A Second SDR for the Desktop – 3


Figure 1Crystals for the Soft66

The Soft66 Lite SDR receiver is quite versatile.  The rock-bound limitation of it is that multiple 192 KHz segments (width depending on audio capability) are selectable, but that’s not really “continuous coverage”.  I thought about modifying it, using a DDS Xtal replacement, but quickly calculated that the cost would be more than the original cost of the radio, and probably be double the original cost.

So, for now I’m collecting a few crystals, but maybe in the future will upgrade the little receiver to DDS.   I mentioned in a few of my posts about this radio, that I wanted to use it for weather fax reception.  While I know I might want/need to have increased sensitivity in a WX-Fax receiver, I am experimenting with the little SDR anyway, to see what it’ll do.  What combination of crystals is a good selection, knowing that I’ll have only a few of them?   Well, in the US (where I live), the best weather fax transmitters are in Boston, New Orleans, Nova Scotia, Pt Reyes, California, and Kodiak Alaska.  Well, OK, Nova Scotia hasn’t been annexed (didn’t they try that once?).  But it’s close enough to be a likely (occasionally at least) possibility for WeFAX reception.

After some futzing around with my calculator, I came to the conclusion that the best all-around crystal for that job is 4.300 MHz.  With 4.3 MHz, I can do the following with the Soft66:

  • ( 4.3 x 2 ) / 2 = 4.3 (close enough to the frequencies for New Orleans (4.318), Boston (4.235), and Kodiak (4.298))
  • ( 4.3 x 3 ) / 2 = 6.45 (close enough to the frequency for Nova Scotia 6.496)
  • ( 4.3 x 4 ) / 2 = 8.6 (close enough to the frequency for Pt Reyes, Calif (8.682))
  • ( 4.3 x 8 ) / 2 = 17.2 (close enough to another frequency for Pt Reyes, Calif (17.151)).

This idea assumes a 192 KHz sample rate (for capture).  The Pi2 + Cirrus board gives me that nicety.  It means that the frequency of the weather fax transmitter has to be within 192/2 = 96 KHz of my radio center frequency (the calculated number in each row).  The center frequency is dependent on the switch settings of sv1, sv2, and sv3 on the Soft66 (which adjust the multiplier as shown).

Side note: at a minimum, I need to use very good antennas with this SDR in order to take up some of the slack. I am currently using a dipole, which hopefully will do service well enough for my experiments. I’ll probably end up with a commercial WX-Fax receiver for the main fax printer here. But, meantime let’s take a look at the diminutive little receiver’s fax-abilities  …

A 3.2 MHz crystal is not a bad choice either.  Here’s the math for that one:

  • (3.2 x 2) / 2 = 3.2 (close enough for Resolute, Canada weather fax at 3.253)
  • (3.2 x 4) /2 = 6.4 (close enough for the Nova Scotia transmitter).
  • (3.2 x 5) / 2 = 8.0 (close enough for the U.K Northland transmitter).
  • (3.2 x 5.333) / 2 = 8.532 (close enough for a second New Orleans transmitter).

(7.030 x 2.5 ) / 2 = 8.7875 MHz, which is close enough to the freqency for the SSB maritime weather and wave forecast for the Atlantic.  It’s transmitted from Norfolk VA, and I can receive it pretty well with the little SDR radio (I’m not very far from Norfolk).  The 7.030 crystal was one of the ones supplied with the radio, and does duty on the ham bands as well as this marine forecast frequency.  The 4.3 MHz crystal I had to purchase separately.

I’m still playing with and experimenting with the setup for WX fax reception using the SDR.  I’ll be posting pics and results shortly.

(To be continued )


Soft66 Lite hardware can be found at:


Note: This author and site is not affiliated with the Raspberry Pi in any way. For information about those projects visit http://www.raspberrypi.org. “Raspberry Pi” is a trademark of the Raspberry Pi Foundation. For info on the cirrus adapter, see the Cirrus Logic website. Cirrus Logic (not afilliated with this site). More into may be found atwww.cirrus.com.  This site has no affiliation with the Soft66 or its website.