Details of the anti-spurs mod (added 45MHz filter)

The spurs can be conquered by a relatively straight forward modification.  The circuit diagram for the mod is shown above and the photo below:

The filter is a 45MHz 15Khz type filter identical to the existing roofing filter in the µBITx and replacing R27 in the original circuit.   Warren WA8TOD encapsulated the filter, transformer, and capacitor in shrink wrap and connected the ground of the filter to the nearby junction of R13 that is grounded.

Warren’s implementation of the filter uses a single transformer on the output of the crystal filter.     A capacitor is also needed between the low impedance transformer output and the center tap of the mixer transformer.  He soldered a 0.1 uF SMD, one side to the output pad of R27 and the other side served as a connection for the transformer low impedance output.  No doubt a through-hole component could also be deployed.

The filter input is attached directly to the other R27 pad.  Warren elected to not use a second transformer because the source is much closer to the filter impedance than it is to 50 ohms.

In Warren’s case the transformer does not dramatically reduce the spurs versus the bare filter. The primary effect, and one that is desirable, is that a much more reasonable level of audio drive is available. The radio is still susceptible to intermodulation distortion because the levels are increased closer to the stock configuration, giving only slightly reduced power output.

Other implementations of the anti-spur filter

A number of others have now implemented the anti-spurs 45 MHz filter and demonstrated that it reduces spurs.   uBITx.net believes this modification should be included in the production of the µBITx immediately, in order to clean up the output on SSB.

Glenn  VK3PE has made his own board to mount the components for the filter mod on, and Nick VK4PP has designed a board (which will be for sale or included as a freebie with his LPF board).  Photos of their board designs follow:

Reference

Conclusions on how to eliminate spurs

Alison KB1GMX has advice to constructors on the spurs on the higher bands:


For bands below 20Mhz spurs are NOT an issue as the low pass filters catch it.

Spurs are only a problem for SSB and frequencies greater than 20mhz.

NOTE: due to the way the uBTX does CW it is never an issue on any band.


The short form is when you mix two frequencies you get a third, in a perfect world.

The diode mixers used are handy but they can present conundrums.  If any of the three ports (IF, LO, and RF) are mismatched (think SWR) The signal can be reflected back in.  Since DBMs are omnivorous in that any port can be input or output and if mismatched both!  This does not include effects of distortion in the source signals.But in the real world things like this exist.

Double balanced mixers also suffer from overload, too many and too strong and you get a plethora of signals.   What that means for lots of simple and complex reasons you can get “spurs” or spurious  outputs that are undesired.

Basic math, addition and subtraction:

If you mix 45Mhz with 73mhz you get 28mhz.  We want that  and the radio needs that.   However if any of the 28 gets reflected back into the DBM where it originated it mixes with 45mhz and we get 17mhz.With those four signals you get mixtures of those like:

  • 73-17=56
  • 28-17=11

Those are “first order” as they do not involve harmonics.  They will be the strongest, but not always equal strength.

Both inputs can have harmonics like 90mhz and 146mhz and the 34 and 56 coming out can have harmonics too.  If you add and subtract all the possibles you get an increasing sea of signals some weak some stronger.  We will not cover the possibles as the first order ones are the most troublesome.

The solution traditionally applied is band pass filters or if it isn’t between 28 and 29.9999 the filter strongly attenuates it.  But you need a band pass filter for most every band… uBitx takes the path of below a certain frequency you only need low pass filters and fewer of them.  And it generally works well especially for 80 though 17M…

But at 20mhz and up the low pass filter passes everything below 30mhz and if you overdrive the rig slightly you get a spur on the tech window on 10m where the spur is 16.5 to 16.7mhz and there is no filter for that.  What makes this worse is some radios are very poor at 10M putting out maybe 2W so pushing the audio to get more invites the problem to be greatly worse.

There is no setting we can safely give that absolutely assures there will
be no problem that is consistent with maximum achievable power.

As a licensed amateur radio operators we are responsible for signal quality and also not generating signals outside our assigned bands.

There are two solutions one is bandpass the other is high pass filter.
Either way the rig must be modified to allow those and there are side effects.

One side effect is you need extra switching not provided.  The other is any filter has a loss though it and that would further reduce power out.

Short of that, keep the power right down on the higher bands and go for it…

Reference