Homebrew SDR HF Transceiver - Listening to the receiver in SDR#

I recorded this video of the I/Q outputs fed into my computer with the demodulation being performed by sdr# (now with audio)

The large central spike is, I think, bleed through from the local oscillator signal.

My next task is to measure the loss in the receiver and also look at whether there is a ground loop problem.

Next post: Introduction to the microcontroller processing.

Homebrew SDR HF Transceiver - Quadrature Divider

We are now at the point where we need to take the signal from the Si5351, divide it by 4 and produce two outputs, one of which is 90 degrees out of phase from the other.

A good tutorial on this process is here although we will divide by 4 rather than 2. The datasheet for the 74HC74 is here. The 74AC74 and 74HC74 are more or less interchangeable, the difference being that one can handle higher frequencies than the other.

There are many circuits on the Internet for quadrature dividers but I used one that is used in the SoftRock Ensemble receiver, here. The only difference being that the outputs to QSD CLK0 & CLK1 are not used, however, note that pin 9 does need to be connected to pin 2 (it took me a while to figure that was the reason I couldn't get it to work). Pin 14 is fed with 5V from the 7805 regulator and pin 3 is fed from CLK0 output on the Si5351. The outputs from pin 5 and 8 are individually fed to the LO ports on ADE-1 #1 and ADE-1 #2.

Below is my version, it looks a bit messy but it works.

The screenshot from my oscilloscope shows the two out of phase outputs from the quadrature divider in yellow and blue. The purple line shows the that these two outputs have been divided by 4 and are at a frequency of 7.100 MHz.

We are now ready to see what this thing can receive by taking the I & Q outputs from the audio LPF's to a stereo jack plug (I to left channel, Q to right channel or vice versa plus ground) and plugging it into our computer to listen to the signals using SDR# but finally a picture of the completed RF splitter, mixer, AF low pass filter, regulator, quadrature divider board.

I was going to mount the Si5351 breakout board onto the two posts top right but I think the board uses imperial screw size holes as my M3 screws will not quite fit.

Next post: Connecting to SDR#

Homebrew SDR HF Transceiver - the Si5351

The Si5351 is "...an I2C configurable clock generator that is ideally suited for replacing crystals, crystal oscillators, VCXOs..." That is to say that it can generate frequencies from 2.5 kHz up to 200 MHz. Adafruit Industries produce a breakout board with the Si5351 chip and other required components installed for under $8, this allows us to interface with a microcontroller, e.g. Arduino, and control the frequencies produced very easily. The breakout board allows us to generate three frequencies but for this project we will only be using one of the outputs. We will produce a frequency that is four times the LO frequency we require, when the signal goes through the quadrature divider (74AC74 flip-flop) it will be divided by four to produce the LO frequency.

Adafruit Si5351 breakout board on the left, Arduino Uno on the right.

The connections are simple: +5V and GND from the 7805 regulator, pin A4 from the Arduino Uno connected to pin SDA on the breakout board and pin A5 connected to pin SCL. 

I have hacked the example program below, it sets the Si5351 to a default frequency of 28.4 MHz, i.e. 7.1 MHz after the divide by 4, and by using the serial monitor window in the Arduino IDE you can change frequency in 1 kHz steps up (1) or down (2), or 100 Hz steps up (+) or down (-). This frequency is output on CLK0 - the next step is to connect CLK0 to the 74AC74 flip flop.

You will need to install the Si5351 library by following the instructions here.

/*
 * si5351example.ino - Simple example of using Si5351Arduino library
 *
 * Copyright (C) 2015 Jason Milldrum <milldrum@gmail.com>
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */

