DDS VFO, Amplifiers, and T/R Switch

This DDS VFO is a simplified PIC-EL. The idea for a simplified PIC-EL originated with VK5TM. The original configuration is at his website: http://www.vk5tm.com/dds/dds.php An excellent website for many good ideas on working with DDS projects.

I worked on the code for several months and got a Transmit Function to work. Grounding an output of a PIC16F628 pin eliminates the offset in the calculation of the DDS word and outputs the exact receive frequency for output to a transmitter.

The Transmit Function makes the circuitry for the transmitter simpler. It eliminates a mixer, filters, and a couple of amplifiers. The output of the DDS goes directly to an amplifier that drives the output device of the transmitter. This lowers the cost and the parts count of the transmitter considerably. The BLT can drive any QRP transmitter with an output of 100ma from the VFO amplifiers. QRP transmitters that need less than 100ma drive will use a 3dB or 6dB resistive pad between the BLT and the transmitter.

The PIC used in this simplified PIC-EL will also work in the PIC-EL boards with the modification of disconnecting the transistor driving the speaker output. That switch is then used for a KEY output that is the transmit function of the DDS VFO, eliminating the offset from the output of the DDS.

The 1st and 2nd generation PIC-EL boards work extremely well with this transceiver as having an outboard DDS VFO eliminates almost all of the birdies and noise of the DDS from the receiver, resulting in a very clean receiver output. I have not tested the PIC-EL III boards, but they should work. This board is reasonably priced and is available from Craig Johnson, AA0ZZ at http://cbjohn.com

The DDS VFO on the receiver board is not that objectionable. Birdies are minimal and is very convenient to have on the receiver. Birdie reduction techniques have been used to reduce them considerably. The band noise on the lower bands covers almost all of the remaining birdies. On the higher bands, above 15 Meters, birdies have not caused problems receiving signals except at one or two frequencies. The problem is most prevalent on 10 Meters, but the band is useable.

PIC16F628/20 and Oscillator

The PIC used is the 16F628/20. The /20 is preferred over the 16F628A. The oscillator used is the internal oscillator with the frequency controlled by a resistor to ground on Pin 16. With a zero ohm resistor (bare wire lead below shown with yellow arrow), the oscillator runs at ~10.4 MHz, which allows both a 24 step mechanical encoder or a 128 step optical encoder to be used.

The internal oscillator is used as it has the lowest emissions outside of the PIC for quiet operation with the receiver. The crystal oscillator was hard to isolate from the receiver board. Pads were placed on the board in case a crystal oscillator is preferred, including pads for the two capacitors used with the crystal. One of ground leads is used to ground the PIC (black insulated wire below), which was an error on the board. Later versions of the board will not have this jumper.

The board has provisions for either a optical or mechanical encoder to be connected. See picture below, First production boards on the left and later production boards on the right.

Notice on the Optical Encoder the negative connection (-) and positive connections (+) on either side of the 1 and 2 connections. Notice on the Mechanical (Mech) Encoder that the middle connections have a ground in the middle between 1 and 2, which is the most common pinout of Mechanical Encoders.

The Switch box is for connections to the Mechanical Encoder shaft push button switch. It is located back from the encoder so that the wires to the switch help hold the encoder upright if the shaft is mounted horizontal toward the front of the board. The wires can be made very sturdy if #14 wire is used or short lengths soldered to #24 wire used to make the connections. See the instructions.

The Switch box is connected to PB3. PB3 is pushed along with the Morse Code Key dit or dah lever (with iambic) or down (straight key) to set the transmit frequency. This makes it quicker/easier to set the transmit frequency. PB3 also moves the cursor to the left one decade position.

DDS Module

The DDS module is the EBay AD9851 board from China. An inexpensive module with the oscillator, power supply filtering and input/output pins on one plug in board. The AD9850 board can be used with a simple change in the code. The AD9851 is used for better performance on the higher bands and is equivalent to the DS60 board used on the PIC-EL.

DDS VFOs Outboard

A lot of other DDS VFOs can be used instead of the onboard DDS VFO. Provisions have been made to tie in other outboard DDS VFOs. More information is available at DDS VFOs with Transmit Function. If you have a DDS VFO already, you can use it with the BLT and save on cost by not ordering the BLT internal DDS VFO.

The outboard DDS VFO output is tied to the DDS IN box and the 5V power can be tied to the 5V box. If you built the onboard DDS VFO as shown in the picture, remove the .01 cap beside the Frequency Counter box (yellow arrow) to isolate the output from the onboard DDS VFO. The DDS Module can be unplugged if you mounted it with SIP in line sockets.

The MOSFET amplifier gain adjust has worked for all the outboard DDS VFOs I have bought, with or without an additional amplifier (MMIC amp) after the DDS Module.

LCD Module

The LCD is a white on blue 16x2 HD44780 display. The background lighting is slightly different than other 16x2 displays with full 12V (no resistor) powering the backlight. If you use other 16x2 displays, check its specification sheet to make sure how they recommend powering the backlight (most require a low value resistor).

Notice in the PCB image below, the V+ (the blue backlight supply pin), the second pin from the top, goes directly to the +12 bus.

Contrast control is set to maximum (CON to Gnd) which works best with the white on blue display of the type used with this kit. .

Shown below is the V2.0 and the slant on the number 16 which are the ones preferred with the kit..

Others don't have a Version number and the number 16 is straight up. These require a 1.2K resistor from CON to GND. These work as well but I think the V2.0 is a better display.

On the first production BLTs, if a resistor is needed between CON an GND, the top trace between the connections is cut and a resistor installed between the connections. A 1.2K resistor was used with the non-version displays.

Later production boards will have provision for either a wire jumper or a resistor placed between CON and Gnd (B pad connections) to set the contrast.

12 Volt bus Cleaning

Two RF chokes are used to keep birdies and noise off the 12 Volt lines. Two ferrite beads are used here. Other values seen on other DDS VFOs are 10uH on up.

3.3 Volts for the DDS Module

Some of the China DSS Modules (most likely the AD9850 modules) have crystal oscillators that are made for 3.3 Volts. They can be found by touching the crystal module with a finger and noting how hot it is running. A very warm oscillator is probably a 3.3 Volt module.

The RF choke is removed and two diodes (1N4000 types) in series are put in the choke's place. The diodes will drop the voltage to 3.4 Volts.

A trace is cut on the opposite side of the module underneath the board between the .01 capacitor and the through hole pad at the V+ pin on the LCD output pin (see picture above). This will isolate the voltage for the DDS Module to the RF Choke side.