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Author Topic: Why does the USB show lower freq than the carrier?  (Read 519 times)
KD6CPA
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Posts: 84




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« on: April 30, 2007, 07:42:35 AM »

New /AG general here.  Passed the general exam 2 days ago.  Have had my Icom 730 for a month or so, listening pending getting the general.  After successfully passing the test, I came home and tried the rig on TX.   I tune in a freq on AM mode (say 7245) to tune lowest SWR on the tuner.  Then when I set the rig to LSB to do a call, why does the transmitter freq shown go to 7246.5?  If it's lower sideband, if it's going to change at all (and I don't understand why it does), I'd intuitively think it should show 7243.5 if the regular AM freq is 7245?  If I want to make a contact on a net that is advertised as held on 7245 LSB, with my rig set to LSB mode, do I tune to 7245, 7243.5 or 7246.5?
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KX8N
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Posts: 543




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« Reply #1 on: April 30, 2007, 07:48:28 AM »

"If I want to make a contact on a net that is advertised as held on 7245 LSB, with my rig set to LSB mode, do I tune to 7245, 7243.5 or 7246.5? "

You keep it in LSB mode, and tune to 7245. You keep it in whatever mode you intend to use - you don't find a signal with one mode, and then switch modes to make the contact.
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KB4QAA
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Posts: 2235




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« Reply #2 on: April 30, 2007, 09:55:43 AM »

The reason is partly explained by the name SSB, Single SIDE BAND.  

AM and FM both tune directly on the center or carrier frequency.   LSB and USB side bands are found on either side of the center or carrier frequency.  To be accurate, in SSB, the carrier and the opposite sideband are removed and never transmitted so we can't use them for actually tuning.  

Military and older ham radios do not adjust for the sideband difference meaning the operator has to calculate the value for the "window" vs. the center center frequecy, by subtracting or adding 1.5 or 2.0 khz.   Fortunately for us, most ham rigs today show the correct frequency if you are in the same mode, but not if you switch between AM - SSB or CW, nor between CW and USB, except for some of the newest digital radios.

So just look at your hand with three fingers side by side and think of CW & AM in the center frequency and LSB/USB 1.5khz on either side.

73, Bill
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W8JI
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« Reply #3 on: April 30, 2007, 11:34:31 AM »

Actually in a good modern radio design the carrier should NOt shift when you go from upper to lower to AM.

In some older radios the carrier used to shift only because they used a single frequency SSB filter and had no way to readjust the VFO to match the new carrier frequency, but there is no excuse for that problem in a synthesized radio.

My FT1000D for example sits right on a given frequency as I push AM, LSB, and USB. Doesn't move a bit. My newer FT-1000MP MKV and other rigs don't move a bit either.

Now on older radios that used a single filter, they sometimes moved the carrier to one side or the other of the SSB filter. That meant on LSB the frequency was about 3kHz higher than when on lower sideband, and AM was on either the LSB or USB frequency.

So what you have is really a design quirk in the radio. If the radio has a synthesizer frequency control there is really no excuse for this happening, except someone just didn't care.

In older radios that used crystal controlled oscillators and VFO's instead of digital synthesizers, it was pretty normal to have a shift.

73 Tom


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W8JI
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« Reply #4 on: April 30, 2007, 11:44:47 AM »

By the way, if the radio is calibrated perfectly and you know the frequency, you set it to the frequency of the net. Not lower...not higher. It does NOT change with the sideband you use nio matter how the radio behaves when you switch modes.

Technically what you should actually do is learn to recognize when a signal is tuned in properly. This would be without burble or warble or a wrong pitch.

You would then tune the radio to be so it SOUNDS perfect on the net control operator's voice, or to the voice of whoever you are calling. You would, ideally, ignore the dial reading and pay attention ONLY to the sound of the voice with the RIT or clarifier turned off.

That's how an experienced SSB operator tunes. An experienced op almost never tunes for a dial reading, except to find the basic spot.

The reason you tune this way, by only perfect natural pitch and no harmonic mixing or warble, is we are not channelized and radios are not all perfect. So you really technically should just adjust for perfect voice on whoever established the operating frequency and ignore the dial.

73 Tom



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KD4RBG
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Posts: 92




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« Reply #5 on: April 30, 2007, 12:25:50 PM »

"The reason you tune this way, by only perfect natural pitch and no harmonic mixing or warble, is we are not channelized and radios are not all perfect. So you really technically should just adjust for perfect voice on whoever established the operating frequency and ignore the dial."

And by doing it this way, you leave open the possibility of entertainment, because you can enter into all sorts of arguments with people about who is and isn't "on frequency".  If you happen to have a frequency counter with recent calibration traceable to NIST, they will tell you that NIST must be wrong.

And they won't be kidding when they say it, either.  

The first rule of Ham Radio is that *nothing* is ever wrong with your gear, it is the other person every time, and to suggest otherwise is a formal Declaration of War.  I learned this lesson putting up repeaters.

Jeff/KD4RBG
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W7ETA
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Posts: 2528




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« Reply #6 on: April 30, 2007, 01:43:56 PM »

Tune in the station you want to talk with.

My current radio has an analog display.  I tune in whatever station I'm interested in.

I can adjust the dial using a calibrater, I then make sure I'm not operating too close to a band edge.

73
Bob
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W8JI
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« Reply #7 on: April 30, 2007, 05:29:51 PM »

"The first rule of Ham Radio is that *nothing* is ever wrong with your gear, it is the other person every time, and to suggest otherwise is a formal Declaration of War. I learned this lesson putting up repeaters. Jeff/KD4RBG "

And the problem is getting worse.

Try this experiment. Find someone, assuming you know the limits of your receiver, who has bad splatter and isn't even really that strong. Then in the nicest possible way wait until he finishes and then call him and mention that he might be a little wide.

Watch what happens.  :-)

It used to be, years ago, that someone would take an OO card or constructive report seriously and try to determine if they had a problem and if they did correct it. Now the helpful person is considered a "policeman" and everyone is discouraged from saying anything negative about the very worse situation.

This forces people into not giving their calls when they make a negative signal report, and has virtually killed the OO program.

It isn't a good trend for our future.

73 Tom

 
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AA4PB
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Posts: 12638




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« Reply #8 on: April 30, 2007, 06:07:57 PM »

The dial frequency is correct, always reading the suppressed carrier frequency. Simply set the frequency AFTER selecting the appropriate sideband or mode.

As others have said, it has to do with placing the upper or lower sideband signal within the IF filter passband. On almost all radios the actual VFO frequency shifts by approx. 3KHz when switching between USB and LSB. In the later designs, the processor offsets the dial reading in order to correct for the shift in VFO frequency so that the operator doesn't see it.

If you want to work a net on 7245KHz, just select LSB first and then dial in 7245KHz.
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W7ETA
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Posts: 2528




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« Reply #9 on: April 30, 2007, 08:25:47 PM »

Tune to net control. It doesn't matter what your display says.
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