#include "si5351.h"
#include "Wire.h"
Si5351 si5351;
char incoming_char;
long serial_input_number, serial_input_number1;
void setup()
{
  // Start serial and initialize the Si5351
  Serial.begin(57600);
  si5351.init(SI5351_CRYSTAL_LOAD_8PF, 0);
  // Set CLK0 to output 7.1 MHz with a fixed PLL frequency
  si5351.set_pll(SI5351_PLL_FIXED, SI5351_PLLA);
  si5351.set_freq(2840000000ULL, SI5351_PLL_FIXED, SI5351_CLK0);
  serial_input_number1 = 7100000;
}
void loop()
{
 if(Serial.available()>0){
    incoming_char = Serial.read();
    switch(incoming_char){
    case '1': //up by 1kHz
      serial_input_number1 = serial_input_number1 + 1000;
      serial_input_number = serial_input_number1 * 4;
      si5351.set_freq((serial_input_number) * 100ULL, SI5351_PLL_FIXED, SI5351_CLK0);
      Serial.println(serial_input_number1);
      break;
    case '2': //down by 1kHz
      serial_input_number1 = serial_input_number1 - 1000;
      serial_input_number = serial_input_number1 * 4;
      si5351.set_freq((serial_input_number) * 100ULL, SI5351_PLL_FIXED, SI5351_CLK0);
      Serial.println(serial_input_number1);
      break;
     case '+': //up by 100Hz
      serial_input_number1 = serial_input_number1 + 100;
      serial_input_number = serial_input_number1 * 4;
      si5351.set_freq((serial_input_number) * 100ULL, SI5351_PLL_FIXED, SI5351_CLK0);
      Serial.println(serial_input_number1);
      break;
     case '-': //down by 100Hz
      serial_input_number1 = serial_input_number1 - 100;
      serial_input_number = serial_input_number1 * 4;
      si5351.set_freq((serial_input_number) * 100ULL, SI5351_PLL_FIXED, SI5351_CLK0);
      Serial.println(serial_input_number1);
      break;
    }
    Serial.flush();   
  }
}

A 7805 regulator circuit used to convert ~12V to 5V is here.

When you run the Arduino sketch you should be able to hear the output from the Si5351 on an HF receiver at 28.4 MHz

Next post: Quadrature Divider

Homebrew SDR HF Transceiver - Adding the ADE-1 Mixers

The ADE-1 is a passive device with an RF and LO input and an IF output. Two ADE-1's are used with the RF port on each being fed with output from port 1 and 2 of the RF splitter respectively (Note: I changed the design of the RF splitter from that which I first published, here is a link to the updated page). The LO ports will be fed from one of the two outputs of the 74AC74 flip-flop respectively. The output of the IF ports goes to an audio frequency LPF (one LPF per port).

The theory behind how the I & Q signals are produced are described in this excellent YouTube video by Craig, devttyS0. In short the LO outputs are produced by dividing the signal from the Si5351 by 4, there are two outputs from the 74AC74 at the LO frequency and one is 90 degrees out of phase from the other. When each one of these is mixed with the RF in the ADE-1 we end up with two outputs at baseband audio frequency, I & Q, one being 90 degrees out of phase from the other as shown below:

The audio LPF that follows each RF output I copied from EA4NZ

Source: EA4NZ

Below you can see a picture of the RF splitter, ADE-1's and audio LPF's. The 7805 and assorted capacitors in the top right is the 5V supply for the 74AC74.

I note from devttyS0's video that the FST3253 device removes the need for the two ADE-1's by implementing a Tayloe Detector - something to be tried.

Next post: The Si5351

Homebrew SDR HF Transceiver - RF Splitter

The RF splitter is used to split the incoming RF into two equal parts. These individual signals are then used as input to the ADE-1 mixers that produce the I & Q signals (90 degrees out of phase). Obviously you don't want to lose too much power in the splitter circuit and you want to ensure that the two output signals are of equal magnitude.

Update: 27 Dec 2017. The splitter design I originally used did not produce zero phase difference. Hence I have updated this page with another design.

I built this splitter based on a design from EMRFD (Experimental Methods in RF Design, Hayward, Campbell & Larkin), specifically the one from the 18 MHz transceiver shown in Fig. 11.14.

The schematic is below:

I used an FT50-43 toroid with 6 bifilar turns.

My built version can be seen in the picture below.

The picture below shows the outputs from ports 1 and 2. They are equal in amplitude and in phase which is perfect.

