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Author Topic: BFO Frequencies (2)  (Read 725 times)
TANAKASAN
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Posts: 933




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« on: February 01, 2008, 08:32:36 AM »

This is a follow on to my previous question about BFO frequencies. I think I have this sorted out but I would like someone to confirm the following:

1) Assume that my transceiver has a 4.91520 MHz I.F.

2) As I find a 650 Hz sidetone frequency comfortable I set my C.W. B.F.O. to 4.91585 MHz and alter the RC components in my sidetone oscillator until they oscillate at 650 Hz,

3) If I receive a station who I want to QSO then I tune my V.F.O. until I hear a 650 Hz beat note in the speaker.

4) We carry out our QSO and we can both hear each other.

The two reasons I am puzzled are as follows.

1) What if the other station has a 500 Hz sidetone and his rig has the B.F.O. set to 4.9157?

2) Some rigs transmit C.W. in the upper sideband and some transmit in the lower sideband, how does this affect things?

I am sorry for all the questions but this is how I learn about the ham radio technology so I hope that you all can help.

Tanakasan
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AC5UP
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Posts: 3908




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« Reply #1 on: February 01, 2008, 08:57:03 AM »

Traditional CW has no sideband(s). It's just a carrier keyed long, short, or not (space).

What you may be thinking of is MCW or AFSK where a tone is modulated onto an SSB transmitter and either keyed or shifted to simulate CW or FSK. As for a difference between two rigs on the detected CW tone frequency, that's strictly a choice at the receiver.

Try this: Tune WWV during a segment when they're transmitting just carrier and clock ticks. Set your rig to CW. Wiggle the big knob. You can choose any beat tone you like, but WWV didn't need to change frequency to 'follow' you up or down the band. Wink
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AA4PB
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Posts: 12905




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« Reply #2 on: February 01, 2008, 10:04:14 AM »

As long as both stations tune for the tone that *their* transceiver is set to receive (most often matching their sidetone) then you will both be transmitting on the same RF frequency. If one station gets it wrong and you both keep retuning with each "over" you may walk up or down the band. The best way to tune in a station after the initial contact is to use the RIT so that you are only adjusting your receive frequency.
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KE3HO
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Posts: 235




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« Reply #3 on: February 01, 2008, 11:06:28 AM »

1) What if the other station has a 500 Hz sidetone and his rig has the B.F.O. set to 4.9157?

This is where RIT is handy. For example, if you call CQ on cw and someone answers you, you should assume that he has set his dial for a pleasant sidetone frequency. If you don't like the tone he is producing on your end, you should tune your RIT, not your main tuning dial. Otherwise, you will be chasing each other up or down the band at each exchange.

In general, the station who makes the initial call should not change his transmit frequency unless asked to by the other station or if mutually agreed to in order to move away from QRM/QRN.

73 - Jim
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AA4PB
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« Reply #4 on: February 01, 2008, 11:37:02 AM »

1) What if the other station has a 500 Hz sidetone and his rig has the B.F.O. set to 4.9157?
---------------------------------------------------
If he tunes so that your signal produces a 500Hz tone then his transmitter should be exactly on your transmit frequency regardless of whether or not you use a different sidetone/IF/BFO frequency. The only problem comes when he has a system set for 500Hz and tunes for a 650Hz tone. Then your two transmit frequencies are 150Hz apart.
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KA5N
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« Reply #5 on: February 01, 2008, 11:37:57 AM »

Radios use different IF frequencies.  Just because your radio has an IF of 4.91520 MHz, doesn't mean his will.  It might be 455kHz or 8.83 MHz or 9 MHz or any of several other frequencies.  So the BFO frequency will depend on the IF.  Usually the BFO is set for SSB so that voices sound natural, neither too bass or too high pitched.  For CW the BFO is set so that the beat note (the one you like) is set in the center of the passband of the filter.  You need to download a program that shows the bandpass of your receiver/filter and insure that the signals are properly located.  There are a number of such programs that use your computer sound card for this purpose.
Allen
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TANAKASAN
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Posts: 933




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« Reply #6 on: February 01, 2008, 02:09:22 PM »

Sorry, 'sideband' might have been the wrong term to use when talking about CW reception. What I was talking about was the option of having the CW BFO either 650 Hz above the filter frequency or 650 Hz below.

Which is the normal way of doing things?

Tanakasan
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AG4RQ
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Posts: 300


WWW

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« Reply #7 on: February 01, 2008, 03:40:30 PM »

"What I was talking about was the option of having the CW BFO either 650 Hz above the filter frequency or 650 Hz below.

Which is the normal way of doing things?"

