Adding het detection to your Ultralight receiver

dhsatyadhana <satya@...>

Hi all:

One thing that ultralights don't have, by definition, is a BFO by
which a heterodyne may be heard to indicate that a weak station is
there. With the Sony 2010 or other communications receiver, tuning
slightly away from 774 khz (say to 775 khz) with the BFO on (i.e., in
SSB mode) will produce a 1 khz het which lets you know that JOUB-
Japan is there. Simply jumping up and down the dial by 9 khz will
conveniently produce a het to show that a TP/TA is present on a given
channel. This can serve as a "spotter" receiver to indicate when a
prize catch can be had on another receiver.

As discussed at and perhaps
elsewhere, you can induce a beat frequency into your Ultralight with
another radio! This is easily done with two SRF-59's (and we all
have at least two of them...). For example:
1. Tune the listening receiver to a station at 730.
2. Place the BFO unit directly on top or underneath.
3. With the BFO unit turned ON, and volume all the way down,
tune it about 50 khz lower (i.e., to 680 khz or so).
The 50 khz IF on the SRF-59 means that, on the BFO unit,
a beat frequency of 730 khz is being produced in order to
tune in 680 khz.
4. Since the 730 broadcast signal on the listening unit and the
730 khz beat frequency on the BFO unit are now colliding with
each other, you should hear a fairly pronounced heterodyne!
5. Move the BFO unit around a bit to adjust the intensity of the het.

The 55-khz (or so) IF on the SRF-59 family works well, because you
can cover nearly the entire dial. For instance, tuning the BFO unit
to 525 khz (its lower limit) produces a beat frequency of around 580
khz, which can then be fed to the listening unit. Contrast this with
using a BFO unit with a 455 khz IF: the lowest beat frequency it can
produce is (455 + 525 =)980 khz, meaning that the listening unit can
only be tuned as low as 980 khz while still being able to take
advantage of the het production.

What I have noticed is that the Sony S5W, with its humongous RF
stage, puts out a huge BFO signal, which may be more convenient if
you are working the upper part of the band - you can be several feet
away and still pick up the beat frequency! As such, the enhanced RF
stage on a super-modified SRF-39 might make it a good BFO unit for an
Eton E100 listening unit.

On thing to watch - you will likely notice that sensitivity on the
listening unit goes down. What appears to be happening is that the
BFO signal from the BFO unit, when received on the listening unit,
causes the AGC on the listening unit to reduce the overall gain. In
other words, if the broadcast signal and BFO signal are about the
same, the AGC will cut the gain in half in order to keep what it
believes is a constant audio level. Therefore, judiciously place the
BFO unit a little farther away in order to keep the gain up while at
the same time still inducing a beat frequency into the listening
unit. Alternately, I suppose you could super-modify your listening
unit to switch out the AGC!

Being somewhat inland and not prone to getting up a 4:30 AM (lying
prone often seems preferable, and unlike John B. I don't have cats to
push me out of bed), I haven't been able to try this out with actual
trans-oceanic signals, and so I would be delighted if someone out
there (Alan in NF??) would give this a try to see how useful it is.

73 - Kevin S
Bainbridge Island, WA


Interestingly I was also thinking about the possibility of using a BFO
with an ULR this afternoon. I discovered an interesting article at a few
minutes before reading your post. It includes a BFO circuit that
someone might be able to adapt to an ULR - most likely the E100.

Richard Allen