Forty-Niner PLL

 ... another 49’er project with a digital VFO

Finished Forty Niner in a box

NOTE : this whole post talks about the popular "49'er" design by Wayne Burdick (N6KR), but  what follows is a universal PLL controller that can be applied to any direct conversion or superhet transceiver, so read on !

Introduction

This project started as a basic 49’er project, based on the QST article by Jack Purdum - W8TEE et al. "A Modular 40 Meter CW Transceiver with VFO".
That article was published in March 2016, and I read it with great interest, thinking “YES! I must build one too” … but I had never found the time to actually do it.

In autumn 2022, I finally got to order a Chinese 49’er kit, and build it with most of the changes described in the 25 page manual that can be downloaded from the ARRL here 

I asked my friend Gil (ONL12523) to write the Arduino software, like he did for the OST Morse Box.

A video on YouTube shows most of the available functions.

The Hardware

The chinese kit came without building instructions, there was only one page, containing the circuit diagram (click to enlarge). Luckily, someone posted a full manual online, click here. 
NOTE : DON'T BUILD the kit according to this manual, unless you only want a 1-frequency XTAL-controlled rig.

Original circuit diagram

Referring to the QST article, some more changes were needed. The AD9850 DDS has become difficult to source, and rather expensive, so it was replaced by an Si5351 module.

We also wanted to do some other stuff like keying the TX from the Arduino, generate a PTT signal to mute the receiver, or generate a variable (and perfect sinewave) sidetone in the Arduino. There is a paragraph further on with more detail of these changes ***

This is the circuit diagram with all of the modifications highlighted in yellow (click to enlarge).

I wanted our project to be “educational”, like the one in the QST article, so it had to be in a clear plastic box. At the same time I wanted it to be environmentally friendly, so it had to be some recycled box, which I found in the form of a case from a video cassette of the old Philips VCR standard from the seventies.

Old Philips VCR cassette (1972)

And this what I made of it. Everything is mounted on a simple L-shaped PCB chassis, which then shoves inside the box, and is fixed with double-sided adhesive tape. The 49’er board is mounted on HEX standoffs, which are soldered to the ground plane, and the perfboard with the Arduino, the voltage regulator and the Si5351 module (and an experimental CW filter) is mounted on some stiff wires that were soldered to the ground plane.

Mounting it all on a simple PCB chassis

The next picture shows the external connections, needed to operate the rig

Connectors on the 49'er PLL

And this is a view of the simple front panel before is was mounted it in the box.

The front panel

If you're in a hurry, you can skip the following paragraph.

*** More details of the hardware changes, see circuit diagram above:

-        Do not install R3, C4, C5, C6, W1 (trimmer pot), Y1 and Y2 (xtal’s)

-        Replace R5 with a 10k resistor

-        Replace C2 with 82 pF

-        Replace D2 with a 5.1V Zener diode

-        Put series LC filter (22µH + 56 pF) where Y1 was

-        Put a 3 pin header where Y2 was, connecting the middle pin to GND.

There is a nice video about building a 49’er with these changes on YouTube, click here 

Further changes we made because we used separate outputs from the SI5351 for the TX and RX oscillator, was disconnecting C10 (82pF) from pin 6 of the NE602, and feed it with the separate TX oscillator signal from CLK0. Also we made C10 1nF.

For the RX signal, the level from the Si5351 is a bit too much for the NE602, so we inserted two series capacitors (both the Si5351 and the NE602 have a DC level on their output or input), with a 220 Ω trimmer pot in between, to adjust the LO level.

D6 was disconnected from the keying line, and connected to a new NPN transistor which will serve as the PTT driven by the Arduino (I mounted it under the 49’er board with a 1k resistor at the base, and covered the resistor in shrink wrap).

In the same way, a keying transistor was put over the keying input on the 49’er, so that also the Arduino kan key the transmitter with a preprogrammed message.
An unlimited number of messages can be programmed, and those can be used as CQ or QSO messages, or as a beacon text.

The sidetone is made by the Arduino, it only needs a simple RC low pass filter.
So you can omit all components of the Twin-T oscillator, and feed the sidetone directly to pin 3 of the LM386. If you had already mounted the sidetone components on the 49’er, just remove resistor R1 to disable it.

I made a simple single stage opamp filter at 600 Hz, this is still wide enough for casual listening to SSB signals, and narrow enough to make CW operating possible.

During tests I blew up the original PA transistor (Q5, D885), and replaced it with a BD139, but mounted it in a socket made from 3 pins of an IC socket. So now I can “plug in” my final transistor like in the tube days, hi. Always handy whenever it blows again …

 Building the Arduino Controller

The Arduino circuit is nothing special, nor is it difficult, I just built it on a piece of perfboard. An Arduino Nano, a 1602 LCD display with I2C adapter (UC-146) to reduce the number of wires, and a Si5351 PLL module, also connected on the I2C bus.
A voltage regulator, and a handful of discrete components complete the circuit.
See title picture for the final result. 

Arduino controller circuit

The software

So far so good, but then came the big disappointment … the Arduino code in the zip file from the ARRL website was far from complete: it didn’t support the RIT function like was promised in the article. The only thing it did was check the band limits for different license classes (not really needed for us in Europe), and store the last used frequency in EEPROM.

Even an updated version of the software I found in this newsgroup  was not what I was looking for. There is another solution by K4KRW, but I found it rather complicated.

So I called in the help from my good friend Gil, ONL12523, who has also written all excellent software for the OST Morse Box.

By using two of the three oscillators in the SI5351 PLL module, we were able to find an elegant solution to above problems.

Our software is now working very well, and has the following functions:
Basic operation with RIT, SPLIT and SPOT function for tuning “zero beat”.
Menu operation to select band, set lower and upper band limits, adjust OFFSET, PTT delay, WPM for the built-in messages and even a beacon function with a countdown timer.

The operation manual describes how to use the rig as a basic Code Practice Oscillator, as a beacon, or as a foxhunt transmitter.

The code also includes a software DDS generator for making a pure sinewave sidetone, the sidetone follows the chosen offset frequency and sounds just perfect.

There is even a built-in calibration routine for the Si5351 PLL module !

All needed files, with hardware and operating manuals can be found on my github page, click here 

Superhet system

Note that this concept can easily be adapted to a superheterodyne receiver, edit the code of the main sketch, and put the IF frequency in line 38:

#define IF 0000000  // enter IF frequency (in Hz) for superhet RX
                    // 0 Hz for direct conversion RX 

As an example, we used the same setup and software for an old project called BINGO (designed by Bernard, F6BCU, see some notes about the Bingo here ).
It worked first class !
This was our version of the Bingo, we built it for 80m CW.

BINGO 80m CW front panel

and the rather complicated hardware ... certainly not a backpack QRP rig !

BINGO 80m CW circuit boards

Some changes were needed in the code, because the Bingo we built used a different LCD display and a professional rotary encoder. Send me an e-mail if you need more info about that (address is good on qrz.com).

73 de Luc, ON7DQ