Tanakasan, the pitch that you choose to receive the transmitted signal is up to you. It all depends what pitch is pleasing to your ears. Some like it at 500, some like it at 600, 700, 800 or 1000. On modern radios like my IC-718, I can adjust the CW pitch to anything from 300 to 900 Hz. I can choose to vary the pitch using my RIT until I like the tone I am hearing the signal at. The signal of the transmitted signal is one thing. The offset (pitch) which is in your receive is another. CW and reverse CW put the received signal either in the upper sideband or the lower sideband. It all depends how you wish to receive the signal you are copying.

73 de Mark
AG4RQ
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VK1OD
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Posts: 1697




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« Reply #8 on: February 01, 2008, 05:33:55 PM »

AC5UP said "Traditional CW has no sideband(s). It's just a carrier keyed long, short, or not (space). "

This is not true, it does have sidebands, the distribution of which depends on the shape of the keying and the keying rate.

You cannot modulate the carrier (keying, whether amplitude and or angular modulation) and then pretend that it does not have sidebands (ie constant amplitude and phase).

It is equally nonsense to suggest as some do that FSK, or other forms of FM, or PM do not have sidebands.

The existence of sidebands is the reason that all these modulated forms have a minimum necessary bandwidth that is greater than zero.

Owen
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W8JI
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WWW

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« Reply #9 on: February 01, 2008, 05:48:23 PM »

Tanasakan,

I think what you are missing is a proper transmitter does not transmit with the BFO frequency as a local oscillator.

Let's look at round number.

Say I have an IF frequency of 9.00000MHz. This is the center of the passband. Let's say it is 500Hz wide.

Normally the BFO would be set just outside the passband, the offset from the center would be your favorite pitch.

If you like 1kHz pitch the  BFO would be either 8.999 or 9.001 MHz. If you like 500 Hz it would be 8.9995 or 9.0005 MHz.

So you tune the signal and at the favorite pitch it is centered in the passband.

You cannot transmit that way however. When you transmit the BFO is shifted up to the filter center. It is at 9.0000 instead of being offset. This makes your carrier on the same frequency as the incoming signal when it is tuned for filter center, or your favorite pitch.

This is why the sideband does not matter. Changing sidebands changes the direction of pitch change as you tune the receiver, but when properly tuned it will be centered in the filter. This assumes also you use a narrow filter, and not a very wide filter.

Whatever the filter the carrier oscillator is set to match the TONE compared to the BFO. This is why it doesn't matter what filter you use or what sideband you use. So long as you offset the carrier properly when going from transmit to receive, the movement equal to the pitch you like, you will always transmit on the frequency you tune in.

As long as the other person follows the same rules you will not 'walk" up and down the band and the "sideband" will not matter.

Does this help??

73 Tom

 

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VK1OD
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Posts: 1697




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« Reply #10 on: February 01, 2008, 05:56:39 PM »

Tanakasan,

The convention is that you transmit and receive on the same frequency, not a rule, but a convention. You expect that someone calling CQ will listen in the same channel as they transmitted, unless otherwise stated.

Your transceiver is / should be aligned so that your receiver channel is centred about the transmitter's carrier frequency AND that a signal on the carrier frequency down converts to your desired beat note frequency.

To achieve this, the effective frequency of the local oscillator(s) needs to be offset from the carrier frequency in receive mode by an amount of the desired beat note frequency.

If your transceiver is properly coordinated in this way, the other station isn't aware of what beat note frequency you are using. A properly coordinated transceiver's sidetone oscillator is also set to the desired beat note frequency.

If your transceiver is properly coordinated, to tune exactly to the calling station's frequency (called netting), set your tx up in CW mode, VOX off, and press the key for sidetone whilst receiving the other station. Tune the VFO for zero beat between the received signal beat note and the sidetone oscillator. All done, you have netted exactly, now re-enable the VOX or use the PTT as preferred.

When it comes to oscillator design, on receive with narrow IF, the carrier reinsertion oscillator needs to be offset from the centre of the receiver passband by the desired beat note frequency. If that would put the oscillator within the filter passband (ie wider IF), move it so that it is 6dB or so down the side of the filter response. Then set the other mixing oscillators so that the dial or display reads the carrier frequency, and the receiver channel is offset by the desired beat note frequency.

Owen
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TANAKASAN
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Posts: 933




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« Reply #11 on: February 02, 2008, 03:46:35 AM »

"You cannot transmit that way however. When you transmit the BFO is shifted up to the filter center. It is at 9.0000 instead of being offset."

Ahhhh!!

That is the information I was missing, I assumed that the CW BFO only changed frequency when you changed mode from SSB, not when you changed from receive to transmit.

Thank you, Tom.

Tanakasan
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