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Author Topic: CW reception, IF, LO, BFO and my confusion  (Read 2789 times)
KC8RFV
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« on: April 05, 2013, 02:00:22 PM »

I am obsessing about my Ten Tec Argosy II and wondering how the magic works when it receives CW.  It is operating perfectly, I am just an amateur trying to understand how the parts work.  Here are the facts for the receiver part of the rig:  superhet, IF of 9.0Mhz, fixed sidetone of 750Hz, CW mode BFO is 9.000750 Mhz.  The PTO/LO feeds a frequency of 5.0 to 5.5 Mhz to the receive mixer to yield the 9 Mhz IF.  I operate almost exclusively CW on 20m.

Like all of you, I've observed that a CW signal at, say, 14.060 Mhz will go from higher pitch to lower pitch as I tune from below 14.06 to above 14.06.  During such tuning, the PTO/LO signal is going from (for sake of argument) from 5.058 Mhz to 5.062 Mhz.  The receive mixer then results in an IF of 9Mhz.  This 9Mhz combines in product detector with BFO frequency of 9.000750 Mhz thereby yielding the pleasant tone of 750 Hz.  I understand.  What is confusing me is this:  this process does not explain the changing tone as tune through the transmitted signal.  Can anyone help me understand?  I'm hung up on the fact that regardless of where you tune, you will have a 9Mhz IF which results in 750 Hz tone no matter your relation to transmitted CW signal. 

I hope this question is clear!  Thanks for help.  I'm trying to deepen my knowledge of receiver design. 

72,
Charlie
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AC2EU
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« Reply #1 on: April 05, 2013, 06:31:49 PM »

Look up heterodyne.

The IF is a fixed "Intermediate Frequency" amplifier that also heterodynes with the VFO frequency to produce the 9 mhz.
In fact your radio is probably "double conversion" in which there is a higher frequency IF before the 9 mhz.
The superheterodyne receiver was an advance from the early "TRF" receivers that required tuning every stage as you changed frequency.
You will get the 750HZ tone because that is the preset offset from "zero beat" which allows you to hear the signal.

On an old receiver that had - 0 + BFO, you could set it at zero to tune until you heard no tone (zero beat) then adjust the bfo for the most pleasing tone.
The heterodyne ripples though the whole system, so to speak, from difference between the VFO and the received signal to the speaker. There are "beats" produced if you are off frequency which affect the 750 hz tone.
If you are perfectly tuned without the BFO, you would probably hear nothing!

I hope that helped.
Pehaps someone else can explain it better...
« Last Edit: April 05, 2013, 06:42:34 PM by AC2EU » Logged

K8AXW
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Posts: 3677




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« Reply #2 on: April 06, 2013, 09:07:18 PM »

Charlie:  It's good that you're interested in learning about what is going on in your rig.  Check out this link to a W2AEW YouTube video on RF mixers.  This is about as clear and easy to understand as it gets.

Note also Alan's many other videos listed at this link.  Each one is equally easy to understand and informative.

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KC8RFV
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« Reply #3 on: April 07, 2013, 07:06:56 PM »

Thanks for all of the help.  W2AEW's youtube videos helped.  I wonder if there will ever be a solid state version of YouTube.  Hi Hi.

After pouring over my books and the circuit diagram for the TT 525D Argosy, I've come to the following conclusions.  This particular rig indicates the LO frequency on the digital readout at all times.  In CW receive mode, a 9Mhz IF is beat against a 9MHz BFO.  So in order to hear a CW signal, one must tune the VFO to get a 750Hz audio tone.  On transmit, the BFO adjust to 9000750 Hz to combine with the same LO frequency.  The end result is that both transmitters transmit on the same frequency.  I think this is a bit unorthodox, but interesting nonetheless.  What is particularly curious is that the digital readout of frequency is 750Hz inaccurate on transmit (in CW and SSB reverse only).  The TT manual specifically states the same.

I confirmed these theories by listening to K2SDR's WebSDR in NJ.  I was able to compare his waterfall sigs with the same signals I'm receiving here in NYC.

This has deepened my knowledge of transceiver design. 

73,
Charlie
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K7KBN
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« Reply #4 on: April 09, 2013, 05:03:18 PM »

Thanks for all of the help.  W2AEW's youtube videos helped.  I wonder if there will ever be a solid state version of YouTube.  Hi Hi.

YouPill?   Grin
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73
Pat K7KBN
CWO4 USNR Ret.
W9GB
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Posts: 2600




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« Reply #5 on: April 09, 2013, 05:59:18 PM »

Quote
YouPill?
Our Brick ...
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