Superhet crystal mixing schemes

Homebrew superheterodyne receivers/transceivers using a computer crystal ladder filter, BFO and  VXO offer the combination of simplicity and stability.  The key to this receiver pattern is the VXO.  I know VXOs are so ’90s, and that I should embrace digital and start banging out DDS VFOs with Arduinos, eBay AD9850 cards and AdaFruit si5351‘s.  I will soon, I promise.  In the mean time I just want to explore computer crystal mixing schemes a little longer.   Scratch-building a DDS VFO necessitates some new practices, such as getting the script you’ve picked from dozens online to compile and download to the controller, for the particular combination of display, Arduino and DDS chip in use.  I’m not saying it’s too difficult, I’m just observing that it isn’t trivial, and if you get stuck you may find yourself limited in your options to unpick it.  Crystals, on the other hand, are real, physical things, remarkable for their sharp resonant characteristics.  And these days they’re almost literally a dime a dozen.

Back to the VXO.  You can bang one out with usable coverage in a few hours.  In Pete Juliano N6QW’s excellent JABOM series (Just A Bunch Of Modules transceiver, QRP Quarterly, Fall 2011) he recommends building JABOM initially with a VXO, just to prove the receiver, before considering tackling a DDS VFO in a second stage of the project.  In so doing, the maker eliminates one source of problems (DDS VFO problems) that might drain his or her time, energy and enthusiasm.  I think of the VXO in this way, as an interim variable frequency oscillator, the one you get the project running with, until such time as you pull it out and replace it with the real thing with all the bells, whistles, and phase noise.

The computer crystal-based superheterodyne  architecture strikes an appealing balance.  It sits at a pivot point between performance and complexity.  A receiver illustrates.  The received signal (let’s say, 7,085 kHz) is mixed with a Variable Frequency Oscillator (VFO)(say, 12,000 kHz), running above the intermediate frequency (IF)(say, 4,915 kHz), yielding among various sums, differences and pass-through signal components, the signal at the chosen IF.  This signal is then stripped of one of its sidebands and shaped by a narrow crystal filter, optionally amplified, and mixed with the fixed frequency Beat Frequency Oscillator (BFO) about 1.5 kHz above or below the IF, in the product detector, to reveal audio.  The audio is optionally filtered and amplified for speaker or headphones.    Varying the VFO frequency tunes the band.  If the VFO is a VXO, the segment of the band covered depends upon how far you can pull or push the crystal.

The VXO, BFO and IF filter can all be made from cheap computer crystals, sold for as little as 10c each in volume.  The two crystal oscillators are relatively pure, stable, and foolproof — a crystal oscillator is about the simplest kind of oscillator you will ever build.  The crystal IF filter, once the most expensive and unobtainable component of a sideband superhet, are being reliably reproduced with just 4 crystals matched only by checking the closeness of their oscillating frequency, soldered with a few fixed capacitors, with acceptable results.   And these two oscillators, each with its buffer, draw a total of around 10 to 20mA, a fraction of the 120mA drawn by a DDS VFO.  The entire superhet receiver built to this pattern can draw under 50mA.

But what about the (barely adequate) coverage of a VXO?  The first few VXO circuits I built, some years ago, were disappointing, swinging only a handful of kHz before they fell out of oscillation.  Good only for netting around the crystal’s cut frequency.  But the Super VXO by JA0AS and JH1FCZ , as used in the Wilderness Simple Superhet Transceiver (SST),  Beach 40 and many other designs, and the availability of computer crystals at frequencies where crystals can be pulled further with stability, has partially addressed this concern.  The 2-crystal Super VXO I built recently gets 50kHz swing at 16MHz with excellent stability.  As the main tuning oscillator in a bench-top receiver, the Super VXO’s limited band coverage will inevitably frustrate.  But not so if all you want is a segment of the band for CW, or portable operating, say, SOTA, where you set up within a narrow range of frequencies, spot, and let the chasers come to you.

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Having decided to pursue this approach, the choice of VXO and IF frequencies for the band of interest is critical.  My TCF transceiver borrowed Peter VK3YE’s mixing scheme — the VXO on 16MHz and the IF on 8.867 MHz puts your receiver on 7.133MHz.  I set up the Super VXO with series inductance and a Motorola MVAM109 varicap, pulling the VXO below 16MHz which resulted in receive frequencies from about 7.070 to 7.120MHz.  The SOTA and Parks calling frequency 7.090 is in the middle of the tuning range.  Perfect.

Inspired by this frequency scheme, I wanted to see what other neat combinations popped out by making up a spreadsheet with locally available crystal frequencies on the rows and columns.  I highlighted all combinations of crystal frequencies that fell within amateur bands in red.

Of course not every red cell is viable.  There are a few rules to follow when identifying workable combinations.  First, you want to keep the VXO frequency above the operating frequency.  Some nasty harmonics and mixing products can occur if the VXO is below the IF, although some schemes do work reliably and have been used by commercial transceivers in the past (didn’t the Yaesu FT200 use 9MHz IF minus its 5.5MHz VFO for 3.5 MHz transceive?).

