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Project TouCans Lab Book: Varactors

 Just a few notes on the varactor that's providing the FSK on the Rockmite and a few more notes on the FSK circut itself. This page will continue to evolve and is related to the slow moving project to try out teletype with Project TouCans. To communicate in most of the 20 meter RTTY band, the rig needs to have a 175 Hz FSK, rather than 500 to 700 Hz. I'm woroking on understanding the FSK circuit to see if it can be modified to give the desired shift.

Short conjecture: I suspect/hope I can place a small variable capacitor around the varactor to bring the frequeqncy shift into the range that will work.

The RockMite achieves frequency shift with its Colpitts oscilator via the following circuit from the manual



and I'm just not sure how this works out. When Q2 closes, it looks like there should be a short to ground.

The Zener, D5 with its 4.7 V reverse voltage, is immediately gone. Using the capacitance chart below, it looks like we gor from 100 pF from the varactor ot on the order of 500 pf depending on how much bias current is able to flow through the crystal.





R11 and R12 are still providing DC bias to the transistor. An interesting note here is that the RockMite in TouCans is powered with 15 Volts, not 12 Volts.

And this is where I get lost. The varactor is a voltage controlled capacitor, but it looks like it's now attached to DC ground, and maybe that's the point. (Is the left lead of the crystal attached to ground?)



This video shows the inductor, which is later replaced by a crystal, being connected to ground.

K6STR on the Rock-Mite group pointed out that while the resistor makes a DC connection to ground for the varactor, it also participates in the AC behavior of the crystal. Cool!


The interesting point for now, to me, for teletype, is that it looks like the frequency shift can be adjusted only by changing out the Zener, which isn't continuously adjustable. 

Here's how the RockMite's varactor's capacitance changes with voltage


Temperature Dependence

And, I found a note about temperature and the response of this family of varactors, (that would be the family that includes the MVAM109.)

"NOTES:
1. The effect of increasing temperature 1.0°C, atat any operating point, is equivalent to lowering the effective tuning voltage 1.25 mV. The percent change of capacitance per °C is nearly constant from -40°C to +100°С."

Did you know that the Tuna Topper in TouCans runs hot? Did you also know that the transmit frequency of TouCans shifts from 10457.5 kHz to 10457.3 kHz as it warms up on a typical day in San Francisco. (I saw the rig go as low as 10457.2 in El Paso, TX.)






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