latest news

Frequency regulation works. Its range has to be reduced though. Now the signals jitter around the setpoint value. The course setting is just too rough. 50 MHz error is no exception. The course setting is simply 500MHz/24Volt * Voltage.I have measured frequency vs Voltage of the VCO it, it is not lineair at the low range and at the high end.

I will make a hardware linearizer. I may store correction values in EEPROM if the linearizer doesn't solve the problem entirely.

blanking works

I have just added blanking. It was needed to remove spurious images during fly-back of the sweep signal.

The Blanking device is a 74HC4066 switch. It is controlled by the micro controller.
The micro controller generates the sweep so it knows exactly when to blank.
It works perfectly. The ghost images are gone.

This weekend I will try to make the center frequency regulator work.

some improvements

This weekend I did not spend much time on the project.
However I was able to make a few improvements.

-Spectrum is not inverted anymore. I have added an inverter using an opamp. I have
changed the software to stop inverting the Sweep signal.
-Added blanking output signal in software. One of the IO pins provides a blank
signal. It is needed for the flyback.

I discovered that the 24 V power supply has a ripple. This caused the spectrum to
jump around a bit. The 24 transformer can not provide enough current.

signals visible

I was able to display the spectrum from a few MHz to more than 440 MHz. A lot of improvements are needed though.
-signal is not stable on x axis. It jumps around. Probably caused by flyback of sweep.
-Signal strength out of tuner is low
-Sweep is inverted. Highest frequency is on the left of the screen.
These are the worst problems. I hope I can fix them in the weekend.

All Modules ready

I just finished the power board. I was mostly straight forward.
The only problem was the -12V. I used a separate 12V transformer.
The output of this transformer was 20V AC. After rectifying it The DC was too high for the 7912. It turned out that this little transformer needs to be loaded.
The 30V for the varicap does not work. The voltage out of the 24V transformer is too low for the zenerdiode wich is actually a 32V zenerdiode. I will use 24 instead. It should be enough as I have tested the tuner using 24Volt.

I build the 38 MHz receiver in a tin can. In fact I have built it on the bottom of this box. I just built the rest of the can around it.

Now all modules are ready. I am trying to connect them and fix the interface problems. I hope that I can display a spectrum tonight.

Power board

yesterday I bought some parts for the power board. Fortunately I had most of the parts in the junkbox.

I need several voltages.
5V for the microprocessor and the divider in the tuner.
12V for the opamps.
-12V for the opamps.
24V for the tuner.
30V for the opamp that drives the varicap in the tuner.

Last night I added to the power board 5V, 12V and 24V.

input impedance counter

Counting the frequency of the tuner worked. However the counter input required a lot of current. I build some amplifiers to boos the signal but is was not enough.

I found out that when the counter was stopped the counter input pin was configured as output. This messesd up signal. PORT A was configured as output. Turning on the counter overrides this setting. However when the counter was stopped the port was set to output again. My codes starts and stops the counter 50 times a second.

My port A has been configured correctly in my startup code. However the HITEC LCD software reconfigured it.

I fixed it, now the counter works fine.

tuner tests

I ran some tests on the UDT-401 tuner. It seems to be pretty stable. It drifts 200KHz. after startup. It drifts slowely so I thing the regulator can keep up with it.

Over the frequency/tuning voltage is a straight line. Only at the high end >21 volt its the MHz/PerVolt is less. So I probably need a compensator.

some pictures

I made some pictures to show the progress.


The red swich selects the function of the rotary encoder. Up selects the center frequency, Down selects the span. Pressing white button icreases the tuning speed.



Software updates implemented today:
-make the encoder more responsive.
-invert the sweep signal.
-reverse the function of the white button.
-increase the sweep frequency.

I have connected the analog board to the digital board. This caused an interface problem. The output of the PWM output is filtered. This filter needs to see a high impedance. The input impedance of the analog boar was too low. I added an emitter follower to take care of that problem. It works but it clips the output signal as the signal out of the PWM can be lower than 0.6 Volt. I think I can fix it by conneting the emitter resistor to -12 V instead of 0V.

Software

I spend a lot of time refactoring the software. It was getting a bit messy.
After cleaning up I added some functionality.

Now the frequency counter works. It can measure the center frequency. It is suppossed to work even when a sweep is in progress. This has to be tested.
I also made a regulator for the center frequency. It compares the actual frequency with the setpoint. I case of an error the software integrator is increased or decreased. This integrator value is send to a DAC. It works but I have to test it with the real tuner to be sure.

In the meantime I found an enclosure for the analyzer. I also drilled some holes for the controls.