Possible useful technologies: Variations in common transistor models (BJT)
Becoming real
From conception to fabrication and measurement – reading this article may require some basic knowledge of electronics.
Reading this article may take several minutes, but you should be able to gain the knowledge you are looking for. Having too many theoretical formulas can lead to symptoms such as crossed eyes, and I understand that, which is why this article focuses on practical applications.
I first came up with this idea in 2019, but due to various academic commitments, I put it off for 4 years. I welcome feedback and corrections from colleagues!
Regularly published articles featuring various types of diodes and transistors.
The BH7CNC STUDIO© can be reproduced, please indicate the original source.
Introduction
Ultimately, you'll probably have to build your own VCO [Doge], and that might require using a variable diode.
We must all avoid this type of organism!Diodes are one of the earliest types of semiconductor devices and are also among the most common andThe most affordable!One of the components, which we all know has unidirectional conductivity, but this article discusses another characteristic – the junction capacitance. There is always an equivalent capacitor across the PN junction, and the capacitance between the two ends of the diode is related to the voltage at both ends. As the reverse voltage increases, the depletion region becomes thicker, and the junction capacitance decreases... By utilizing this feature, variable-capacitance diodes came into being. This article does not delve deeply into the underlying principles, but focuses on practical techniques.
Although variable-capacitance diodes (also known as "varicaps") are now relatively inexpensive, with products like BB112 and 1SV2638 costing only a few cents each, if you require high consistency, please purchase dedicated varicap diodes. To be honest, finished varicap diodes are really good – they're cheap and reliable. Why bother messing around with common transistors and rectifiers? However, it is possible to use common transistors and rectifiers as varicaps if they have equivalent characteristics, such as the junction of a TVS diode, a BJT, or a JFET. Basically, any PN junction or PIN junction will work (乐).
So, the question is: how do we measure this small capacitor (ranging from a few pF to several hundred pF)? The standard bridge circuit isn't really suitable for this task. However, these variable capacitors are often used in applications such as tank circuits, high-gain amplifiers, and tuning circuits. Therefore, we can try to replicate the usage scenario by building an LC circuit!
But, we don't know if the inductor is standard (we also need to consider temperature drift and its own distributed parameters). In that case, we can just buy a standard inductor! After seeing the price (shocked), "Okay, let's try making a relatively stable inductor ourselves first, then measure its precise value or use error-elimination methods.
The experimental design for this experiment.
Construct an LC oscillator, assuming that L changes very little or remains constant. Measure the oscillation frequency f using a NanoVNA and then calculate the capacitance C.
f = 1 / (2π√(LC))
By changing the reverse voltage Vr at both ends of the variable capacitance tube (typically between 1V and 10V), we can further determine the range and ratio of variation for the variable capacitance tube.
Raise a few practical questions
1. Regarding inductors: The equivalent circuit is R in series with L, followed by C in parallel. Therefore, the self-resistance R primarily affects the Q value and has little impact, as there is an amplifier. In this case, the stray capacitance Cl' is problematic. This capacitor typically ranges from a few pF, but if we don't replace the inductor, we can calculate the value using a slightly more precise capacitor and perform addition/subtraction to eliminate errors. Ideally, use a shielded core inductor, but the stray capacitance will increase. Alternatively, you can use a closed magnetic path inductor to reduce the impact (magnetic ring). Make sure the wire is as thick as possible to minimize heat-induced temperature drift (of course, you can turn on the air conditioner to cool this inductor, depending on your preference).
2. Regarding capacitors: A capacitor can be treated as an RLC circuit, but in this case, we are not dealing with scientific instruments. Therefore, it should be considered as an ideal capacitor.
3. Regarding PCB distribution parameters: MHz level, low frequency, similar to capacitors, the distribution parameters of various conductors are all within acceptable error margins and do not require in-depth study.
4, Regarding impedance matching: As long as a measurable signal is produced for measurement purposes, the actual matching process becomes less important. However, for a more professional approach, it's still possible to match to 50 ohms.
5. Regarding the equipment: The Nano has a TCXO (temperature-compensated crystal oscillator) which, in my opinion, is sufficient for basic use. However, if you want something more advanced, you could use a professional spectrum analyzer or vector network analyzer. But I don't think it's necessary. Another cheaper option would be to purchase an OCXO (on-chip crystal oscillator). You can then use the frequency output of the OCXO and measure the output frequency with your own oscillator to perform mixing. With this, you can build a receiver!
6. Regarding accuracy: If you want to improve it? Yes! You need to pay more! The experimental error should be as small as possible, and the acceptable range is a matter of opinion; perhaps achieving J-level (±5%) would be sufficient, or maybe M-level (±20%)? As long as it works, there will always be adjustable components for designing variable-capacitance tubes.
Adjustments based on practical considerations
In summary, standard components – expensive! Building an oscillator circuit – complicated! So, I thought about using a direct measurement method, as shown in the following figure:
The T1-T2 ports can be used for measuring with Nano, or they can be used as measurement ports for a Wheatstone bridge. Regardless, they are the measurement ends.
C1 uses a 10µF (50V X7R) capacitor;
D1 is the diode under test (as shown in the diagram, it's a common 4148).
R1 and R2 are current-limiting resistors that also isolate the measurement circuit from the DC bias.
Let's analyze this circuit briefly: In reality, the capacitor C1 and diode D1 are in series, and commonly used transistor-based capacitors are not usually greater than 200pF (this is an estimate based on experience). So, if we assume the maximum reverse capacitance of the transistor is 200pF, then 10μF in series with 200pF equals 199.996pF. This means that this error is much smaller than other errors, and a smaller capacitor actually leads to greater precision! The theory exists; let's start with the practical implementation!
Practical implementation/production
First, complete the necessary preparations. Since other software has copyright issues, I'll use Excel to create the charts. The following image is just an example:
Later, we will use this chart to illustrate the variable volume characteristics of each pipe.
Given that most variable capacitors are measured at 1 MHz, and different frequencies will result in different capacitance readings, I am also using a 1 MHz frequency for measurement here. Using Nano to measure allows me to change the frequency. The S11 port can also be used to measure the current frequency's Rs (equivalent series resistance), which can be used to calculate the Q value (although this may not be very useful).
Then, I used Nano to simply view the port properties of S11:
Connect a 22 pF capacitor:
Connect a 1000 pF capacitor:
It seems to be as expected, so I can draw a jig template now.
With this setup, both patch testing and direct insertion can be performed. Then, the stability power supply needs to be tested and the data recorded.
Test data
Another period of time has passed, I don't know how many days it has been.Once the distribution parameters have been calibrated using the board, during actual measurements:
Here's a simple test to release two versions:
To be continued...
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