LP Pan versus P3 Panadapters

[Edward R Cole]

I do not have a P3, so my comments are based on my experience using
the SDR-IQ on 144-MHz using a 144/28 converter to receive 2m-eme signals.

The sensitivity of the display is tied to the sampling rate (FFT/BLK
size on the control panel of the SDR-IQ).  With maximum sampling of
262,144 I get a bin size of 0.42 Hz.  This also referred to as the
RBW (resolution band width).  The narrower the FFT routine in the sw
is, the more sensitive the display is.  It is analogous of using
narrow filters to hear very weak signals.  But one cannot copy CW
using bandwidths under 25-Hz due to filter "ringing" effect, so the
panadapter display can be several times more sensitive than what one
hears, depending on what the sw permits.

I usually run my SDR-IQ at 65,536 or 32,768 samples/sec with RBW of
1.7 or 3.4 Hz.  WSJT sw that is used for eme has a RBW of 4.3 Hz (I
believe) and that will display signals 10-dB weaker than can be
heard.  WSJT has a max span of about 4-KHz.

 From what I am reading from those who are running the P3, RBW is
dependent on span size.  This is true for the SDR-IQ, as well.  The
figures I have quoted is for a span of 100-KHz.  If I expand to
190-KHz then 262,144 produces a RBW=0.75 Hz.  So if the P3 is looking
at the whole 20m band (350-KHz), for example, it will have less
sensitivity than narrower spans.  I would guess that a panadaptor is
used for different reasons on HF than it is for operations like eme;
signals are not as weak as a rule.

I made sensitivity tests that were the subject of a paper at
Microwave Update 2005 on the applicability of the SDR-IQ for eme.  I
compared it to my FT-847.  I fed both radios with a signal generator
able to generate down to -172 dBm and found that for a span of
100-KHz the FT-847 with WSJT sw could see -152 dBm signals.  The
SDR-IQ could see -166 dBm signals at the same span.

There were two preamps feeding both radios so this explains about
30-dB of the overall sensitivity (the FT-847 is spec at -122 dBm
SSB).  On air tests using real eme signals verified the results.  I
hope this sheds some light on the topic, if only as background info.


73, Ed - KL7UW, WD2XSH/45
======================================
BP40IQ   500 KHz - 10-GHz   www.kl7uw.com
EME: 144-QRT*, 432-100w, 1296-QRT*, 3400-fall 2010
DUBUS Magazine USA Rep [hidden email]
======================================
*temp
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[David Woolley (E.L)]

Sorry, but the only way of handling this well is proper interleaved style.

Edward R. Cole wrote:
>
> The sensitivity of the display is tied to the sampling rate (FFT/BLK
> size on the control panel of the SDR-IQ).  With maximum sampling of
> 262,144 I get a bin size of 0.42 Hz.  This also referred to as the RBW

I believe that one of the early articles was talking about professional
systems that actually run interleaved FFTs with frequency offsets, so
several bins overlap and are averaged.

I suspect it is true that all amateur radio power spectrum estimation is
done with a single FFT.

> (resolution band width).  The narrower the FFT routine in the sw is, the
> more sensitive the display is.  It is analogous of using narrow filters
> to hear very weak signals.  But one cannot copy CW using bandwidths
> under 25-Hz due to filter "ringing" effect, so the panadapter display

The real reason is that the sidebands of fast CW extend beyond 25Hz.
The first order sidebands of 12 wpm morse are at +/- 5Hz, so, with 25Hz
bandwidth you have lost most of the signal (as against the carrier) at
more than 30 wpm.

> can be several times more sensitive than what one hears, depending on
> what the sw permits.

What you are comparing with the pan adapter is the ability to hear the
carrier, not the modulation.  The effective bandwidth of the ears is, I
believe, less than 25Hz at typical sidetone frequencies, so whilst you
may not perform as well at detecting the carrier as the FFT, you will
perform much better than the ability to recover the modulation.
>
> I usually run my SDR-IQ at 65,536 or 32,768 samples/sec with RBW of 1.7
> or 3.4 Hz.  WSJT sw that is used for eme has a RBW of 4.3 Hz (I believe)
> and that will display signals 10-dB weaker than can be heard.  WSJT has
> a max span of about 4-KHz.

There is non-coherent integration going on here as well.  You get a 10
log10 (n) improvement in SNR of the carrier by averaging n samples.  As
far as I know the EME reflection process spreads the spectrum and
prevents coherent integration of even very slow morse.
http://www.proaxis.com/~boblark/wksig1.htm seems to suggest something
around 7Hz/GHz of spreading.
>


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