Topband: Re: Minimum discernible signal ?

[Marijan Miletic]

W8JI wrote:

>That's fine Tony, but you have to consider the spacing of the test
>signals used in the ARRL tests. Virtually all receivers today have a
>roofing filter that follows the first mixer. This filter is generally 10 or
>more kHz wide, and most often has a somewhat poor shape factor.

Roofing filter should be wide enough for narrow FM mode found in all modern
HF RX today.
Monolithic filters have a reasonable shape factor.  Spurious response is
damped by LC circuits.

>It's a "cheap" filter, and a wide one, but it makes a significant
>difference in test results. Testing outside the BW of the roofing filter
>takes all later stages, including mixers and amplifiers, out of the picture.

I strongly recommend W0IYH QEX article on price/performance issues in RX
design.  It is obvious that "cheap" filter does a very good job under
prevailing operating conditions especially in USA!

>While wide spacing IMD and blocking DR tests are useful if you
>have a ham "down-the-road" operating 30 kHz or more away, there
>should *always* be a test well within the roofing filter BW of
>receivers. As far as I know, the ARRL does not do this.

RSGB does it even in English.

>If the testing party does not test or publish narrow-spaced tests
>(spacing much less that the BW of the roofing filter) we have no
>idea at all how the receiver will work in normal CW or SSB
>conditions. Any blocking or IMD numbers, if taken at more than a
>few thousand Hz spacing, are almost meaningless in the
>conditions we typically face on amateur bands below 50 MHz.

We strongly overstate ham bands overcrowding.  Statistics show it as the
quietest parts of HF.

>For example, my FT1000D looks very good in ARRL tests, yet the
>receiver was almost useless in the very first CW contest I operated
>with it. I heard all kinds of bloops and bleeps that weren't really
>there, because of design flaw in the noise-blanker that allows the
>noise blanker to mix signals ****even when the noise blanker is
>NOT turned on*****!!!!!

The same type of balanced noise blanker is used in almost all modern HF RX.
Strong DC diode bias should prevent limited nonlinearities within roofing
filter passband.

>I looked at that rig, and wondered why on earth I bought it.
>Fortunately the fix for that only took a few cents worth of parts and
>a few hours of my time, and now it is a decent receiver.

>However, if the modified FT-1000 and the stock FT-1000D were
>compared in the ARRL test...they would look the same.

There is an even cheaper fix with a single resistor preventing speaker bump
after the headphones are disconnected but nobody is perfect...

>I look at the roofing filter BW, and the test signal spacing, and if
>test signals are NOT both placed within  the roofing filter BW of the
>receiver I ignore the results (unless they are bad, in which case I
>know closer results will be even worse).

ARRL results give a good clue of front-end robustness and everything else is
much easier to cure!
Front-end filters mentioned by K3BU are the most expensive.

>Maybe, but it would have little to do with the DSP system vs
>analog filters. Digging into noise requires all stages are quiet, and
>the bandwidth is as narrow as possible. A 50Hz filter will ALWAYS
>make the MDS look good, even if you add it after the receiver.
>Noise (white noise) level is directly related to the BW of the receiver.

This relation is much more complex due to the presence of active components
with additional noise contribution and wideband design of modern electronic.

>The question is how narrow a BW can you really use in normal
>conditions, and how "smooth" is the noise. In practice, I find a 250
>Hz filter is about as narrow as I can ever use and not have major
>problems tuning the station. If there is "rough" noise, like QRN or
>power leaks, a narrow filter can make things worse. My ears do a
>much better job than any filter (including DSP) will in that case,
>and during the summer I often use a 1.5 kHz filter or wider on CW.

Human brain and ears performance is often neglected in the above discussions.
We are more sensitive to filter ringing then wideband noise...

>Narrow filters ALWAYS tend to ring out the noise, and blurr it over
>the signal. That's true no matter what the type of filter is, although
>smooth flat filters that are properly designed can be better, for a
>given low attenuation BW, than a crummy poor design.

DSP filters are much better here as ringing is caused by phase delay variations.

>> I always thought that if two rigs show a MDS of -140dbm for
>> instance, that they would both hear the same? Or is there more to IF DSP
>> then the MDS figure represents?

>It sounds to me like the 50 Hz filter "inflates" the MDS. If you tack
>a 50 Hz wide audio filter on a regular receiver, the MDS normally
>goes down.

MDS will improve considerably because noise energy is reduced while the signal
level is the same!
However, it is not a very pleasant to listen to single signal only.  Evolution
optimized our hearing for speech with some overhead left for music :-)

>This is a 160 meter reflector. My 160 meter noise floor, after pre-
>amplifiers, on a quiet winter night when the band is open to Europe
>is about -117 dBm measured at 3.5 kHz bandwidth. With a 350 Hz
>bandwidth, the "off-the-air noise floor" of my system is -127 dBm.
>With a 35 Hz filter it is -137 dBm.

