Topband: Re: Minimum discernable signal

[Tracy, Michael, KC1SX]

Hi Tom,
      While I don't read the topband list, Dean Straw mentioned you had a
follow-up post in this thread.  Let me address some things:

> 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.

While the Product Review table that appears in QST is 20-kHz numbers only,
we do test 'contest-type' rigs (and some others) over a swept range of a
couple-hundred kHz down to 1-kHz for both blocking and IMD.  The graphical
results of these tests is contained in the Expanded Test Result Report,
which may be downloaded by ARRL members and obtained by mail by non-members.
For anyone planning on plunking down several grand for a rig, this report is
well worth the read <plug, plug>.  :-)

> Testing outside the BW of the roofing filter takes all
> later stages, including mixers and amplifiers, out of
> the picture.  

Not necessarily.  You have to remember that the roofing filter has a very
gradual roll-off in most cases and signals "on the skirts" still get a
significant amount of energy passed along to following stages.

Also, having seen a number of these swept responses, I can say that
_generally speaking_, the relative performance of several rigs at 20-kHz is
indicative of how they compare at closer spacings as well.  However, since
the data is available, there is no need for a ham to speculate about
particular rigs.

> 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.

Then you haven't been reading Product Review closely enough.  :-)

> 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.

So you are saying you should never have problems on HF from stations more
han 10 kHz away?  In a multi-multi setup, you can bet this isn't the case.
Even in a single-rig shack, this can still occur, although it is far less
likely if the rig's performance is decent to begin with.

> 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*****!!!!!

Ah, but now this is a slightly different issue.  As I pointed out,
conditions on a test bench are only an approximation at best, especially
when steady-state signals are the only ones conveniently available.

While we don't currently have any pulse type noise or pulse type signal
(e.g., CW) tests, I frequently give thought to ways to implement such.  If
you have any suggestions that might help, please email me direct.

> 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.  

That's only because such narrow filters typically have a significant
insertion loss or passband shape change.  If you can narrow up your passband
width and still keep the signal centered you will improve MDS, but it isn't
"inflated" in any way - MDSes at different bandwidths can't be compared
directly - that's why we stick with a 500 Hz BW in Product Review.
 
> This is a 160 meter reflector. My 160 meter noise floor, after
> preamplifiers, 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.
>
> To give you an idea of signal levels, W4ZV (the loudest station
> I normally hear when beaming Europe) is normally -30 dBm here.
...
> 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.

While you could use a rig like that, you don't have to.  With 20-dB of
attenuation kicked in, you'll still hear at the band noise level and only
need 80 dB of DR in the receiver.

> IMD and blocking are important at test spacings of a
> few hundred Hz!

That's why our Expanded Test Reports also include an in-band IMD test
(signal levels of S9 and S9+40 or S9+60 depending on the rig, with 700 Hz
spacing between them).  As this also gives one an idea of the rig's audio
distortion on SSB, it can be a real eye-opener.

> That's why my primary receivers are now Drake R4C's with 600 Hz
> roofing filters ...

Hmm - I wonder how much it would cost a manufacturer to put in a switchable
(mode dependent) roofing filter (that is a serious thought - I'd like to see
better receivers on the ham market too).

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

This I can't agree with based on keying waveforms I've seen.

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

This I do indeed agree with and that gives me a thought - since this data is
in the ARRL expanded reports, perhaps I should make it SOP to send the
manufacturer a complementary copy (couldn't hurt, I figure).

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

Unfortunately, most hams don't have the dough for such an rig.  However, I
think it can be done without twice the dough - it just requires the right
attention.  

Bells and whistles do come in handy sometimes though.  Not too long ago, I
was sitting in front of the ICOM IC-781 over at W1AW and I was tuning around
20 meters.  I was experiencing some interference from the second harmonic of
an operator on 40 in another room at the station (I'll spare the details,
but he was using a homebrew transmitter).  The noise blanker on the 781 can
be set to wide or narrow and has an analog adjustable level.  By fiddling
with it, I eliminated the majority of the interference.  That was a good
feature to have in that circumstance.

> 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.

Umm, that's what the two-tone transmit IMD that appears in QST's Product
Reviews gives you.  Of course, actual IMD varies with different tone
spacings and number of tones, but some sound-card based software for digital
modes has IMD reporting capability for received signals so at least the
operator on the other end can tell you that you are IMing.

> They didn't say, or you didn't quote, how weak the signal was.

It wasn't mentioned, but just to clarify - it wasn't the _Lab_ that
performed this procedure.  It was the fellow who used the rig on the air for
the Product Review.

> > How bad can the dynamic range be?
>
> Bad.

Of course, that's all relative to a particular station set-up, so I can't
really address this.

...
> The last time I looked A/D conversion with 
> enough speed and bits would cost a thousand bucks.
> That would buy a lot of crystal filters, which when
> properly designed are just as good in BW and don't
> suffer overload!

That would be about 10 filters, just one less than the number of slots in
the FT-1000MP and Mark V.  A couple advantages of a DSP filter are the range
of BW adjustment available and the ease of passband shifting, but there are
others too.  A high quality ADC such as you mention could be coupled to a
front-end that doesn't crunch either, but the cost is not reasonable for the
ham market yet.  In my personal opinion, it is inevitable that the cost of
these devices will come down and the ham market will benefit substantially.


73, Michael, KC1SX
mtracy@arrl.org


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