Total posts : 45366
The formula for the SSTran synthesizer was covered here: http://part15.us/node/692
The European version uses a 3.6 MHz reference crystal. The formula is similar, except that the multiplier is 9 instead of 10:
Frequency in kHz = 9 * (S4_value + 1).
Since you want a frequency of 207 kHz, the S4 value should be 22. The binary equivalent of this is 10110. In the SSTran, the least significant digit corresponds to switch 1, and the most significant digit corresponds to switch 8 (of S4). Note that in the SSTran, a “1” is represented by “OFF” and a “0” is represented by “ON”. Thus the switch settings on S4 would be as follows:
Switch 1: ON
Switch 2: OFF
Switch 3: OFF
Switch 4: ON
Switch 5: OFF
Switch 6: ON
Switch 7: ON
Switch 8: ON
Note that switches 6, 7, and 8 need to be set to the equivalent of “0” (ON). I hope this isn’t too confusing.
I tried this with my unit, which uses a 4 MHz crystal and therefore tunes in 10 kHz steps, and it worked! However, the frequency came up at 230 kHz instead of 207, due to the X10 multiplier instead of X9. So the synthesizer part isn’t too difficult to get working.
Unfortunately, you are not finished! You will probably need to increase the value of L8, in the collector circuit of Q5, to produce a high impedance at this lower operating frequency. In addition, you’ll need to increase the inductance values of one or more of the output loading coils L4-L7 in order to get the output circuit to tune to resonance. Alternatively, you could short all of these out with the switches in S5 and use an external loading coil. This would probably be the best way to go.
Here in the U.S. we have a licence-free band from 160-190 kHz, so interested experimenters could give it a go. I don’t think the range would be nearly as good as on the high end of the Medium Wave band, but it might be interesting to try.
Classical Music and More!