[RTTY] Bad AFSK

[Joe Subich, W4TV]

Dick,

Unless you were observing the bulletins on *Monday* night local, don't
be making any kind of statement about "local" interference.  I've been
doing this quite a few years - I was OO Coordinator in Ohio 30+ years
ago and an OO for many years before than - I can certainly recognize
the difference between audio harmonics in an AFSK (and PSK) generator
and "local interference".  I can also recognize symptoms that appear on
one signal and not on other signals of similar strength nearby.

That capture was made with an Elecraft K3/P3 - the P3 is an IF spectrum
analyzer - its pick-off point is prior to any narrow filters and prior 
to any AGC action.  Its results are not subject to misalignment (the
improper receive mixer settings) and/or transceiver AGC action which
makes "measurements" made with the waterfall of typical user digital
software useless and inaccurate.

 > I think you owe the Joe Carcia, NJ1Q, the W1AW Station Manager and
 > this reflector a retracton for your public comments about the
 > "terrible" W1AW signal and your statement "….One simply *can not*
 > run "all sliders to the max" and "all knobs to the right" with AFSK."

Since you called out the station ... I was not able to see a signal
- either RTTY or PSK - on 20 meters at 2300Z Tuesday.  I did look at
the 80 and 40 meter PSK signals and noticed that they show the same
problem of insufficient carrier suppression exhibited by the 20 meter
signal (although they do not show the "audio harmonic" issues which
were present on both the 20 meter AFSK and PSK31 bulletins).

The 80 meter broadcast (www.subich.com/80M_carrier_-35dB.jpg) shows
carrier only about 35 dB below the PSK level.  The 40 Meter broadcast
(www.subich.com/40M_carrier_-22dB.jpg) is even worse with carrier only 
about 22 dB below the PSK level as well as what appears to be 120 Hz
"hum" (note the two "humps" above the 40 meter PSK signal was another 
(interfering) signal - it appeared and disappeared independently of
signal being observed).  Even if one assumed a 6dB "back off" from
the CW/PEP level to prevent ALC action, carrier suppression of less
than 40 dB is not exactly "good engineering practice."

Modern DSP based transceivers are capable of essentially infinite
carrier suppression.  Absent excess IF gain and ALC, a properly aligned
analog balanced modulator should provide more than 40 dB of carrier
suppression and proper placement of the carrier on the skirt of the
transmit IF filter should attenuate any residual carrier by another 10
to 20 dB.  With an "S9" (-73 dBm) signal, the carrier of any properly
aligned SSB transmitter should be in the background noise at most urban
or suburban receive locations on any frequency below 25 MHz.

> The Station Manager uses a custom interface hard wired to the digital
> ports on the rear - not via the Mic connector as implied in your
> message.

The Icom "ACC" port is not immune to over drive any more than a mic
input; the only difference is that the clipping level is somewhat
higher.  However, Icom generally specifies pin 4 of the ACC connector
at 100 mV RMS - not substantially higher than the level of an electret
mic.  The effects of audio clipping in the mic amplifier or modulator
are quite distinctive and unmistakeable just as insufficient carrier
suppression is clearly visible on a spectrum analyzer but not on the
waterfall of most digital software - particularly when the receiver
is tuned to the same carrier frequency as the transmitter.

I stand by my observations and measurements - were I an active OO today
I would have sent a notice to the station involved for all three 
transmitters.  The signals do not exhibit good engineering practice
and there were no unaccounted for sources of interference to make the
observations ambiguous.

73,

    ... Joe, W4TV


On 1/29/2013 9:48 PM, Dick Kriss wrote:
> Joe Subich, W4TV
>
> It was pretty obvious you were talking about a W1AW with the "terrible" AFSK signal. There are no W1AW RTTY bulletins on Tuesdays so I listened to the early BPSK-31 bulletin on 14.095 and on 18.102.5 using a Kenwood TS-590S with a direct USB interface to fldigi-3.21.67 on an iMac with OS X 10.8.2. The W1AW signal was on frequency on both bands with solid print, clean looking BPSK-31 signals with none of the harmonics or birdies you mentioned.
>
> I am aware of the W1AW setup with Icom transmitters and the station manager tries very hard to be sure each band dedicated transmitter is correctly adjusted with no ALC showing. The Station Manager uses a custom interface hard wired to the digital ports on the rear - not via the Mic connector as implied in your message.
>
> I think you owe the Joe Carcia, NJ1Q, the W1AW Station Manager and this reflector a retracton for your public comments about the "terrible" W1AW signal and your statement "….One simply *can not* run "all sliders to the max" and "all knobs to the right" with AFSK."
>
> You may want to check your setup for local interference causing the harmonics and birdies you observed. There was no evidence of harmonics or birdies on the early 29-Jan-2013 PSK-31 bulletin.
>
> Dick AA5VU
>
>
>
>> Date: Tue, 29 Jan 2013 10:38:45 -0500
>> From: Joe Subich, W4TV <lists at subich.com>
>> To: MMTTY at yahoogroups.com, RTTY <rtty at contesting.com>
>> Subject: [RTTY] Bad AFSK
>> Message-ID: <5107ED05.6010405 at subich.com>
>> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
>>
>>
>> Elecraft have taken the lead on FSK bandwidth - thanks to W7AY, K0SM
>> and W0YK among others.  Lets hope other manufacturers follow quickly.
>>
>> Yesterday evening as I was looking for some RTTY activity to try the
>> new Elecraft "narrow FSK filter", I stumbled across another terrible
>> AFSK signal.  It just happens to belong to a well known club so I
>> won't give the call here but I have contacted the trustee.
>>
>> However, I've posted a screen capture from my P3 that clearly shows
>> what happens when over driving the mic input of an SSB transmitter at:
>> http://www.subich.com/BAD_AFSK.jpg (the last spike at about 14.0972
>> is a local birdie).
>>
>> Note this transmitter has not only "second harmonic" energy at
>> +1 KHz/340 Hz shift and "fourth harmonic" energy at +2 KHz/680 Hz
>> shift, it also shows insufficient opposite sideband suppression (or
>> possibly IMD in the PA - it's impossible to tell which).
>>
>> One simply *can not* run "all sliders to the max" and "all knobs to
>> the right" with AFSK.  Any manufacturer or individual who says to
>> do so is giving you bad advice.  Again, unless you have the tools
>> to make absolutely sure you are generating clean AFSK, *use FSK*,
>> the clicks are far less objectionable than 2nd/3rd audio harmonics
>> that are suppressed by -18 and -15 dB respectively.  This capture
>> was made during a fade - signal levels were generally much higher
>> and even the spurious signals were well above -73 dBm (S9+).
>>
>> 73,
>>
>>     ... Joe, W4TV
>
>
>