Re-Crystaling a 10MHz Softrock Receiver

A few weeks ago I helped Nir Israeli diagnose a problem with his Softrock RXTX 6.2 transceiver. The transceiver is crystal controlled and covers portions of the 10MHz and 7MHz bands. While looking at the schematics I realized that Tony Park, the designer of the kit, used a 40.5MHz crystal to cover the 10MHz band. I had a partially built Softrock 6.2 receiver, and I decided to convert it to 10MHz using a 40.5MHz crystal that I ordered from GenesisRadio (along with 9 other crystals; they offer good frequency selection and low prices). The receiver is the “upgraded” version, which means it has lower noise and higher gain opamps than 6.2 receivers for lower frequencies. I only partially built it because I tried to use it with an Si570 synthesizer and a switched bandpass filter; this setup did not work so well.

I removed the components I added to the board for the experiment, wound the toroids for 10MHz, installed them, installed the oscillator circuit and the new crystal, and tried it out. It works very well with WSPR (it again took me a while to figure out the fiq setting, but I eventually got it right). I can now cover most of the 10MHz band with this unit. It joins the 7MHz receiver I used previously for WSPR, the 14MHz receiver (which unfortunately does not cover the WSPR sub-band), and the Si570-driven Ensemble II RX receiver.

The use of the 40.5MHz crystal should make the receiver more sensitive than the crystal that Tony Park ships with the kit, because the original 13.5MHz crystal relies on subharmonic sampling (it does not sample the radio signal at every cycle, but only once every 3 cycles). I did not compare them directly, however. I am not sure why Tony used the 40.5MHz crystals in transceivers but not in receivers.

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