Next post: Adding the ADE-1 mixers

Homebrew SDR HF Transceiver - Band pass filter

The band pass filter (BPF) is used to filter out strong signals from outside of the frequency band of interest. I have built a 7MHz BPF so the receiver will initially be limited to the 40m band. However, to cover other bands a BPF will be required per band and switched in and out of circuit as the receiver is tuned to the different bands.

I used a circuit that I found on VK4FFAB's site, I initially built this as input to a NE602 mixer where the impedance needs to be 1500 ohms but for this project replaced the 270 & 18 pF capacitors with 220 & 56 pF ones to create 50 ohm output.

The circuit schematic is below, the output is chopped off but is just to the right of the 18pF capacitor with the output of the 18pF capacitor carrying the signal to the next stage.

Credit: VK4FFAB

The Elsie (filter design tool) plot for this is:

My built version looks like this:

Having built the circuit I connected my Wideband RF Noise Generator to the input of the RF amp and the output of the RF amp to the input of the BPF. I then connected my oscilloscope to the output of the BPF and used the FFT function to plot the frequency that the filter passes through. 

The result is shown below. The yellow line is the signal from the wideband noise generator, the purple line shows the response of the filter centred nicely on 7.1 MHz. 

The variable capacitors in the circuit can be used to fine tune the centre of the filter. If you don't have an oscilloscope then a cheap'ish device like the Yaege FC-1 can also be used. I have one of these and it can be used to measure very low power. Using a short piece of coax connected to the BNC connector with two crocodile clips or similar connected to the centre and braid at the other end of the coax you can connect to the output of the BPF and ground. If you input a very low power at 7.1 MHz into the BPF you can then tune the capacitors in the BPF to give maximum power output meaning that the circuit should be tuned to pass most power at 7.1 MHz.

You can also use the FC-1 to check that the RF amp is amplifying an input signal.

Next post: RF Splitter.

Homebrew SDR HF Transceiver - RF Amp

The RF amplifier is the first component of the receive chain. It is used to amplify the incoming signals and this circuit provides about 7dB of gain, a little over multiplying the incoming signal by 5.

The amplifier is called a Termination Insensitive Amplifier (TIA) and is taken directly from the 2009 paper describing TIA's by W7ZOI & K3NHI. In the paper this amp is the one in Figure 6. The benefit of a TIA is that it is not affected by the termination impedance of the preceding or following stages so we have one less potential issue to think about. In the final transceiver I will build a bidirectional TIA but for now and to get things moving on the receive side I settled on this design. KK9JEF has a good video showing how TIA's work.

Directly copied from W7ZOI & K3NHI's paper the circuit is:

Credit: W7ZOI & K3NHI

In my version I used ceramic capacitors for the 0.1uF bypass capacitors rather than the electrolytics shown in the circuit. The 4.3K ohm resistor is not a standard value so I used a 1K ohm and a 3.3K in series (1K + 3.3K = 4.3K)

The input to the amp is the connector shown on the bottom left of the circuit and the output is the one on the right hand side. Below is a photograph of my build.

You can see that I have used an SMA connector on the output port. Since I took the picture I have also added an SMA on the input. I've decided that I like using these as it means I can disconnect and reconnect modules easily. I bought a bunch of sockets and crimp plugs (for use with RG174 coax) on ebay. However, there is no reason as far as I know why you shouldn't solder the coax centre and braid to the board directly as I have done many times before.

As an FYI here is a close up of the SMA connector on the board. I just scraped the copper away with a knife and checked with a multimeter that the copper the centre pin is soldered to was not connected to the rest of the board.

Finally I connected a signal source to the input (AD9850 and Arduino, see here) and using my oscilloscope measured the gain at several different frequencies. Yellow line is the input and blue line is the amplified output. At 10 MHz the gain is about 8dB and you can see that at 14 MHz the gain has started to drop.

3 MHz

7 MHz

10 MHz

14 MHz

Next post: Band pass filter.

Homebrew SDR HF Transceiver - Introduction

In 2016 I spent some time working on a VLF/LF receiver by I2PHD that was fully software defined, see here. The challenge of this approach is the speed of the ADC and subsequent processing that is required within a microcontroller. There just were not enough clock cycles to implement a waterfall display for example.

