[hpsdr] NPR of Mercury (was: Boat Anchors..PreAmps)

[Alex VE3NEA]

The maximum NPR of 85.4 dB quoted by Adam was computed [1] for an ideal 16-bit 
ADC whose noise floor of -98.09 dBFS [2] is determined solely by its 
quantization noise. Perhaps ideal ADC's were out of stock when Mercury was 
prototyped ;-) because the radio uses LTC2208, a real 16-bit ADC with a noise 
floor of only -78 dBFS [3]. This makes a huge difference as far as the NPR is 
concerned.

I have re-computed the maximum NPR using the same equations as in [1], but 
taking into account the actual noise floor of the ADC. The new chart [4] has the 
same curves as in Figure 2 of [1], with two more curves added for the real 
ADC's. The theoretical maximum NPR that can be achieved with LTC2208 is 66.4 dB.

For comparison, LTC2206-14, a 14-bit ADC used in Perseus, has a noise floor 
of -77.3 dBFS [5]. Its maximum NPR is 65.5 dB.

The values measured by Adam do not indicate any excessive IMD in the preamp of 
Mercury. To the contrary, for some reason they exceed the theoretical maximum 
for a real ADC.


References:
[1] www.analog.com/static/imported-files/tutorials/MT-005.pdf
[2] http://en.wikipedia.org/wiki/DBFS
[3] http://www.linear.com/product/LTC2208
[4] http://dxatlas.com/misc/npr.png
[5] http://www.linear.com/product/LTC2206-14


73 Alex VE3NEA




-----Original Message----- 
From: Larry Gadallah
Sent: Tuesday, June 05, 2012 3:15 PM
To: lester at veenstras.com
Cc: hpsdr at lists.openhpsdr.org ; lstoskopf at cox.net
Subject: Re: [hpsdr] Boat Anchors..PreAmps

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

I was reading Adam, VA7OJ's excellent reports on his NPR testing of
various receivers including Mercury (see
http://www.ab4oj.com/sdr/hpsdr/hpsdr_npr_va7grr.pdf) and I was
intrigued by this comment:

"The theoretical maximum NPR for a 16-bit ADC is 85.4 dB, as compared
to 74.01 dB for a 14-bit ADC.
Thus, the HPSDR receiver exhibits a significant deviation from the
theoretical maximum value (much
more so than the Perseus, whose highest NPR value in my August 2011
test was 75 dB at 5340 kHz.)

The RF preamplifier in the Mercury receiver is in the RF signal path
at all times.  A 20 dB pad is switched
in at the preamp input for the “Preamp out” function. Apparently, the
noise loading provokes sufficient
IMD in the preamp to degrade the NPR as much as 12 dB below the
theoretical maximum value. (The
optimum noise loading value decreases by 19 dB with the attenuator
out, which reflects the inserted 20
dB attenuation pretty closely. )"

I'm not sure I grok the meaning of "noise loading" in that comment,
but I'm assuming that he is implying that the attenuator contributes
enough noise to trigger some nonlinear operation of the preamp? This
leads to the question: Is the current design of leaving the preamp in
the circuit at all times and switching in/out an attenuator preventing
Mercury from approaching the theoretical NPR capabilities of the
LTC2208? Would it be better to completely switch the preamp in/out of
circuit as needed?

Cheers 


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