[Elecraft] BFO Dumb Question

Sat Mar 2 20:29:00 2002

Jason

Technically, there really isn't a BFO.  The BFO was something you used back
in the days where we always did AM detection.

Suppose you have a signal at 7040.  Let's also suppose that your VFO will
operate at 7040-4915=2125 KHz.  Now the VFO and incoming signal are injected
into the double balanced mixer resulting in the signal being centered in the
4.915 MHz IF passband.  So far, so good, the crystal filters whack out any
nearby signals and the only thing making it to the product detector is the
original signal, now translated to the IF frequency.

Here's where it gets weird.  The product detector needs a local oscillator,
close to the 4915 IF frequency.  However, it shouldn't be EXACTLY 4915 (as
it sounds yours might be).  If it were, then when you were tuned exactly to
the signal so that it is at maximum strength through the IF, you would be
exactly zero beat in the audio amp,  zero beat here meaning zero beat, zero
frequency, below what you can hear.

However, if the local oscillator were, say, 700 Hz above 4915, at 4915.7, or
below, at 4914.3, you would hear a 700 Hz tone.

Let's suppose your LO was working at 4915.7.  As you tune down, 2 things are
happening.  First, the signal in the IF is moving up.  Say you move the VFO
down 200 Hz, to 2124.8.  Now the signal is at 7040-2124.8=4915.2 in the IF.
The tone you hear is now lower, only 500 Hz.  But there is something else
going on.  The signal in the IF is starting to move away from the center of
the IF passband, so it's getting weaker, too, as it runs into the sides of
the filter.  Depending on your filter settings, by the time you get to, say,
2124.3, not only is the tone so low you can't hear it, but it has gotten
very weak, hopefully inaudible.  As you tune farther, the tone will start to
increase again, as the signal in the passband crosses 4915.7, but if the
radio is aligned properly, the signal is so weak you can no longer hear it.

The exact same thing happens if the LO were at 4914.3, except it would be
backward.

You may hear a very strong signal on the other side of the "null", but
unless the signal is extremely strong, you shouldn't hear it at all on a
properly aligned receiver.

Hope this helps
72/73 de WB8RCR    http://www.qsl.net/wb8rcr
didileydadidah     QRP-L #1446 Code Warriors #35

----- Original Message -----
From: "Jason Hissong" <[email protected]>
To: <[email protected]>
Sent: Saturday, March 02, 2002 7:57 PM
Subject: [Elecraft] BFO Dumb Question

> I have a silly question...
>
> I notice that tuning up and down the CW subbands, I here the CW signal on
> both sides of the BFO frequency.  For example, tuning up the band, say I
> start on 7022.0.  I start to hear a CW signal at 7022.44.  I keep tuning
up
> and I can zero beat him at 7022.71. Tuning up, the CW signal disappears at
> 7023.4.  I zero beat him again on 7023.93.  Then disappears again at
7024.5.
> Which side is he on?  (i.e., which frequency is he transmitting on).  I
> think 7023.4.  If that is the case, which side of the BFO should I zero
beat
> on him?  I understand what the BFO does and what I hear in the speaker is
> the difference BFO frequency.
>
> I do not know why I have never thought of this before.  I think in the
past,
> I zero beat on the higher frequency (in my example, it would be the
7023.92)
> without giving it much thought.
>
> Sorry for the completely silly question... you can reply direct to me...
> (including flames :)
>
> Thanks
> Jason N8XE
> K2 #1270
>
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