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

Wed Jun 6 12:59:02 PDT 2012

Hi Adam,

When NPR is measured in an analog receiver, the extra noise in the idle channel 
is caused mostly by the combinational products that that are created because the 
amplitude characteristic of the radio is not a perfectly straight line. The 
magnitude of such noise is a function of the spectral density of the loading 
noise. Because of this, the NPR for the analog receivers is defined, quoting 
your article, as "the ratio of the noise power in the notched band to the power 
in an equal bandwidth adjacent to the notch".

In the RF sampling receivers, the extra noise is dominated by the clipping noise 
which depends on the total power of the input signal (or noise loading, in case 
of the NPR measurements), not by its power density. It is the use of the total 
power vs. power density that causes confusion.

What happens if you repeat your measurements but change the loading noise 
bandwidth from 5.6 MHz to, say, 2.8 MHz? for an analog receiver, you will get 
essentially the same results. For an RF sampling receiver, the results will 
"improve" by 3 dB, because you are measuring the spectral density of the loading 
noise, and at the same noise power that is required to produce clipping, the 
spectral density has doubled. To get rid of this fake improvement, you will have 
to subtract 3 dB from your measurement. Similarly, when you are testing a 
receiver with a bandwidth of 61 MHz using the noise with a bandwidth of 5.6 MHz, 
you should subtract 10 * Log10(61/5.6) = 10.37 dB from your raw result, this 
will immediately turn the unrealistic NPR of 74 dB into something more 
plausible.

Does this all make sense?

73 Alex VE3NEA

P.S. I used Graph (http://www.padowan.dk/graph/) to plot my charts.

-----Original Message----- 
From: Adam Farson
Sent: Wednesday, June 06, 2012 3:07 PM
To: HPSDR Reflector
Subject: Re: [hpsdr] NPR of Mercury (was: Boat Anchors..PreAmps)

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

Larry, Alex,

Many thanks for bring me into the group. I joined because I would like to
follow this thread, and also out of a general interest in direct-sampling
SDR technology.

The criterion I use for determining the optimum noise loading point in a
conventional receiver is that proposed by I2VGO. In which the optimum point
is that where the noise level at the audio output rises by 3 dB relative to
the noise floor. I found that when testing the Perseus and the HPSDR, I
could not use this criterion because the ADC clipped before the audio output
increased by 3 dB (as noted in my NPR article).

http://www.ab4oj.com/test/docs/test_npr.pdf  Slide 28

Thus, for the SDR tests I used another criterion proposed by Gianfranco
Verbana I2VGO,  i.e. a noise loading level at which the ADC just did not
clip over a 10 sec. interval:

http://www.ab4oj.com/test/docs/test_npr.pdf  (I2VGO presentation, Slide 36)

and read NPR directly off the SDR’s spectral display:

http://www.ab4oj.com/test/docs/test_npr.pdf  (Slide 33)

My results for the Perseus were very close to those presented in Slide 35 of
I2VGO' presentation.

http://www.ab4oj.com/sdr/perseus/perseus_notes.pdf (p. 12)

In the ADI MT-005 paper (referenced below), Eq. 16 gives a correction factor
which must be applied if the bandwidth of the noise loading is less than
fs/2.  For the Perseus, where fs = 80 MHz, this correction factor was +8.6
dB in my test setup with a 5.6 MHz noise band limiting filter. Now if I take
Alex’s calculated NPR value of 65.5 dB and add 8.6 dB, I arrive at 74.1 dB,
which is the theoretical value I quoted and is also quite close to my
best-case measured value of 75 dB.

Looking forward to your further comments.

Cheers for now, 73,
Adam VA7OJ/AB4OJ

---------- Forwarded message ----------
From: Alex VE3NEA <alshovk at dxatlas.com>
Date: 6 June 2012 08:08
Subject: [hpsdr] NPR of Mercury (was: Boat Anchors..PreAmps)
To: hpsdr at lists.openhpsdr.org

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

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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Larry Gadallah, VE6VQ/W7                          lgadallah AT gmail DOT com
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