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Author Topic: SW-40+ birdy on 7.110  (Read 10818 times)

Posts: 18

« on: September 28, 2013, 12:24:14 PM »

Years ago I built a Small Wonder Labs SW-40+ transceiver kit but never used it much other than to confirm it worked. This weekend I decided to dust it off and noticed a loud noise at 7.110 which persists after disconnecting the antenna.

Has anyone else had this problem or have any suggestions on how to fix it? I re-wound L1 and installed a different C7 which put the radio in the lower end of 40 and the birdy went away but when I put in a different C7 to go back to the novice subband the noise is right there at 7.110. It's very loud and painful with headphones on, even with the gain knob all the way down.

I googled around but only turned up some false leads. There must be lots of these kit radios setup for the novice subband and I wonder if they have this problem. I wouldn't mind if it was at 7.095 or 7130 but right on the QRP calling frequency hurts.


Posts: 17476

« Reply #1 on: September 28, 2013, 06:40:08 PM »

Typically such birdies are due to the relative frequencies of the VFO and the BFO.  What
is the IF frequency of your kit?  (I ordered the board for the predecessor 40-40 transceiver
from FAR circuits, then stuffed it with parts from the junkbox, so I know mine isn't typical.)

The problem usually appears when, for example, the 3rd harmonic of the VFO beats against
the 5th harmonic of the BFO, giving an audio note.  (There are many combinations that will
do this, which is why choice of the IF is a major factor in designing a superhet receiver.)
I have a similar problem with a 80m receiver around 3.640 MHz, where the VFO on 3.185 MHz
beats with the 7th harmonic of the 455kHz BFO.  Reducing the harmonic content of the BFO
would help, but so far it hasn't been a problem.

In your case, the "simplest" fix might be to change the VFO frequency to beat against the
signal frequency from the other side:  for example, in my case I could put the VFO on 4.095 MHz
instead.  (Unfortunately, that is still a multiple of the BFO frequency.)  Depending on the
combination of frequencies that is causing a problem, this might at least shift the spur to
a spot where it isn't bothersome.

Another approach would be to replace all the crystals with another frequency not too far
away (for example, 4.000 instead of 4.196, etc.) and readjust the VFO frequency accordingly.
The basic circuit works over a fairly wide range of IF frequencies, from perhaps 1 or 2 MHz
up to 9 MHz or so (though the selectivity widens a bit at the higher frequencies.  I used some
surplus crystals around 5.7 MHz in my unit.)  This takes a bit more work, but there are enough
standard crystal frequencies to choose from that it shouldn't be very expensive.  The only changes
are the crystals, the VFO tuning components, and perhaps the capacitor and inductor in series
with the BFO and TX carrier crystals to get the right offset relative to the filter.
« Last Edit: September 28, 2013, 06:47:39 PM by WB6BYU » Logged

Posts: 18

« Reply #2 on: September 28, 2013, 07:41:42 PM »

Thanks for the response, The IF is 4MHz. All of the crystals are marked 4.000Mhz.

I found this site with descriptions of nearly every component in the radio which is helpful.

How do I change the VFO frequency to beat against the signal frequency from the other side? It would be great to shift the noise 20Khz in either direction. Is it as simple as changing a capacitor or another component with a slightly different one? I have a general idea of how these radios work but I don't know much about making changes beyond alignment.


Posts: 17476

« Reply #3 on: September 28, 2013, 10:28:25 PM »

C2 through C10, along with L1, set the VFO frequency.  You probably can get by with just changing
one or two capacitors, or the coil, to shift the tuning.

With a 4 MHz IF the VFO would tune 3 - 3.1 MHz to cover 7000 to 7100kHz.  To cover the same band
from the other side would require that the VFO tune 13 - 13.1 MHz.  You'd probably want to change
both the coil and some of the capacitors to do that, and it would affect the frequency stability (it is
more difficult to make  a stable oscillator at a higher frequency.)  It's probably easier to change all
the crystals to 4032, 4096, 4194 kHz (all standard values) and then just tweak the VFO a bit

In this case, with a 4 MHz IF the oscillator is on 3.11 MHz when you hear the spur.  I don't see a
likely combination of harmonics that will do that:  if it were the 4th harmonic of the VFO beating
against the 3rd harmonic of the BFO the spur should be closer to 7.000 MHz.  It's possible that
the BFO crystal has a spurious output - unfortunately it is difficult to tell without a spectrum
analyzer (which I'm guessing you don't have handy.)  You can try just replacing the BFO crystal
with one of the others to see if that helps.  But it may be worthwhile understanding the
mechanism that is causing the spur before we resort to shotgunning parts to try to make it
go away.


Posts: 18

« Reply #4 on: September 29, 2013, 05:57:33 AM »

I understand now, learning a lot which is one reason to build and work on your own gear.

I took a turn off L1 which changed the frequency the dial tuned to, I changed C7 to compensate and bring the radio back to the tuning range I want. 7.095-7.125 but the noise is right there at 7.110

This morning I bought a new set of 5 4032 crystals on fleabay. Would I need to replace all of the crystals or can I leave the Y1 through Y3 which make the narrow bandpass Cohn filter for signal selectivity? I'll replace Y4 and Y5 first and see what happens.

It will take a week or two to get the new crystals and get them installed, will report back what happens after they're installed.


Posts: 17476

« Reply #5 on: September 29, 2013, 08:41:35 AM »

All the crystals need to be the same frequency.  If the ones in the crystal filter don't match
the BFO crystal, you'll only hear signals in the passband if you hearing goes up to 32 kHz.
If the TX crystal isn't changed, your TX frequency will be 32kHz off from your receive.

The comment about just changing just the BFO crystal was to swap it with another on
the same frequency, in case that particular crystal had a spurious response that was
causing your signal.

Posts: 18

« Reply #6 on: September 29, 2013, 11:26:55 AM »

I swapped crystal 4 and 5 with each other, operates the same as before. Will have to wait for the post man to deliver the 4032mhz crystals and I'll give those a try.

Posts: 18

« Reply #7 on: October 05, 2013, 12:00:53 PM »

I changed the tuning range to be from 7.030 to 7.065MHz where the radio works perfectly. After spending a few days listening to the lower end of 40 I found there is more CW there, less QRM, and while most of the code is too fast for me not all of it is beyond what I can copy.

Plan to leave it there for now, someday the crystals will show up and if I decide to switch back to the novice sub-band I'll install them.

Thanks for the help.

Posts: 2

« Reply #8 on: October 10, 2013, 06:56:44 AM »

I would spend some effort finding out what is causing the problem before making modifications to the receiver.  What happens "most of the time" is valuable to know, but it is not a valid troubleshooting method.  How about disabling the BFO, or changing BFO frequency temporarily to see if the unwanted signal disappears or changes?  The use of a tuning wand should let you know right away which circuits are involved and whether you have an in-band mixer product.  Write down your LO, VFO and BFO frequencies and see if you can mathematically pinpoint how your 7.11 MHz signal is being produced.  If it is produced by a high order harmonic of one of the oscillators, why not tack in a simple series resonant LC filter across it at the offending harmonic?  That approach would keep it simple, and keep you focused on what you really need to do.
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