Crystals in Softrock LTE combined kit
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I really don't understand what is happening with the crystals for 20 meters operation.
I understand that the counter needs a frequency that is four times that of the received signal to produce the two out of ?phase signals needed to produce the I an Q outputs.and I read that sampling is done at the 3rd overtone. ?Without a scope or spectrum analyzer I am having trouble trying to picture what the freq. is at the detector produced by the counter. ? The crystal supplied for 20 Meters is 18.73 megahertz. ?? Is the crystal operating at the third overtone to create the 56 Mhz signal or is something else going on? ? ?I think I better buy a scope to really see what is happening. |
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On 20m and 30m, the Softrock Lite II uses 1/3 subharmonic sampling. That means that the final sample rate isn't 14 MHz on 20m, it's 14 divided by 3. The actual rate with that crystal is 4.6825MHz for a center frequency of 14.0475MHz. The receiver covers all the usual QRP frequencies with a 48KHz sound card and nearly the entire CW portion of 20m with a 96KHz sound card.
On Fri, Apr 11, 2014 at 12:05 PM, <jackmargolis@...> wrote:
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The use of subharmonic sampling is not so much to allow the mixer to operate at a lower rate. The switches in the QSD work reasonably well up to 10 meters, and they are used that way in the Ensemble RX II and Ensemble RXTX without subharmonic sampling. The issue in the Lite is that fundamental crystals for frequencies above 30 MHz (remember that the crystal has to be for 4x the operating frequency) are not readily available at a reasonable price. Overtone crystals, which used to be the main way to generate higher frequencies, are no longer easy to find and are not inexpensive. They have mostly been replaced by crystal oscillator modules that use lower frequency crystals and PLL multipliers. Cheap modules are fine for computer clocks but are unsuitable for ham radio LOs because they have not been designed for low phase noise. In fact, computer clocks are often INTENTIONALLY spread to lessen the size of narrow radiated emission peaks. Using an overtone crystal would also require adjustment of the oscillator stage and measurement with high performance test equipment to make sure the oscillator was actually functioning on the correct overtone, negating the goal of a simple and quick build.
If there were enough demand for a Lite variant that would be useful on the higher HF bands (17m through 10m) it might be possible to do a design that incorporated a fixed ratio PLL multiplier in the LO design. You would want to avoid a 4X multiplier because that would place the base LO right at the intended operating frequency and leakage of that signal would be hard to avoid. A 3X or 5X multiplier might be suitable if appropriate inexpensive crystals are available; the Lite uses inexpensive crystals that are mass produced for use as clock oscillators for microcontrollers and their associated peripherals.
On Fri, Apr 11, 2014 at 4:16 PM, Mike Collins <mikecol@...> wrote:
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Jack
Shirley Dulcey KE1L