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Modifying the K5BCQ Si570

Function Generator Kit to an

Optical Encoder

Perhaps one of the current "bestbargains" is the K5BCQ Si570 FunctionGenerator Kit for use as a LocalOscillator in homebrew ham radioprojects. This kit has been widely usedwith the the Soft Rock SDR kits andhas many unique features and isdescribed at the K5BCQ website here.

For my application with the 20 MeterMMIC based SSB Transceiver, I

needed an output at either 5.0 MHz (LO + IF at 9.0 MHz) or 23.0 MHz (LO IF at 9.0 MHz). The kit produces a

square wave output which can berounded up to a nice sine wave using aow pass filter. By choosing the LO onhe High Side (23 MHz) a Low Pass

Filter with a cut off frequency of 32MHz was chosen. By doing this thisopens up the possibility for other bandssuch as 160 M (L0 = 11 MHz), 75 M(LO = 12.5 MHz), 40 M (LO = 16.0

MHz), 30 M (LO = 19 MHz), 20 M(LO = 23 MHz), 17 M (LO = 27 MHz)and 15 M (LO = 30 MHz). I haveactually done listening tests (by passinghe transmit board) and found this to

work nicely.

A note here regarding sidebandnversion: with the LO above the 9.0

MHz IF, to receive LSB requires theuse of the 8.9985 MHz Carrier

Oscillator and for USB requires the9.0015 MHz Carrier Oscillator. This isn reverse of the situation where the LOs below the operating frequency.

The kit as supplied by K5BCQ is fullycomplete including the knob for theboard mounted mechanical encoder andhe serial display. The kit as supplied

can be customized to include separatinghe display LCD from the main circuit

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board. One is still left with a problemof the mechanical encoder which to meappeared to have a limited life time andeven though mounting holes are nowstandard on the main circuit board,having a front panel tuning knobpresented certain mounting challenges.

I had concluded that for my application

hat 1) changing over to a more durableoptical encoder and 2) having thatencoder remote from the board wouldprovide me a greater degree offlexibility in the application to myhomebrew transceiver. TheMicocontroller Unit used with the kitoperates at 3.3 Volts and thus an opticalencoder must provide a zero voltcondition to one of the two MCU inputpins to simulate the mechanical switch

o ground.

I happen to have a very high endmechanical encoder with 512 CountsPer Revolution. This seemed like agood unit to runs some tests with thegenerator. Before doing that I contactedhe manufacturer of my switch (US

Digital in Vancouver, WA) and askedabout using the switch to simulate themechanical encoder. It was suggestedhat I create a voltage divider from a 5

volt source so that each channel (A &B) would see 3.3 Volts and as theencoder is rotated the Channels wouldalternately go to zero depending on Aeading or lagging B. A schematic of

optical encoder with the voltage dividers shown here. The unit was temporarilynstalled simply by tack soldering the

wiring to the mechanical encoder. Tomake this work with both in the circuit,one needs to find a point where the

mechanical encoder is in between thestates. That being done the opticalencoder worked perfectly. The 512 CPRs too fast! I happened to have some

128 CPR units that were purcahsed fora stepper motor project and theseseemed to have just the right feel, albeitcertainly a lot better than the 512 CPRunits. I would think that just about anyoptical encoder would work with theK5BCQ kit so long as the voltage

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divider was wired as shown in theschematic.

The as supplied mechanical encoderalso has a push in switch functionality,meaning that in some of the operationshe mechanical encoder is pushed in

while the encoder is rotated to the rightor left to select certain digits or

operations. This is a neat feature buthis too I identified as placing a lot of

wear and tear on the small mechanicalencoder. My encoder did not have aswitch, so I simply wired up amomentary push button switch andacked soldered it to the switch pins on

mechanical encoder.

There are panel mounted opticalencoders that have a built in push

button switch (Bourns and others) andhe prices are in the $30 range. Thatsaid I can live with the separate pushbutton switch and the surplus opticalencoder. I am sure the choice for themechanical encoder w/switch wasclearly one of economics as these unitsare in the $2 to $3 range.

Having such success with the opticalencoder I removed the mechanical fromhe board and permanently wired in the

optical encoder and the switch.I nowhave the flexibilty to package my radiowithout being constrained by the boardmounted encoder. Again the K5BCQkit is a superb bargain!