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Author Topic: bfo  (Read 4020 times)
KE5PYF
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Posts: 15




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bfo
« on: August 01, 2012, 02:35:48 PM »

Hello all

Is it necessary to zero beat with new transceivers?

Tnx David
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PA0BLAH
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Posts: 0




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« Reply #1 on: August 01, 2012, 02:56:47 PM »

In general in ham radio CW stations in QSO work on the same frequency. With old and new transceivers.

That is to save frequency space, and also to avoid QRM, when another station is thinking your transmission frequency is free (not occupied) while you are listening to your QSO partner on a different frequency.

So zero beat means interference frequency zero between the carriers, but you can't copy Morse code with zero beat, so the side tone generated when you are zero beat with your QSO partner is 600 to 800 Hz, due to the fact that your BFO is not zero beat.

When you have a direct conversion receiver you have to tune for an audible beat. That is possible left or right on the frequency scale of your receiver, of the carrier frequency of your QSO partner. When you start transmitting you are hence 600 to 800 Hz higher or lower than his carrier, He re-tunes, and so you walk step wise through the CW band. So start low in the band and you end up high with a multi changeover QSO. Aren't you?

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VA7CPC
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« Reply #2 on: August 01, 2012, 05:48:19 PM »

With new transceivers (in CW mode), the rule is:

. . . When the received signal's pitch is the same as the pitch of your own sidetone, you are "zero-beat" with the received signal.

So you "zero-beat" by tuning the rig so that _his_ pitch is the same as your own sidetone pitch.

Most rigs have some way of generating an audio tone at your own sidetone pitch, without transmitting -- sometimes called a "spot tone" feature.

Most modern IF filters are sharp enough so that you don't have to worry about being "on the wrong side of the BFO".  [Explaining what that means requires yet another post . . . ]

           Charles
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K4KRW
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Posts: 99




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« Reply #3 on: August 05, 2012, 10:16:12 AM »

Some transceivers like my FT-897D have an LED indicator that flashes with the signal when you are zero beat.  I really wish all transceivers had this.  It works perfectly and is very user friendly.  You can add the feature yourself.  I have added it to my QRP rig.

This is the kit I used.
http://www.wb3aal.com/Pages/K6XX/K6XXCWIndicatorKit.htm

73 - Richard
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STAYVERTICAL
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Posts: 875




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« Reply #4 on: August 05, 2012, 03:22:59 PM »

Hi David,

As you can see from the above good replies, the answer is yes.
But how you do that depends a bit on the mode and the transceiver.
If you observe the frequency of your new rig, and switch modes from USB to CW for example, you may notice a change in dial frequency.
This is commonly around 700 or 800 hz, depending on the new rig - and how the menu option for the CW offset is set up.

For SSB the zero beat is obviously when the voice sounds most natural (assuming R.I.T. is off).
In many cases guys set their rigs to integer values of frequency 7.0700 rather than 7.732 for example.
So you are pretty safe going for integer values in these cases because modern transceivers have good frequency control.

When using CW, as others have said better than I, you match the tone of the incoming signal with that of your CW offset (700/800hz).
This means your transmitter frequency will be the same as the incoming signal.
However - sometimes you will hear some DX stations say UP or DOWN 2 for example.
This means they are listening higher or lower 2Khz for replies.
Most new rigs have two VFO's and in this case you set the transmit VFO to where you want to transmit and use the other VFO for receive - split mode.

For PSK31 operation - you normally just set your transceiver TX/RX frequency to the beginning of the segment, for example 14.070Mhz.
The dial is then generally not touched, and you use your digimode program to select the station you want to qso with.
PSK31 (and most other digimodes) expect USB transmission.
USB is only an agreed on sideband, and there is nothing to stop you and another station both using LSB - but USB is the normal mode.

Another factor which comes into play are filters.
On modern transceivers, each mode employs filters of different widths and passband placement.
For SSB many filters centre on 1500Hz for example and are from 1.8 to 2.4Khz wide.
For CW, filters centre from 600 to 800Hz and are typically from 250 to 500Hz wide.
For Digimodes, filters centre from 1000 to 1200Hz and are typically 500Hz wide.

However - modern transceivers have a universe of menu settings which enable you to change many of these filter settings.
So, a good read of your manual is going to pay dividends in the long run.

I hope this post has not confused you, and wish you good dx.

73 - Rob
« Last Edit: August 05, 2012, 03:26:42 PM by STAYVERTICAL » Logged
K8AG
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Posts: 352




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« Reply #5 on: August 08, 2012, 12:55:41 PM »

The K3 has a feature that can auto tune for zero beat.  Actually seems to work better than I expected.

73, JP, K8AG
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GILGSN
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Posts: 208




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« Reply #6 on: August 12, 2012, 08:36:30 AM »

Hello,

I use an iPod app called "Spectrogram" to zero-beat any station to my sidetone. You can see the station's tone on the screen, and turn your tuning knob until the line hits your sidetone frequency mark. Very easy to do.. For an LED zero-beat circuit, look here: http://home.windstream.net/johnshan/cw_ss_zerobeat.html

Gil.
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PA0BLAH
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Posts: 0




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« Reply #7 on: August 13, 2012, 06:45:22 AM »


Pretty sloppy design Gil. For R1  the series resistor with a Zener diode with value 6.2 volt, he advices a value that sets the zener diode "dry " at 15,3 mA (not MA because that means Megaampere).

According to the spec sheet of this Touch Tone Decoder, ( http://www.datasheetcatalog.org/datasheet/nationalsemiconductor/DS006975.PDF ) the used current without the load (yellow-LED) current is typical without and with lock between 8 and 14 mA (milliampere) Add the current of he load of the yellow LED estimated 10 mA, and you can watch that "typical" during lock (burning LED) the Zener stands dry , and hence the Vcc voltage is not 6,2 V and hence al the parameters are different.

All stuff of no code technician licence requirements, Isn't it?

Who cares. We are all sucking hams, aren't we? Only buying equipment and not anymore able like our predecessors, to create their own equipment from parts, just around in the junk box.

The developer of the circuit probably explains "It works here, I marketed a kit, and if it doesn't work at your location after assembling, on which fact you can be very proud, because only the most stupid guys buy  kits, and so I never ever get a complaint, because they fail to assemble it, you must have made a stupid error."  "Stupid questions do not exist, so your question does not exist, All sat"  "The overpaid components in the kit, with which  I paid may luxury world wide cruise vacation, are factory guaranteed. So contact Intel, Vishay and so on to file your complaints.  You must be a pretty dumbo, a typical kit-buyer, kind of after 40 years retired Morse number station copyer, I suppose; so shut up please instead of wasting my valuable time" .

So not my words, I am a friendly Dutch guy, all Dutch guys are friendly, the most friendly people you ever can meet, but it are the supposed thoughts of the United States kit marketeer.

When you are still reading, and want some additional comments about lock-time and bandwidth of the touch tone decoder, and miss the brains to find out for yourself from the refered specsheet, I am at your service, all for free for the asking.

And Oh yes, excuse my crippled English, not my mother tongue . Dahdah stays for milk but also for melk. When you are a typical proud kit builder just as K8AXW, reading and understanding my message may require a lot of ham spirit as he said before in the forum.

So better annoy it, like  K0CBA says he does, and ask other guys around here what the meaning is, saves time.

Bob
« Last Edit: August 13, 2012, 06:53:53 AM by PA0BLAH » Logged
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