[RTTY] Bad AFSK

Wed Jan 30 07:18:53 EST 2013

Joe,

Your setup must be much better than my Kenwood TS-590S.  On Tuesday evening I did not see any of the stuff you observed.  The W1AW signal looked normal to me. If there is a problem with the W1AW Icom transmitters, I feel sure the Station Manager will take corrective action. 

I have seen some bad looking signal on the bands but W1AW was not one of them.

Dick AA5VU

On Jan 29, 2013, at 10:10 PM, "Joe Subich, W4TV" <lists at subich.com> wrote:

> 
> 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
>> 
>> 
>> 