|Adding an Optical Encoder to the K5BCQ Frequency Generator Kit|
The K5BCQ Frequency Generator Kit (link) is undoubtedly one of the most amazing and excellent kits for those wishing to provide an extremely stable Local Oscillator signal to a homebrew radio or electronic projects. With the purchase of the full kit you receive everything you need including the Si570 Frequency Generator IC, the LCD Display and even a small knob to tune the mechanical encoder which is integrated on a virtual postage stamp size circuit board. The kit as delivered includes an somewhat generous LCD display which can be configured to plug inot the very small circuit board. So one mounting option has everything more or less mounted to the front panel. While that makes for an intergrated package, it does consume a large amount of front panel real estate and does somewhat limit how the frequency generator/display is physically implemented into the project.
A word or two about the capabilities of this frequency generator. For one it can be used straight through as a frequency generator. But the lower frequency limit for the IC is advertised as 10 MHz. The same Si570 chip is used in the "softrock" SDR kits and there it is being used down to about 3.5 MHz. That is not so much of a problem when used as a local oscillator where the IF is say 9.0 MHz as that would now enable a first conversion to 1.0 Mhz (10-9 =1). One of the features is that the frequency can be offset up or down from what is actually output to what is displayed. In the 2011 application the IF frequency is 4.9152 MHz and the LO is operated at 23.0452 MHz but the display reads 18.130 MHz. Right in the 17M band. That same offset holds for all of the bands so engaging the programming push button and calling on another bank of frequencies will hold that offset
For a project I built in late 2009 and for another one in 2011 I wanted to have the capability of just mounting the LCD display in one location , which I should mention the kit does come with connetors so you can do that; but also to have the tuning encoder underneath the display while all of the electronics that generate the frequency are remotely mounted. Having no sense to say this can't be done I proceeded on a path to make that happen. Later I realized that the remote mounting came with a few bonus points which I will cover later.
In looking at the schematic that came with the kit, the mechanical encoder supplied actually is two devices in one. The first is the obvious encoder itself which responds by generating signals to raise or lower the frequency depending on whether it is a right or left rotation of the encoder. The second function is to provide a "push in type" mechanical switch that is used as a part of programming the frequency generator and for changing the memory banks where various frequencies are stored. Eliminating the mechanical encoder now means supplying not only the encoder signal to the Micro Controller Unit (MCU) on the circuit board but also a separate momentary push button switch must be added "somewhere" on the front panel to replicate the programming functionality.
Shown below is the front panel of a radio built in late 2009/early 2010 that was the first implementation of the frequency generator. Pretty cool huh? The red button is the programming push button.
Next is a view of the Frequency Generator remoted main board and electronics interface
Below is the arrangement used in the JABOM 17/20M QRP SSB transceiver built in 2011
So how do you swap out the mechanical encoder/pusb button on the K5BCQ kit and use a mechancial encoder? There are two cases, of which I have done both. Case one is where you have a built kit and have already installed the mechanical encoder. Try as I might I could not unsolder and remove the installed mechanical encoder and push button. Thank God that he made wire cutters. Trust me save yourself time and grief by simply cutting the assembly off of the board right at the component and not the board. This will give you five short pins to solder the external electronics to the board. So life is good. In case two where the part was not installed on the main board, I installed PC Board headers and simply plugged into the board. In the case of the encoder I used a "keyed' three pin header so I would maintain the protocol --turn to the right and the frequency goes up and the reverse it goes down. This was done because of a lot of testing I was doing and there was a frequent plugging and unplugging. Such a keyed connector is not mandatory --it was solely for my benefit.
Now I must confess that I had no clue how to swap an optical encoder for a mechanical one. But I knew enough to place a phone call to U S Digital Electronics who manufactures high end encoders and just happens to be not too far from where I live. Probably it helped too that several years ago I purchased an encoder from them and had a PO reference number. So in looking at the MCU pins 15 and 16, a signal has to be provided to engage the controller. That signal level is 3.3 Volts. The typical optical rotary encoder operates at 5 Volts. So the applications engineer at U S Digital said all you need is a voltage divider scheme so that what comes out of the encoder is translated to 3.3 volts. Ahh Ohms law.
So we looked at 5 volts and how do you translate that to 3.3 Volts. I usually do things the hard way. So I said lets us work with something I can use my fingers for. So if you multiply 5 X 2 you get 10 (ahhh the number of fingers) now if you multiply 3.3 X 2 you get 6.6 --so I rounded it to 7. So I needed about a 10: 7 ratio to get me 3.3 V (or about 1.43: 1). Looking in my junque box I found a 4.7K and a 2.2K resistor. Adding those together I get 6.9K and if I pick off the volatge at the 4.7K resistor I get 1.47 which is 3.4 Volts. I called that close and went on my way. I used a local 5 Volt regulator to supply the voltage to the encoder. The momentary push button switch is connected on one end to Pin 4 and the other end to ground. You have lots of options on the optical encoder and these are frequently found on eBay. Try to pick one with a lower CPR (Counts Per revolution) such as 64 or 128. The higher counts such as 256 make the tuning too fast.
Shown below is the fianl schematic.
Earlier I had mentioned some bonuses from this arrangement. The first is that you are not limited to a single method of installation. Second the knob supplied is really small for my fat fingers. I like BIG knobs and thus you see a big knob in one of the photos. The third bonus is that you can remote the programming switch to any convenient location and I must tell you I was not too good at push and rotate with the supplied mechanical switch. My approach uses two hands and I personally like that better. I also suspect the MTBF (Mean Time Between Failures) is of a shorter span with the mechanical switch and bearing in mind it is a bear to remove -- an optical encoder replacement is a simple plug in to the interface board.
Noteworthy on the K5BCQ site is a link to a technical evaluation of the kit performed by Clifton Laboratories. The data is most impressive! Follow the K5BCQ build instructions and you will have no problem. It is a well done kit!