[GreenKeys] The things I worry about.
Richard Knoppow
1oldlens1 at ix.netcom.com
Mon Sep 14 12:36:23 EDT 2015
I think it can be confusing. I see two things: one is that the only
important factor in the _received_ signal is the amount and direction of
the shift. The other is that if a conventional FSK generator, with
fixed output frequencies is used those frequencies must pass through the
SSB transmitter. Most transmitters have some limitation on the
bandwidth of the base band, i.e. audio. Typically, the passband is less
than 3Khz wide with a high audio frequency cut off of around 3 Khz. The
low frequency cut off is usually around 300 Hz and not a limit. If the
old 850 Hz shift is used with an audio frequency exciter the tones are
2125 and 2975 Hz. The mark frequency is too high for most transmitters
and will be at least attenuated, if not cut out, by the filter. If the
generator has variable center frequency fixing this is no problem, just
shift both tones down. Same with narrow shift keying except the amount
of shift needed may not be as much.
Now, the issue becomes one of how to specify the "carrier"
frequency. It must be described in a way that makes it possible to find
it. In fact, there is NO carrier for an SSB carrier suppressed signal
but the convention is to assume the carrier frequency based on the tones
reproduced by the receiver. In the case of speech this is evident by ear
(although some voices may be difficult to tune) but for RTTY it must be
defined in some agreed fashion. That is, if you have a group of
stations, say a net, each using the same amount of shift, the receiver
will be tuned to place the tranmitted tones in its passband and also
must produce the tones the RTTY adaptor requires. That means the
_indicated_ carrier frequency of the receivers may not all be the same
for the same transmitted signal. Perhaps the answer is to specify the
"carrier" as the center frequency of the RF signal and the frequency to
be tuned to to produce the correct tones from the receiver be determined
for each receiver based on its characteristics. Of course, I am writing
mostly about old fashioned detectors with fixed frequencies and not the
computer based ones that can track both mark and space frequencies over
a large range.
Also of course, on the transmitting side I am assuming the method
of feeding audio tones into a conventional SSB transmitter and not one
driven by a frequency shift oscillator.
On 9/14/2015 9:05 AM, Alf Fisher wrote:
> I find it all a bit confusing and, if someone could do it, a couple of
> diagrams may well make it simpler to understand where the various
> frequencies fit within the filter passband on USB and LSB.
> Secondly, I thought that 2125 and higher tones had gone out and we now
> use low tones of 1445 /1275 to make a better fit within the filters of
> SSB rigs.
> The higher frequency tone being the mark and downshift to the lower
> frequency tone on space.
> I guess its easy to transmit "upside down" with the wrong combination
> of tones and sidebands.
>
> 73
> Alf
> G3WSD
>
>
> -----Original Message----- From: Ralph Mowery
> Sent: Monday, September 14, 2015 4:09 PM
> To: greenkeys at mailman.qth.net ; Jeffrey D Angus
> Subject: Re: [GreenKeys] The things I worry about.
>
> When running the sound card (audio input) to a ssb transceiver there
> are 3
> ways the frequency has been described in the past. The first is the
> acutal
> MARK frequency as read by a frequency counter or such . The second
> and not
> very common is to split the mark and space frequencies, that is the
> frequency that is in the middle of the two tones. The more perferred
> way is
> to use the dial frequency of the transceiver, but only if using the MARK
> frequency of 2125 Hz as fed into the transceiver while operating in the
> Lower Sideband mode. If using USB or some other tone for the Mark, then
> you have to recalculate the 'dial' frequency. If using the 3 rd
> method, you
> set your audio input for a 2125 Hz mark frequency, transceiver to lower
> sideband, and the Space tone to be higher by whatever shift you want
> to use.
>
> While not too common now there used to be the esepression LSMFT from
> the old
> Lucky Strike cigarette commercials. Low space means fine teletype. The
> space frequency was ( and should still be) actually lower than the mark
> frequency on the low bands.
>
> Someone discovered that you could feed audio tones into a SSB transmitter
> and if only one pure sine wave tone was sent, you could not tell if it
> was
> actually audio or just a carrier (not counting the 30 or 40 db of the
> other
> sideband and carrier was surpressed). By using LSB the Mark tone
> comes out
> of the transmitter higher in frequency than the space tone. If using USB,
> then the space tone is higher in frequency. By using LSB the same
> demodulator/modulator could be used for VHF FM and AM rtty as was used on
> the lower bands.
>
>
> If someone is using the 200 Hz shift, the Mark stays the same at 2125,
> but
> the Space is shifted up 30 Hz. This means that when tuning in , you
> shift
> your receiver up 15 Hz to center the tones. This will then throw off
> your
> transceiver by 15 Hz if you are using 170 Hz shift. The beter way
> would be
> to tune in the Mark tone and use the RIT of the transceiver for best
> demodulating of the 200 Hz shift.
>
> With a SSB transceiver, any tone pair could be used that is around
> 170 Hz
> apart. However you want them to be high enough so the 2 nd harmonic
> of the
> lowest tone is over 2500 Hz or so if possiable. That lets the side band
> filters filter out more if there is any distortion of the wave form.
> YOu do
> not want the highest tone (normally Space) to be over about 2200 Hz as
> this
> starts to filter out the Space tone and it will be weaker than the Mark
> tone. For the ones using the 850 Hz shift as the old military gear, you
> need to adjust to lower tone pairs. I forgot what the pairing is, but
> there
> was a 'standard' called the LOW tonee where the Mark was somewhere
> between
> 1000 and 2000 Hz.
>
> I am not sure what tones the PK-232 actually uses, so you will have to
> calculate where to call the frequency.
>
>
>
> ----- Original Message ----- From: "Jeffrey D Angus" <jdangus at att.net>
> To: <greenkeys at mailman.qth.net>
> Sent: Monday, September 14, 2015 1:37 AM
> Subject: [GreenKeys] The things I worry about.
>
>
>> Running RTTY.
>> Assuming 170 Hz shift, and 2125 Hz mark tone.
>> What frequency do you tell people you're at?
>> If your receiver is in lower sideband mode and your marking frequency is
>> 7097.875 KHz, do you tell people you're transmitting on 7100 KHz. That
>> would be where the suppressed carrier isn't if you're running SSB AFSK.
>> Do you tell them you're transmitting at 7097.875 KHz because that's
>> where the mark tone is. Or do you tell them you're at 7097.790 KHz
>> because that's the mid point between your mark and space frequencies,
>> especially if you're running true FSK and there isn't a suppressed
>> carrier?
>>
>> What do you tell them if you're using an AEA PK-232 and you're too lazy
>> to change the default RTTY shift from 200 to 170 Hz?
>> Which tone is 30 Hz off frequency, or are both tones 15 Hz off
>> frequency.
>>
>>
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--
Richard Knoppow
1oldlens1 at ix.netcom.com
WB6KBL
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