Locator KP05UK

Modified R2 receiver
I am making a "R2" DC receiver for the 40 and 80 meter HAM bands. It is possible to make it work on the other short wave HAM bands but for the moment I chose 40 and 80 meter because of the Swedish QRP contest. I also intend to make a QRP transmitter so it will be a transceiver.
The original article of the R2 Receiver was published in the January issue of QST 1993, and it can also be found in THE ARRL HANDBOOK"
Some components as variable shaft capacitors has more or less gone out of market but many new nice components have arrived instead.
To build a good VFO with reasonable quality with standard on the shelf parts is not impossible, but to build a DDS VFO with a LCD frequency display is much easier.



The DDS - Local oscillator
I have used a DDS circuit from Analog Devices (AD9832). It is very small, it is housed in a 16 pin SO capsule and it contains all the necessary parts to build a good synthesizer. It is clocked with a 25 MHz clock and deliver frequencies from DC to about 10 MHz with a resolution of about 0.01 Hz.

The DDS circuit is controlled from a micro controller which also updates the frequency on a LCD module.

The local oscillator frequency is chosen to be 56 MHz because I use a divide by four 90 degree frequency divider. To make the synthesizer to output this frequency I used a VCO and a simple PLL circuit to lock it to the DDS. To change frequency from 7.0 to 3.5 MHz I simply divide the oscillator frequency by to to 14 MHz.

Figure 1.   90 degree frequency divider and mixer

As mixer I have chose the standard SBL-1 from Mini-Circuits. When used in the circuit in figure 1 i have measured -95 dBm carrier leakage on 3.5 MHz. The mixers is switched with a square wave current from the 74AC74 circuits.

The antenna signal passes a band pass filter and a power splitter before it retch the mixers.

On 80 meter and probably on 40 to you don't need any preamplifier because the antenna noise level is higher than the noise in the receiver.




To reduce the bandwidth before it reach the first amplifier stage and still hold the mixer IF -out at 50 Ohms you need a diplexer. It consists of a 2 mH inductor and a 50 Ohm resistor in series with a 680nF cap to ground.



The Preamplifier

The pre amps are made with two AD797 which have very low noise when connected to a low impedance source. In the original R2 design they used discrete transistors in an relative complex GB circuit but I think this is better.



Phase shifters

I used the original connection for the phase shifters and it works well. I use 3 NE5532 OP-amp's. I have used a lifted ground for the amplifier chain because it is easier to split 12 Volts in two with a TL2436 than to get an other power source.



Low pass filter
To begin with I used the same circuit as in the handbook for the loupes filter. I plane to use an active filter with turn able cutoff frequency. This will give me a sort of pass band turning.


The original R2 have no AGC but to not damage your ears you need at least an limited. I have planes to make some test with AGC but I think it is better without AGC than with a bad one.



LCD  and controller

To show the frequency I am using a LCD module (1x16 char). It works well.
To control the frequency I am using a two channel digital shaft encoder. They are relative expensive so one way to make an inexpensive one is to modify an "old" computer mouse.
I use an 80C752 micro controller to decode the shaft encoder, display the frequency and update the DDS circuit to right frequency. It works well.
The reason I chose the 80C752 is because I found it in my junk box. The controller have built in A/D converter so I can easy read output power and other analog signals and display it on the LCD.  I will change micro controller to 87C749 because the 87C752 have a I2C buss that I don't need.

To adjust the frequency for TX/RX offset I simply ground a pin on the controller and the program updates the DDS to a suitable frequency offset.

The Transmitter


Related links:
Analog Devices:  data sheet for the AD9832, AD797.

N E w    2000 01 20

Demo software.

Check it out, it have a 2.5 kHz SSB filter.
connest the two channels from the R2 receiver to left and right of your soundcard (Line input)

This is a very simple demo. I have only tested it on my computer.

Download it   and put it in a directory together with the bitmap file. It runs directly without any installations.

I have made some experiments with the R2 receiver connected to the PC sound card.
By connecting the output from the first amplifiers after the mixers to right and left input of the souindcard (AUX.) it is possible to make really good receiver with real time spectrum display.

The software consist of a 1024 points complex FFT and then a mask for the desired frequencies, then a IFFT.

If you listen to this it will sound bad, the FFT need a window function.
With a window like Hamming it works better but you have to update only half the 1024 samples every time and add ti together at the end.
The window have to be symmetrical in the way that the sum of the last 512 samples and the new 512 samples will give a constant value.

I will explain this better later or put a working program on the web.

The filter is very easy to change and it is sharp with minimum ringing.
The Spectrum is so fast that a slow CW signal is readable on the screen.

Any interest or comments are welcome.

The receiver is not fully made yet but I have listen to it and it sounds great!
If you think that this is interesting or if you have any questions pleas send me a e-mail.