Frequency conversion circuit designed and fabricated using DBM

Design of DBM circuit

The structure of DBM circuit is simple. The following points should be noted when designing:

▲The impedance of each input and output terminal should be matched. Here, the impedance of each input and output terminal is designed to be 50Ω. If the match is not achieved, reflected waves will occur, which will destroy the balance of the DBM circuit and generate unnecessary radiation interference. In order to achieve matching, an impedance matching body (pad) can be inserted as shown in Figure 8. Although this will attenuate the signal by about 3dB, as shown in Figure (b), even in the most severe occasions, the SWR is less than 3 and the reflection can be reduced to less than half.

Figure 8 Effect of using impedance matching body (Using this matching resistor will attenuate the signal by 3dB, but it can reduce the impact of mismatch.

(Using this matching resistor will attenuate the signal by 3dB, but it can reduce the impact of mismatch.)

▲To select high-frequency diodes with consistent characteristics, the most ideal diode has a forward voltage of 0V, which is actually impossible. Therefore, if four diodes with consistent forward voltage characteristics are used, the working principle can also be close to ideal. Of course, it is best that the lower the forward voltage, the smaller the capacitance between electrodes .

ND487C1-3R pin polarity

Figure 9 Characteristics of Shottky Barrier Diode Quard ND487C1-3R [Taken from NEC's specification sheet] (four diodes with identical characteristics are connected internally)

Here, the Shottky Barrier Diode Quard ND487C1-3R (NEC) is used. Its characteristic diagram is shown in Figure 9. Four diodes with the same characteristics are connected internally, which is a special product for DBM. From the characteristic diagram, it can be seen that the input power of the oscillator input to the input terminal 2 is +3dBm (about 2mW) ~ 12dBm (about 16mW). At this time, the conversion loss is 5.5dB, plus the loss of 3dB of the impedance matching body, the total loss is 8.5dB. ▲ Transformer for DBM Transformer can use a toroidal core (Toroidal Core) or a manganese-cadmium core used for TV UHF. Here, the toroidal core FT37-#43 of Amidon Company shown in Figure 10 is used, and 0.26mm diameter enameled wire is used to make 3 layers of 4 turns to make 1:2 transformers T1 and T2.

Figure 10. DBM transformer manufacturing process (using enameled wire for triple twist winding)

Design of high frequency filter

The signals output by DBM are (fin+fosc) and (fin-fosc). By using a high-frequency pass filter, only (fin+fosc) can be passed, while (fin-fosc) can be attenuated. Figure 11 shows the design formula and data of the high-frequency pass filter. It is called a fixed-K filter, and the input and output impedance can be 50Ω and the cut-off frequency can be 50MHz.

(The L and C values ​​can be calculated from the formula for the K-type filter. Since the current value is very small, there is no problem with current capacity.)

From the formula in the figure, we can get the coil value L=0.159μH. This can be wound 8 turns with 0.5mm diameter enameled wire on Toroidal core T37-#10. From the calculation formula, we can get C=63.7pF. Here, C=68pF is used. Fabrication and adjustment of frequency converter The overall structure of the frequency converter is shown in Figure 12. Figure 13 is a printed circuit board. In the DBM part, the ground pattern is expanded. Connect the diode ND487C1-3R and the transformer T1 and T2 with the shortest distance. The adjustment method is as follows. First, as shown in Figure 14, connect the frequency counter to the emitter of the transistor Tr1 of the oscillation circuit, and then adjust the trimming capacitor 20pF to make the oscillation frequency 10MHz. If this trimming capacitor is adjusted to the maximum and the oscillation frequency is also above 10MHz, then connect a 10pF capacitor in parallel to the back of the substrate.

(If the fIF signal level is too small, the DBM circuit will not work properly. Here, it is set to +3dBm based on the characteristics of the diode.)

(The frequency is adjusted to 3 times the frequency, and the output level is 3~10dBm. When the local oscillation level of dBm is low, the conversion loss will increase.)

Figure 13: Printed circuit board of frequency conversion circuit (The printed circuit copper foil pattern of the DBM part is surrounded by ground copper foil. The part without components can be used as a VXO circuit of a simple SSG.)

Next, use an oscilloscope connected to point B (using a 10:1 test rod), and alternately adjust T4 and T5 to maximize the 30MHz waveform of the 3x frequency. In order to reduce the false image component generated by the resonance circuit of the 3x frequency circuit, a 2-stage tuning circuit is used. After adjustment, when the power at point B is above +3dBm (2mW, the voltage at 50Ω load is about 0.32V), the conversion loss of DBM is 5.5dB. When the power at point B is below +3dBm, it means that the conversion loss has increased, but as long as it is above -2dBm, it is still practical.

Characteristics test of the fabricated frequency converter The characteristics of the frequency converter can be tested using SSG and an electric field strength meter as shown in Figure 15(a). Figure (b) shows the test using SSG and a 50MHz receiver.

(If there is no electric field strength meter, you can also use SSG and receiver, and attenuator as shown in (b) to get the correct conversion loss value.)

▲Conversion loss and frequency characteristics The conversion loss of the frequency converter is 9dB. Since a resistor matcher with an attenuation of 3dB is added to the output terminal 3, the conversion loss of the DBM itself is 6dB. According to the manufacturer's characteristic diagram, it can be known that the conversion loss is 5.5dB, which is still within the design specification of 10dB conversion loss. The frequency characteristics can be tested up to 200MHz, and a completely flat characteristic can be obtained.

▲Dynamic range characteristics Figure 16 shows the characteristics of input level to output level. Based on this data, the maximum value of the signal level that can be used can be determined. The maximum value of the signal level that can be used in the DBM circuit is as shown in the figure, which can be expressed as a 1dB suppression point. This is when the output is saturated, causing the original four-output signal level to drop by 1dB and become the actual output.

Input: output characteristics of the frequency converter made in Figure 16 (When the signal level applied to the frequency converter by SSG is below -5dBm, no distortion occurs. The conversion loss is 9dB.) Since the input signal level at which the 1dB suppression point occurs is -2dBm, the maximum output signal level of the frequency converter is -11dBm. Recently, although the price of the diode DBM unit sold by the manufacturer is very low, it is meaningful to make it yourself in order to actually understand the working principle of the DBM circuit.