Modifying the K5BCQ Si570 Function Generator Kit to an Optical Encoder


Perhaps one of the current "best bargains" is the K5BCQ Si570 Function Generator Kit for use as a Local Oscillator in homebrew ham radio projects. This kit has been widely used with the the Soft Rock SDR kits and has many unique features and is described at the K5BCQ website here.

For my application with the 20 Meter MMIC 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 be rounded up to a nice sine wave using a low pass filter. By choosing the LO on the High Side (23 MHz) a Low Pass Filter with a cut off frequency of 32 MHz was chosen. By doing this this opens up the possibility for other bands such 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 have actually done listening tests (by passing the transmit board) and found this to work nicely.

A note here regarding sideband inversion: with the LO above the 9.0 MHz IF, to receive LSB requires the use of the 8.9985 MHz Carrier Oscillator and for USB requires the 9.0015 MHz Carrier Oscillator. This is in reverse of the situation where the LO is below the operating frequency.

The kit as supplied by K5BCQ is fully complete including the knob for the board mounted mechanical encoder and the serial display. The kit as supplied can be customized to include separating the display LCD from the main circuit board. One is still left with a problem of the mechanical encoder which to me appeared to have a limited life time and even though mounting holes are now standard on the main circuit board, having a front panel tuning knob presented certain mounting challenges.

I had concluded that for my application that 1) changing over to a more durable optical encoder and 2) having that encoder remote from the board would provide me a greater degree of flexibility in the application to my homebrew transceiver. The Micocontroller Unit used with the kit operates at 3.3 Volts and thus an optical encoder must provide a zero volt condition to one of the two MCU input pins to simulate the mechanical switch to ground.

I happen to have a very high end mechanical encoder with 512 Counts Per Revolution. This seemed like a good unit to runs some tests with the generator. Before doing that I contacted the manufacturer of my switch (US Digital in Vancouver, WA) and asked about using the switch to simulate the mechanical encoder. It was suggested that I create a voltage divider from a 5 volt source so that each channel (A & B) would see 3.3 Volts and as the encoder is rotated the Channels would alternately go to zero depending on A leading or lagging B. A schematic of optical encoder with the voltage divider is shown here. The unit was temporarily installed simply by tack soldering the wiring to the mechanical encoder. To make this work with both in the circuit, one needs to find a point where the mechanical encoder is in between the states. That being done the optical encoder worked perfectly. The 512 CPR is too fast! I happened to have some 128 CPR units that were purcahsed for a stepper motor project and these seemed to have just the right feel, albeit certainly a lot better than the 512 CPR units. I would think that just about any optical encoder would work with the K5BCQ kit so long as the voltage divider was wired as shown in the schematic.

The as supplied mechanical encoder also has a push in switch functionality, meaning that in some of the operations the mechanical encoder is pushed in while the encoder is rotated to the right or left to select certain digits or operations. This is a neat feature but this too I identified as placing a lot of wear and tear on the small mechanical encoder. My encoder did not have a switch, so I simply wired up a momentary push button switch and tacked soldered it to the switch pins on mechanical encoder.

There are panel mounted optical encoders that have a built in push button switch (Bourns and others) and the prices are in the $30 range. That said I can live with the separate push button switch and the surplus optical encoder. I am sure the choice for the mechanical encoder w/switch was clearly one of economics as these units are in the $2 to $3 range.

Having such success with the optical encoder I removed the mechanical from the board and permanently wired in the optical encoder and the switch.I now have the flexibilty to package my radio without being constrained by the board mounted encoder. Again the K5BCQ kit is a superb bargain!