The 7MHz SSB / CW radio receiver has a Fine Tune
First of all, thanks to your fellow members for your interest in this project. This project is the same circuit as the 14 MHz version (https://hs8jyx.blogspot.com/2013/06/14-mhz-vfo.html), but we made a new circuit board to simplify the sealing box. For a frequency of 7 MHz the frequency is quite constant. There is no frequency slider so much that I can not accept it.
The 7 MHz frequency will be able to communicate well in the night. Members who have not played this frequency yet. Do not be surprised if the day can not get anyone. Or get a strange noise.
Download the PCB at https://drive.google.com/file/d/0BxgJ9QX1BWUyWnBVbE1BSlJfR1E/edit?usp=sharing.
1.5 uH coil using 19 copper coil on T37-2 (red) coil (round counter of spindle https://15meter-radio.blogspot.com/2013/02/toroid.html )
The L1 coil can be wrapped around 21-22 on T50-7 or 22-23 on T50-6 (this is because the coil can be squeezed or loosened. Number of rounds can change a little)
(Example of reception I do not have a 7 MHz antenna. Use a long cable instead of an antenna.) Figure down the device and the connection.
Wide Bandwidth - Narrow What determines
From the figure is our VFO circuit. Adjusting the frequency of the VFO circuit can be done in two ways: adjust the L (coil) and adjust the C (capacitor). Here we use the C modulation using the capacity. At 1N4001 diodes, the higher the voltage range, the wider the bandwidth. (Ie 0-8 volts is wider than 0-5 volts, for example).
C2 and C3 also determine the bandwidth. up to a certain point, it does not matter L1 C1 will affect Bandwidth much. That if I Bandwidth widened to the L1 high value C1 less (frequency remains the same, but Bandwidth increases ) that this is coming from the C in the diode will affect the frequency, if connected to a C. a few examples receiver 7.1 MHz (to practice it), we assume a diode 1N4001 in tune with the 2-25 pF (by tuning voltage maximum and minimum).
Frequency Resonance Formula
Circuit 1 uses a low coil (using 1uH). We need to use a C value of 502 pF (500pF + 2 pF of the smallest diodes) to obtain a 7.1 MHz frequency.
At the lowest voltage, we get a frequency of 7.100 MHz ( 1uH // 500 + 2pF)
when the maximum pressure is not frequency MHz is 6.950 (500 +, 1uH // 25pF)
a bandwidth = 150 KHz,
the circuit uses two coils over (use 2 uH)
we need to use the C 502 pF (. 249pF + 2 pF diode, as a minimum) to 7.1 MHz frequency
when the minimum voltage to frequency 7.100 MHz (1uH // 249 + 2pF
p. When the maximum voltage, we have a frequency of 6.800 MHz (1uH // 249 + 25pF) Bandwidth = 300 KHz
Would an op amp like MCP601, MCP606 or maybe MCP618 work?7. The design is strange in that respect. The LM386 makes an extremely poor pre-amplifier so why the designer used it is a mystery. It probably does need an amplifier stage but a conventional low-voltage op-amp would work far better. I would replace it with a TL071 or similar and add the extra few components to bias it properly.
Unfortunately, no for two reasons, first is they can't survive supply voltages as high as 8V, second is they have no internal bias circuits. The LM386 is unusual in that it's inputs can be taken down to ground level but it suffers very high noise levels and quite severe distortion as a consequence. You could use one of those if you connected its supply to the 5V regulator output and added some bias resistors. For example add a 100K resistor from the + input to ground, a 100K resistor from the + input to 5V and a 47K resistor from the output pin to the - input. You might have to play with the feedback resistor (47K one) to get the gain as you need it. If you research differential amplifiers it will explain how they work.Would an op amp like MCP601, MCP606 or maybe MCP618 work?
Again no, it might work but you would be exceeding their current rating. You could use any of the 1N400x series of diodes though.Unfortunatelly I don't have 1N4001 yet. Can I use 1N4148?
The types and values are not critical but don't stray too far off the value. In engineering talk "tune for maximum smoke", in plain language, find a weak signal somewhere near the middle of the band and adjust them for best strength.And are those variable capacitors at antenna input crucial? How do I tune them?
Yes, even 10m but I would expect some degrading of sensitivity as the frequency increases. You would almost certainly have to change the tuning inductor and reduce the VFO feedback capacitors to tune higher than 40m.Would it be reasonable to receive 20m/80m with circuit with changed VFO capacitor value and the input filter?
I am using the x10 from the start, but my ground connection wasn't indeed very close. I will try again soon.The waveform on mixer pin 6 should be stable but you must use a low capacitance scope probe (x10 probe) and ground it close to the VFO to see the waveform properly.
I'm not sure why the LM386 is getting hot but as stated earlier, it isn't a good choice of pre-amplifier. I would guess it is unstable, probably because it is driving a capacitive load, adding a resistor of about 47 Ohms between pin 5 and the 10uF output capacitor should fix that.
Good design practice is to add a 100nF ceramic capacitor across the LM386 supply pins (across the 100uF capacitor beside it).
A small amount of heating, especially in the TDA2030 is normal. I would guess the change in 'static' sound is good, if the LM386 was oscillating it would produce some strange sounds of it's own. You are probably hearing normal background noise now and it was wrong before. The 47 Ohm resistor will make almost no change in the volume or tone quality of the sound, it just adds some isolation between the output of the LM386 and the capacitor across the volume control because like most amplifiers they become unstable when driving a capacitive load.Then I added the resistor (47 as you said), and now the LM386 is not heating, and TDA2030 is heating only a little.
But I feel that the sound static changed slightly....
Years ago there was a color coding system but when Asian production ramped up, it was ignored and these days the color means nothing. However, bear in mind the value of the trimmer capacitor depends on the number of 'vanes' on the shaft and the area interleaved between them. They have a maximum value when all the vanes are intermeshed and a minimum value which is from proximity when they are widest apart. That means for the most part that you can use a higher value than specified if you adjust it to a lower value. Equally, if the value isn't going high enough, you can add a fixed capacitor across it. For example, if you needed 50pF and you had a 20pF trimmer you could add 40pF in parallel with it so the range covered say 42pF to 60pF. I assumed 2pF as the minimum trimmer value.lso, now I have the 50pF input antenna trimmers left to solder, but I realized that my trimmers from drawer have no value description, they are just in different colours. One is yellow and one is brown.... one of them is 40pF and second is 60pF... is there some kind of single-colour code for them? Or maybe I can use just 27pF caps instead of them...
The only reason for using the potentiometers is they allow wires to run to the controls if they are mounted off the board. A variable capacitor will certainly work and may be a solution to the tuning problem but remember that all wires and PCB traces have inductance so if you connect the the variable capacitor 'off PCB' the wire lengths will have an effect on the frequency. Also, if you do use a variable capacitor, make sure the 'body' and the shaft are connected to the circuit ground or it will seriously go off frequency if you touch it. If it isn't obvious which side is ground, do a continuity test between the shaft and connecting pins, only the ground side should show conductivity.What do you suggest to do next? I've been thinking about adding a variable capacitor to VFO. Maybe replace the "Main Tune capacitor" with variable one...
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