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

Wed Jun 6 13:57:45 PDT 2012

Adam,

This sounds like a great plan! Please keep me informed of your findings.

Personally I like the alternate method of NPR estimation described in your 
paper, where you compare the noise level in the idle channel and outside of it 
on the band scope directly, without all intermediate steps. Of course, the noise 
loading must be scaled by the noise/receiver bandwidth ratio as in the other 
method.

73 Alex VE3NEA

-----Original Message----- 
From: Adam Farson
Sent: Wednesday, June 06, 2012 4:27 PM
To: 'Alex VE3NEA' ; 'HPSDR Reflector'
Subject: RE: [hpsdr] NPR of Mercury (was: Boat Anchors..PreAmps)

Hi Alex,

Many thanks for your comments. I shall repeat my NPR measurements on the 
Perseus, using different noise bandwidths. I am using Telco surplus Wandel & 
Goltermann noise generators fitted with 1940, 3886, 5340 and 7600 kHz notch 
filters and their companion band-limiting filters (2048, 4100, 5600 and 8160 
kHz, respectively). I have two of these instruments, one with a 12.5 MHz total 
noise-band and the other with 25 MHz. These values are with no band-limiting 
filter inserted. What I can do is test at 25 MHz, then insert band-limiting 
filters and apply the corresponding correction factors.

I can see that the ideal NPR test setup for a direct-sampling receiver is one in 
which the applied noise-band cuts off at fs/2. Still, though, the receiver's 
preselector will limit the bandwidth of the applied noise, so to measure the 
ADC's NPR the preselector should always be out, or a correction factor of 10* 
log10 ({fs/2}/preselector BW) should be applied. To determine whether passive 
IMD in the preselector is a significant factor, both configurations should be 
tested and the results compared.

Cheers for now, 73,
Adam VA7OJ/AB4OJ

-----Original Message-----
From: Alex VE3NEA [mailto:alshovk at dxatlas.com]
Sent: 6-Jun-12 12:59
To: Adam Farson; HPSDR Reflector
Subject: Re: [hpsdr] NPR of Mercury (was: Boat Anchors..PreAmps)

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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