[minima] Progress report - Debugging and tuning the minima
- From: Thomas Sarlandie <thomas@xxxxxxxxxxxxx>
- To: "minima@xxxxxxxxxxxxx" <minima@xxxxxxxxxxxxx>
- Date: Mon, 17 Feb 2014 20:29:06 -0800
Hello everyone,
I am happy to report that my minima is alive and has caught some (local
test) signals. I finished building the RX stages last week but could not
really test it until today. It took me a few hours to manage to listen to
something. I think a lot of people will go through the same troubleshooting
steps and have the same questions so I will collect replies and update the
wiki.
First, a few questions:
- How do you tune the emitter voltage on q1/q2 in the KISS mixer? I could
not see anything about this and I have tried moving it without any strong
impact on the signal strength.
- How do you tune the two BFO capacitors? I have used a frequency meter to
place both around 20mhz. Right now I have the LSB BFO at 20.006 and the USB
BFO at 20.002. I am not sure what exact values I should aim for but I am
having a hard time making a precise measurement anyway at this frequency.
Is there a technique to do that while listening to a signal? I know Mark
used his spectrum analyzer but I have not made that jump yet and I guess I
am not alone.
- What is the audio amplification we should aim for in the final audio
stage? I have replaced it by an LM386 for my tests with the standard x20
amplification scheme but I can barely hear the noise out of it. I can add
the capacitor go to x200 but I want to make sure that the problem is not in
my previous stages.
(Note on the LM386: I used one because I had never used one before and it's
an experience I had to do - It's on a separate board and will get replaced
by transistors at some point - Don't kick me off the group!)
- What is the amplification I should get in the rf pre-amp? With an input
signal of 36mV, I get an output of 180mV (so x6 amplification) which seems
pretty low for a darlington pair. I am also surprised to see the same
voltage peak-to-peak on q5 base and a5 emitter. I have tried changing it
and checked the resistors around. Is this normal?
[image: Inline image 1]
Now for your information, this is how I debugged my minima. I hope some
will find this useful. Comments welcomed.
I started with the audio amp. I built the lm386 on a separate board with a
little speaker and when I powered it and touch the input of the circuit, I
was very pleased to hear a local station (AM I am guessing). I have to
admit that I considered just stopping there! This audio amp is effectively
the first receiver I built from scratch but I decided it was not a radio if
I could not tune it. The minima will remain my first!
So I went on to the audio pre-amp. Disconnecting the wire that I had placed
between the BFO mixer and the pre-amp and just using it as an antenna, I
was able to get a strong signal again in the speaker. Just a wire was not
enough to get a radio signal on the lm386 board so I knew I was amplifying
the input signal and went on to the other stages.
I had tested the BFO after building it but the output seemed very low. I
made some measurement this time and found the signal at the output to be
approximately 7uW or -22 dbM. This seemed wrong so I double checked the Ve,
Vb, Vc of all the transistors. Q10 bias was at 0.5V and after some
investigation I realized I had connected the 100ohm resistor to the base of
q10 instead of it's emitter. I fixed that and I know have a 0.5 mW at the
output of the BFO.
Testing the RF pre-amp proved more difficult. I do not have a signal
generator. I found two techniques that other builders might enjoy.
1 - Using a nearby transceiver: With my kx3 set to 14.197, pressing the
tune button will cause a signal to come through the LPF, into the KISS
mixer and I get a signal at approximately 20 mhz in and out of the crystal
filter.
This tells me that my LPF connections are ok, the mixer is working and this
was a major breakthrough today!
2 - Using the local oscillator. Tune your minima to 00.003 Mhz. This will
cause the VFO to move to 20.000 Mhz (14.197 + 00.003). You can then listen
to it in the speaker.
I used technique #2 for the rest of my tests to avoid transmitting a "test"
signal.
With technique #2, I got a signal with a peak-to-peak voltage of 36mV at
the emitter of q3. I then get 110mV on Q3's collector, 177mV on q4 emitter
and again 170 mv on q4 emitter.
For those of you with digital oscilloscopes, I also discovered today that
the FFT function was working pretty well to play with the mixer. Attaching
the probe to the input of the crystal filter, I can see my signal (sent by
the kx3), the Si570 frequency (attenuated) and the mixed frequency. You can
then move one of the signal and see the mixed signal move around. Not very
precise but a fun experiment.
Now I have tried tuning to 10.000 but I do not get anything on the output.
It's dark now and I will not find a strong signal on 20m. I will look on 40
but it seems to me that the sensitivity of my minima is way too low. More
testing ideas or other suggestions are very welcomed!
Thanks everyone, this has been an extremely challenging projects and it's
great to be able to share it with the group!
73,
thomas
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