Recently I have been looking at the YouTube channel of Charlie Morris, ZL2CTM, who has built a software defined radio (SDR) using a Teensy microcontroller. Charlie doesn't give a whole lot of details about how to build this radio but I decided I wanted to try it out. Edit: I have also been looking at VE3MKC's blog, I thought VE3MKC and ZL2CTM were the same person but it appears that they have been doing the same thing independently. My thanks to them both for the inspiration and for the information they have published.

Given my experience with the VLF/LF receiver and the ST Microelectronics boards I used as well as the fact that I have three of these boards here I decided to build my version ZL2CTM's HF SDR receiver and transmitter that will be standalone, i.e. it will not require a PC to operate.

The block diagram below shows the main components of the transceiver although as most components are shared between transmitter and receiver I haven't bothered to put the transmit components in the diagram. When I come onto that area I will add them. I have included a description of the function of each block.

Originally I had not planned to blog about this build but a post on Hackaday bought forth some comments on Twitter:

Consequently, I couldn't really not blog about this project.

I could say that all the individual parts of this project are simple enough but I frequently come across projects or circuits that leave out descriptions of a 'simple' part of the circuit because the experienced builder just cannot believe that not everybody knows what they know. Consequently, I will try to explain each section as I go along or provide a link to a resource on the Internet that does a much better job of describing it than I ever could. All components except the ADE-1 are through hole components and I will build Manhattan style. The ADE-1 can (just) be mounted in this way. After I complete this project I would like to try to build the same circuits using only SMD parts as a follow up.

My choice of the STM32F746-Discovery board is mainly because I have one here. It costs about $55 and, although there does seem to be a bit of a lead time in the UK at the moment (December 2017), it is still in production and is fantastic value for money. It contains an ARM Cortex-M7 microprocessor, ADC's (analogue-digital controller), DAC's (digital-analogue controller), ethernet, micro-SD card interface, audio line-in/out and a 4.3" colour LCD-TFT screen with capacitive touch input among other features. You'd be hard pressed to find a 4.3" capacitive touch LCD alone for the same price on e-bay.

If I hadn't found that ST have recently licenced DSP Concepts Audio Weaver software for use royalty-free on ST devices this project wouldn't be going ahead but the Audio Weaver software is fantastic. It will enable me to design the software audio processing parts of the solution on a PC (sadly it doesn't work on Linux, even under Wine) and tune these in real time while they are deployed to the STM32F746 device. When the tuning is completed the code can be deployed standalone to the microcontroller. In fact, at the moment I think that the hardest part of this whole project is going to be building the graphical user interface, for which I will use STemWin, another piece of software that ST licence for their customers to use royalty-free.

What will you need if you plan to join in with me in this project? I am going to make the assumption that you have built some electronic circuits or kits before, firstly I'll build the RF components that produce the I/Q outputs that will be fed into the STM32F746:

Soldering iron and other tools you use to build circuits
Several pieces of single sided copper clad board like this
MeSquares to enable Manhatten style construction
RG174 or similar thin co-axial cable to link the RF stages
Si5351 breakout board, available from Adafruit and other resellers
74HC74 D-type flip flop
2 x ADE-1-24 frequency mixers or the non-SMD but more expensive equivalent (SBL-1)
A handful of 2N3904 transistors
There will also be some resistors, capacitors and ferrite cores required that I will detail as I go along.
The STM32F746-Discovery board from your favourite electronics reseller, Farnell & RS stock them in the UK amongst others or direct from ST

If you already have an oscilloscope with FFT function then I suggest you build a wideband RF noise generator which will be useful to test the RF amp and bandpass filter. I built one like this because it uses the ubiquitous 2N3904 transistors but many designs are available on the web.

Finally, a warning. This will be a learn as you go along project and if you follow along you need to be prepared to trouble shoot and fault find. As it stands at the moment I believe that it will be possible to build a transceiver as described above. What I do know is that we will all learn something and prove that not all of us are "Radio Amateuring like it's 1975"!

Next post: The RF amp.