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Author Topic: Frequency Offset in Homebrew Phasing Transceivers  (Read 651 times)
HA6SST
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Posts: 110




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« on: November 13, 2007, 12:54:59 AM »

OK, I need some help here. When using a homebrew transceiver based on a BFO and filter(s) it's easy to work out where you need to tune to work a station. You alter the VFO until the voice sounds about right then you give them a call, the BFO which is offset from the carrier does all the work.

What happens if your rig uses phasing for SSB/CW reception?

Let's use CW as an example. I tune around and find  station W1XYZ. With my rig in USB mode (there is no CW mode in a phasing rig) I alter the VFO until I get a pitch which sounds comfortable, in my case about 700 Hz. Looking at the rig I see that the frequency display reads 14.005. Should I then transmit on 140043, 14.005 or 14.0057 so that he can hear me?

HA6SST
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KE3WD
Member

Posts: 5689




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« Reply #1 on: November 14, 2007, 08:12:17 PM »

Not quite following how you would key a CW transmit if you are in USB mode...
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HA6SST
Member

Posts: 110




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« Reply #2 on: November 15, 2007, 01:31:36 PM »

There are two options to generate a CW signal when you have a phasing transceiver. Option one is to generate a very clean sinewave at (for example) 700 Hz then feed this into the modulating stages. Option two is to apply a keyed DC bias to the balanced modulators and this will generate a carrier.

Experimental Methods in RF Design has details of this but they do not mention the offset problem.

HA6SST
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KB1GMX
Member

Posts: 780




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« Reply #3 on: November 16, 2007, 06:13:14 AM »

On a phasing or other DC RX zero beat frequency is BFO
frequency.

So for CW you have an interesting case but it's simple and works.  If you tune for a 700HZ tone the BFO will
be offset by 700hz and if you transmit CW by offsetting
the balanced mixer you will have the correct offset.
This is the easy way for a phasing TX to generate CW
(small DC injection to the balanced mixer).

If you inject a 700hz tone to CW key you miss the
correct freqeuncy as the BFO already has the RX offest
of 700hz and your either 1400hz or 0hz offset as a result.  This is the hard way for phasing CW generation.

EMDRF does cover this but not as an example.  Usually
if you are doing Phasing RX and CW TX you do nto need
the excess hardware of a phasing TX (only needed for SSB).  However if you are doing multimode TX then to get CW (or am or DSBSC) out is easy and covered in references outside the EMDRF text.

Allison
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WB6BYU
Member

Posts: 13335




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« Reply #4 on: November 17, 2007, 07:55:04 PM »

Sounds like you need a straight answer...

If you are using a USB phasing receiver and your frequency is reading
the injection frequency (which should be the same as the suppressed
carrier frequency of a USB station properly tuned in) then when you are
set to 14.005 the signal you are listening to is actually on 14.0057.
Because it is the upper sideband, hence above the carrier frequency.

But the frequency readout on many receivers is already offset, so when
you have the received signal centered in your passband the readout
shows the actual received frequency (and the frequency you will transmit
on when you press the key) not the effective BFO frequency that the
signal is beating against.  To properly answer your question we have to
know what the frequency readout in your radio is actually displaying.

Probably the easiest way to do this in a phasing transceiver is to generate
a very clean 700Hz sine wave and feed it into the audio input as if it
came from the microphone.  That makes it easy to change the offset
to match a particular audio filter.
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