Second, you want to avoid placing the IF or BFO fundamental or harmonic frequencies too close to the operating frequency.  The VK3YE scheme I used may suffer from this… when receiving on 7110, the BFO at 8,867 is 1,747kHz away, which may put unnecessary near-signal pressure on the receiver’s front end if the receive bandpass filter is too wide or not properly tuned, or BFO shielding is not adequate.  With the key down, the BFO can leak through the transmit mixer and into the PA, resulting in a bad (illegal) spurious out of band emission. Of course, good bandpass filtering cleans up mixer outputs.  Something to be aware of, because a lot of simple superhets use minimal filtering to avoid complexity and alignment.

Other rules… try to place the VXO frequency reasonably high to maximise crystal pulling for usable coverage, because the higher the frequency of the crystal, the further it will pull.  And finally, keep the IF between about 2 and 12MHz where, I have read, computer crystal filters work best!

Taking these constraints into account, the spreadsheet highlights a few prominent combinations.   The VK3YE scheme of 16MHz VXO and 8.867MHz IF for 7.133MHz is a good one.  Another combo that stands out is that used by TB5X… VXO on 12MHz and IF on 4.915MHz gives a difference of 7.085MHz. This may be ever so slightly better than the VK3YE scheme as the BFO is an additional 3,38KHz away from the receive frequency (2,085KHz).  On the other hand, the VXO is 4MHz lower, so less coverage.

Other noteworthy schemes in my spreadsheet… a VXO on 14,318 with a 4,200 IF puts you on 10,118, perfect for 30m CW.  In fact it’s the SOTA calling frequency.

And for 80 meters, if you choose an IF at 7,680, a VXO on 11,250 puts you on 3,570. But wait, there’s more. A second pair of VXO xtals on 11,340 puts you on 3,660. If you could pull the crystal pairs down by 60 to 80 kHz, a simple switch might allow you to cover most of the band.

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The feasibility of these schemes depends on how far and in which directions you can pull the VXO crystals without losing stability. That’s the place to start if you want to build to any of these schemes, the VXO.  All of the rocks in my spreadsheet are in a local (VK) supplier’s catalogue, so availability should not be a problem. So all I have to do now is to buy a bunch of crystals, knock up a VXO and see what swing I get.   Let me know if you try this yourself

Some rigs that use VXOs in superheterodyne configuration:

• Peregrine: VXO 22,000 and IF 3,932 gives 18,068 (17 meters)
• TB5X 40m receiver, 12,000 VXO and 4,915 IF for 7,085 kHz
• Wilderness SST (Simple Superhet Transceiver) CW monoband transceiver, designed in 1998 by Wayne Burdick N6KR (of Elecraft fame) and Bob Dyer KD6VIO.  In April 1998, Wayne Burdick wrote to QRP-L that ‘it still amazes me that there are standard microprocessor crystal frequencies available that get us close to the QRP frequencies on all three bands’.  His frequency schemes are:
  • For 40m use 3,932 IF and 11,046 VXO for 7,046 (3,932 may not be readily available in VK)
  • For 40m use 4,000 IF and 11,046 MHz VXO for 7,114 (in my spreadsheet, use 11,059 VXO for 7,059)
  • For 30m use 4,194 IF and 14,318 VXO for 10,124 (in my spreadsheet, so these crystals are available!)
  • For 20m use 3,932 IF and 18,000 VXO for 14,068 MHz. (3,932 may not be readily available in VK).
• ILER and EGV transceivers at YouKits (EA3GCY):
  • EGV-40: IF 4.915 and VXO 11.981 (claims a 40kHz spread)
    ILER-17 Mk2: IF 4.915 and 14.900MHz VXO (this is a custom crystal)
    ILER-20: 3.276MHz IF and  11MHz VXO (up to 14.276); 11.046MHz VXO (up to 14.320MHz)
    ILER-40 Mk2: IF 4.915 and 12.031MHz VXO (up to 7.105);  12.098MHz VXO (up to 7.185).

If you know of other schemes, post details in a comment.

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A SuperVXO was kitted by the 4-State QRP Group for a while.

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On a page written in 2007, Pete N6QW writes about:

perusing the Mouser Catalog of Computer Crystals… I discovered that certain combinations would place signals right in the amateur bands. Care had to be exercised about unwanted mixing products but there are still many opportunities.

My first thoughts were for a 7.3728 MHz four pole crystal filter mixed with 11.0 MHz, 11.042 MHz, 11.059 Mhz and 11.228 MHz VXO’s. That results in spots around 3.628, 3.67, 3.686 and 3.885 all in the 80M phone band.

If you take a 10.7 Mhz Crystal in a Super VXO with a doubler and mixed with a 7.3728 IF that puts you on 28.8Mhz –10 Meters.

If you take a 9.0 Mhz IF and mix it with a 5.185 VXO that gives 20 Meters and similarly if one uses a 12.288 Mhz VXO that is 15 meters. I did purchase all of those crystals but first decided to use a 4.9152 MHz IF Filter (K2 uses the same) and mix it

This page is an excellent homebrewer’s notebook, documenting the evolution of this transceiver design.

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The spreadsheet is available in the vkzlqrp Yahoo group files in folder VK3HN.  If you can’t get it from there post a comment.