Frequency conversion circuit designed using DBM and its production part 1

The following trial example is an example of adding a frequency converter to a standard signal generator (SSG) with a maximum frequency of 50 MHz to increase its frequency to 80 MHz.

Using DBM in a multiplication circuit

The heart of frequency conversion is the multiplication circuit, in which a double balanced mixer (DBM) is used. The principle of DBM circuit as frequency conversion can be proved by experiments.

FIG5 shows a block diagram of a frequency converter.

Figure 5 Block diagram of an actual frequency converter

(The output signals of the DBM circuit are only (fin+fiF) and (fin-fiF). By passing this through a high-frequency filter, only (fin+fiF) can be taken out.)

There are two input terminals on the DBM, fin and fosc. Fin is the SSG signal that can reach up to 50MHz, and fosc is the 10MHz signal of the crystal oscillator that is tripled to 30MHz.

Thus, the sum and difference of two frequencies will appear at the output of DBM, namely (fin+fosc) and (fin-fosc) [when fin

Thus, the output frequency fout of the frequency converter becomes (fin+fosc), that is, the frequency fin of the SSG can be increased by fosc (here 30 MHz).

Design specifications of the fabricated frequency converter

Table 1 shows the design specifications of the frequency converter. The impedance of the input and output terminals is set to 50Ω so that it can be connected to other high-frequency devices.

Table 1 Specifications of the fabricated frequency converter

(The most important thing about this circuit is that no unnecessary interference signals appear; therefore, the intensity of the unnecessary radio waves is specified to be below -10dB.)

The output frequency after frequency conversion is fout=fin+fosc=fin+30MHz

Therefore, assuming that the frequency fin of SSG is 2M~50MHz, fout will become 32M~80MHz.

If fosc=40MHz, then fout becomes 42M~90MHz, and the frequency range is too wide, so it is not considered.

(This is to obtain the spectrum when fout=50MHz. It can be seen that when fosc=40MHz, it is more difficult to attenuate fosc-fin=30MHz.)

The output frequency after frequency conversion is fout=fin+fosc=fin+30MHz

Therefore, assuming that the frequency fin of SSG is 2M~50MHz, fout will become 32M~80MHz.

If fosc=40MHz, then fout becomes 42M~90MHz, and the frequency range is too wide, so it is not considered.

Figure 6 shows that in order to obtain the spectrum when fout is 50MHz, fosc = 30MHz in Figure (a), so (fosc+fin)/(fosc-fin) = 50/10 = 5, and the 10MHz attenuation of (fosc-fin) can be used by the high-frequency filter connected later.
However, when fosc = 40MHz in Figure (b), (fosc+fin)/(fosc-fin) = 50/30 = 1.67

Therefore, the 30MHz of fosc-fin can hardly be attenuated.

In summary, for the frequency characteristics of the high-frequency filter, if the unwanted radiation intensity is to be reduced to below -10dB, the following condition must be met: (fosc+fin)/(fosc-fin)>2

Here, fosc is selected to be 30 MHz so that the conversion loss when the input signal is converted into the output signal is -10 dB or less.

Working Principle of Diode DBM Circuit

The DBM circuit is a circuit that can multiply two input signals. The DBM circuit can be composed of diodes or transistors. The following describes the case where the DBM circuit is composed of a simple diode.

The working principle of the diode DBM circuit is shown in Figure 7. A small amplitude signal is input from input terminal 1, and a large amplitude signal is input from input terminal 2.

Figure (a) shows the case where the signal at input terminal 2 is positive. This large amplitude signal turns diodes D1 and D2 on. When D1 and D2 are turned on, the small amplitude signal input from input terminal 1 flows to T2 in the direction shown in the figure. Therefore, the small signal at input terminal 1 appears directly at output terminal 3.

Figure (b) shows the case where the signal at input terminal 2 is negative. At this time, diodes D3 and D4 will be turned on, and the small signal flow at input terminal 1 will become reversed, that is, the signal at output terminal 3 will become a signal with a phase opposite to that of the signal at input terminal 1.

Therefore, by using a large amplitude signal at the input terminal 2, the diode can be turned on alternately, and the position where the diode becomes SW changes, that is, the conduction direction changes.

The DBM circuit is a multiplication circuit. Therefore, when the input terminal 2 becomes positive, it is equivalent to multiplying by 1, and the phase of the signal at the input terminal 1 is output directly, and when the input terminal 2 becomes negative, it is equivalent to multiplying by -1, and the phase of the signal at the input terminal 1 is inverted and output.