:h-hartfuss at t-online.de] 
Gesendet: Mittwoch, 17. August 2011 10:51
An: Helmut
Cc: hpsdr at lists.openhpsdr.org
Betreff: Re: [hpsdr] Dither on Mercury-EU

 

Hi Helmut,

I don't think that this is completely correct. The process is conducted at
the ADC by dithering over 2 or 3 quantization steps. Of course this results
in an S/R reduction. 

However, the ADC is not the first stage in the receiver system. The first
stage is a low noise preamp with 20 db gain. 

The overall noise temperature of the receiver is determined by the noise
temperature of the first stage plus a minor contribution from the second
which is the noise temperature of the ADC, however, divided by the gain of
the first. That's cascading rule!

You can easily calculate that it is almost impossible to increase the
overall noise temperature by an order of magnitude just by increasing the
ADC noise figure by say a factor of 4.

73, Hans, DL2MDQ. 

 


-----Original Message-----
Date: Wed, 17 Aug 2011 09:35:49 +0200
Subject: Re: [hpsdr] Dither on Mercury-EU
From: "Helmut" <dc6ny at gmx.de>
To: "'wully'" <wully at bluewin.ch>, <hpsdr at lists.openhpsdr.org>

***** High Performance Software Defined Radio Discussion List *****

Hi Alfred,

enabled dithering reduces the signal-noise-ratio at least by 6-10 dB. If
less, something is wrong with the efficiency of dithering. My two
mercuries-eu show similar values.

73 Helmut, DC6NY


PS: Perhaps we meet us 40m to continue


-----Ursprüngliche Nachricht-----
Von: hpsdr-bounces at lists.openhpsdr.org
[mailto:hpsdr-bounces at lists.openhpsdr.org] Im Auftrag von wully
Gesendet: Mittwoch, 17. August 2011 09:07
An: hpsdr at lists.openhpsdr.org
Betreff: [hpsdr] Dither on Mercury-EU

***** High Performance Software Defined Radio Discussion List *****

Hi all

First, I would like to add my congratulations for Phil Harman for this 
award!

I have noticed a strange behavior of Mercury-EU, which might indicate an 
individual board failure:

Antenna input with 50 Ohms terminated, no antenna. When switching on 
Dither I notice an increase in background noise level of between 7 and 
11 dB depending on the band! Typical noise value without dither is 
around -130dBm (slightly better than tapr-mercury).

Doing the same experiment on my two Tapr-Mercuries, the increase of 
noise is hardly noticable with a value of 1 dB.

Since I don't have two Mercury-EU, I can not compare.
Normally, I am running the HPSDR with Dither off, so this is only a 
"theoretical" problem, but it is interesting anyway.

Any suggestions are much appreciated.

73, Alfred, HB9EPU
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