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.
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.
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.
LCD and controller
To show the frequency I am using a LCD module (1x16 char). It works
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.
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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
If you think that this is interesting or if you have any questions pleas send me a e-mail.