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Sunday, December 15, 2013

20/40 BITX Build Update #1: VFO Success


I've started construction on my second BITX transceiver, this one for 20 and 40 meters.  As with the previous rig, I decided to take the VFO first, but this time I pledged not to surrender, not to wimp out with a VXO or (worse!) a DDS.   No, this time it would be a real LC VFO.

I took seriously all the admonitions in the tech literature about the fickle permeability of ferrite and iron powder, so this time I used an old-fashioned air-core coil.  It is wound around a cardboard tube.  The tube was previously the bottom portion of a coat-hanger from the dry cleaner.  I wound 40 or so turns on this core, then measured the inductance:  5.6 uH -- that looked about right.

For the oscillator stage I used a Colpitts circuit very similar to the one in the original BITX schematic (but I am hoping I won't need the varactor diode fine tune mechanism).   I had on hand a nice Heathkit 19-146 pf variable cap with an internal 4:1 reduction drive.  Not wanting to pluck rotor places out of this beautiful part, I had to calculate the series capacitance that would yield a frequency spread of about 175 kHz.  It turned out to be 40 pf.  Then I had to figure out how much capacitance to put in parallel with the variable.  Well, it all ended up like this:


There was a bit of trial and error in the process of getting the VFO to cover the desired range.  A big help in all this was an on-line reactance calculator.  I found this one to be very useful:
  
http://www.1728.org/resfreq.htm

I found that the VFO is more stable if I reduce the voltage from 12 volts down to about 8.  Also, I found that when evaluating the stability, it is better (psychologically!) just to use a stable superhet receiver instead of the frequency counter.  The counter may appear to be jumping around a lot, especially if the signal you are monitoring is not very strong.  Just listening to it at zero beat on the superhet is very reassuring.

I followed the Colpitts oscillator with the FET buffer and two stage BJT amplifiers from page 50 of Doug DeMaw's QRP Notebook (page 50).  I now have the requisite 7 dbm signal.  And it appears to be quite stable. 

I plan using this with an 11 MHz IF, with the VFO running at around 3.875--3.700 for 40 meters and 3.175 -- 3.355 for 20.   I plan to use a small relay to switch in some additional capacitance to move the VFO down to the range for 20 meters.


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10 comments:

  1. Bonjour Willaim,

    It is Wonderful job on coil please allow us a comment on du zener silicon part. We should put capactor
    for shunt of noise to ground and give a voltage chance to stabalize. All much zener niose go to buffer in that 1.

    avec mes amitiés

    Phillipe Coutier

    ReplyDelete
  2. Bill - are you sure about your reference ...page 50 of Demaw's QRP Notebook?? Nothing even close to that in mine....First Edition 2nd Printing. I'd like to use that buffer/ amp combo but can't find it.

    Steve VE7SL

    ReplyDelete
  3. johan hvilsted, denmark
    sent on Android

    typo in schematik? very high current will heat 2n222
    2r2 put emitter near earth and most of signal go to earth by emitter node.
    make emitter r higher and maybe choke

    ReplyDelete
  4. Steve -- Yes, you are right. It is in "W1FB's QRP Notebook" page 50. "W1FB's Design Notebook" is a different book. (But he has a somewhat similar "universal VFO" in the Design Notebook on page
    123"

    ReplyDelete
  5. Johan: Thanks. Yea, that was an error with the pencil. It should be 2.2K in the emitter circuit. I fixed it on the blog.

    Phillipe: Thanks for the comment on the Zener. I will work on that this weekend. There is actually a .1 uF cap to ground. (I had the schematic drawn incorrectly in the version you saw.)
    73 to all.

    ReplyDelete
  6. Bill - I guess there must be different versions of "W1FB's QRP Notebook" then as there is nothing like that on page 50 in mine. I don't see anything in the book at all that uses a JFET buffer followed by two BJT's.

    Steve / VE7SL

    ReplyDelete
  7. Bill - apparently my book is "QRP Notebook by Doug Demaw" and published by the ARRL. Different than the earlier (and much better) "W1FB's QRP Notebook". My apologies for the confusion!

    73 Steve / VE7SL

    ReplyDelete
  8. The counter jumping around doesn't forebode well. You may have a phase noise issue. I always put a low noise LDO right at the VFO circuit not only to stabilize the voltage, but to clean up any noise. If if you don't have a low noise LDO, just plunk in a 78L08, it's better than not having it in there.

    73's, David

    ReplyDelete
  9. David: It is much less jumpy now. I think part of the apparent instability came as a result of the output from the buffer being close to the minimum input signal needed by the counter. When I built the amplifiers, the signal into the counter was much stronger, and the display matched the stable signal I was hearing on the receiver. 73 Bill

    ReplyDelete
  10. Bandswitching always introduces some level of trouble. Yes, a relay allows the control to be right where it's needed, but if the contacts get dirty, there goes your stability.


    One of the neat things about solid state devices is that they are cheap, and take up so little space. You might as well just build two VFOs, and switch them using power to each of the two oscillators. Use a dual gang variable capacitor, so the same knob tunes both VFOs. (Or not. Separate VFO knobs means you could change bands but not lose the frequency on the other band.)
    You could probably use the same buffer, have the oscillators feeding the buffer in parallel. Even if both need a buffer, again it won't take much money or space.

    Design is always a tradeoff. I was shocked by one transceiver maybe 20 years ago in QST, where the author used a separate crystal filter (commercial) for transmit and another for receive. But he said he had the filters, and pointed out that it simplified the transmit/receive switching. I wouldn't go that way with filters, but sometimes adding extra parts simplifies other things.

    It's not silly to use a three terminal regulator for a single stage, that's what they were meant for and if it helps the rest of the design, it's a good thing. This isn't the days of voltage regulator tubes, needing a big resistor and space for the tube and socket, and the cost.

    Michael

    ReplyDelete

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