I stated long ago that the claims of nV sensitivity for 160m RX is excessive.
We can now all see that 20 dB attenuation is optimal on any 160m SSB RX.

>To give you an idea of signal levels, W4ZV (the loudest station I
>normally hear when beaming Europe) is normally -30 dBm here.
>DF2PY is normally (average night) -80 dBm. Of course the weakest
>Europeans are down at or below noise floor.

The strongest EU will be audible even with 40 dB attenuation...

>I need a blocking DR of more than 100 dB at CLOSE spacing if I'm
>going to work a weak signal within six kilohertz of Bill. If I don't
>have that DR, it's my problem. I give a "hoot" about the blocking
>DR at a wider test spacing that the entire DX area of the band!

I would use crystal filter for 160m DX window as the simplest solution.

>Contests are even worse, as we have multiple -50 to -20 dBm
>signals just a few hundred Hz from noise-floor signals. IMD and
>blocking are important at test spacings of a few hundred Hz!

Even a good headphones are important!

>That's why my primary receivers are now Drake R4C's with 600 Hz
>roofing filters and 125/250 Hz eight-pole IF filters, diode mixers,
>and MMIC IF amplifiers.

Why use GHz MMIC at IF?  AD-603 looks much better to me...

>I have a blocking DR with 300 Hz spacing
>(both signals in the passband of the roofing filter) of almost 150 dB now.

This is hard to believe as narrow CW crystal filters have a very poor shape
factor.  I wouldn't even mention AGC strain, AF mixing etc. R4C IF and image
rejection leaves a lot to be desired.

>The system is now totally limited by the spurs of closeby
>transmitters, rather than internal defects.

Where is all the internal oscillator noise gone?

>> I enjoy working CW-QRP and digital mode QRP ( PSK31) and if these claims
>> about the IC756 Pro are true, then it should be a real winner for weak
>> signal work. Since I'm not a contester, I'm not worried about the less
>> than perfect blocking dynamic range of the Pro.

IC-718 may be as good for the above needs!

>Contesters and DXers should be worried. And if digital modes get
>more crowded, you might be too.

We worry in EU all the time.  We expected some improvement after the end of
Cold War but only Woodpecker is gone.  BCI keeps beaming MW toward us...

>Manufacturers pay far too little
>attention to transmitter waveshape and bandwidth on CW,

Considering the CW future, I am surprised they do CW waveshaping using DSP.

>harmonic distortion and IMD in transmitters when the audio system
>is used, and close spaced performance in receivers.

Both Yaesu-MkV and IC-718 show considerable improvements!  I anxiously wait
for TS-2000 data.

>I'd pay twice the amount for good RF performance than I would bells and
>whistles and 9 thousand filters.

You get exactly that cheaply with DSP.

>Harmonic distortion in SSB transmitter audio and modulator systems will be
>particularly important with new digital modes piped into the mic plugs on 
>radios, and it is NEVER measured.

It is our fault with blatant AF overdriving in 99% of the cases!

>> Lab stated that they were able to pull out individual CW siganls in the
>> middle of pile-ups using the 50hz filter setting. They also said that they

>So what?. Pulling an S-9 signal out of a plie up is nothing. Pulling
>an S1 signal out of a bunch of 40 dB over signals is great. They
>didn't say, or you didn't quote, how weak the signal was.

Good op. can get 60% of CW signals in the 5 minutes pile-up, DSP decoder
can get 80%, so what?

>> were able to pull QSO's 200 and 300hz apart during the CQ World Wide 160
>> Meter CW Contest. How bad can the dynamic range be?

>Bad.

This has nothing to do with dynamic range until signal strengths are very
high or more then 30 dB apart.

>The fact stations spaced 250 Hz apart can be "pulled-out" is
>absolutely meaningless. I can do that with a FT101E or a *stock*
>R4C, and both are horrid receivers that I would never consider
>using. What I want to do, and what most of us want to do, is be
>able to hear a noise floor signal a few hundred Hz from strong
>stations when multiple strong signals are present.

Life is difficult even with only two close strong signals...

>It also would be nice if someone tapped an engineer on the
>shoulder on occasion, and reminded him that key clicks are not a
>necessary part of sending CW.

If the engineer knows what CW is these days :-(

>DSP filtered radios, without a narrow IF filter upstream of the DSP,
>simply won't do that. The last time I looked A/D conversion with
>enough speed and bits would cost a thousand bucks.

They are already at 14 bits up to 30 MHz.  Just a few more years...

>That would buy a lot of crystal filters, which when properly designed are
>just as good in BW and don't suffer overload!

There is no way to avoid ringing in crystal filters!  They also overload above
mW levels with catastrophic consequences.

>Hopefully someday manufacturers will someday look at what is
>important and improve operation on crowded bands.

I doubt with a present ham population trends especially on HF.

73 de Mario, S56A, N1YU


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