How to conduct accurate testing with a spectrum analyzer?

The selection of characteristic impedance of radio frequency coaxial cable mainly depends on factors such as power capacity, attenuation intensity, and processability. However, the characteristic impedance corresponding to the maximum power capacity and minimum attenuation performance is different. The reason why 50 Ohm characteristic impedance is usually used in the radio frequency field is that the above factors are taken into consideration. In other words, the power capacity and attenuation performance corresponding to the 50 Ohm characteristic impedance are not optimal. In terms of attenuation performance alone, the characteristic impedance of 75 Ohm is much lower than that of 50 Ohm, but its application field is relatively specific and is mainly used in the signal transmission of radio and television signals.

This article will talk to you about the signal test under the 75 Ohm system. For the test of this signal, if the spectrum analyzer supports the 75 Ohm system impedance, it can be tested directly. However, most general-purpose spectrum analyzers only have a 50 Ohm system impedance. How to conduct accurate testing?

If the spectrum analyzer only supports 50 Ohm system impedance and directly tests signals such as radio and television, then the tested signal level will inevitably have a large error due to impedance mismatch. In order to improve the level test accuracy, an impedance matching circuit must be introduced.

Can directly connecting a 25 Ohm resistor in series improve the level test accuracy?

Of course, it is considered here that the resistor can cover the operating frequency range. As shown in Figure 1(a), when a 25 Ohm resistor is connected in series, the impedances are matched at the reference plane Ref. 1, but at the reference plane Ref.2, the impedances are seriously mismatched and the return loss reaches - 9.5 dB. In contrast, when the source is directly connected to the spectrum analyzer for testing without any matching, the return loss at the test reference plane is about -14 dB. It can be seen that directly connecting a 25 Ohm resistor in series is not as good as the direct connection test. This local matching method cannot effectively improve the test accuracy.

(a) Impedance matching via 25 Ohm resistor

(b) Real impedance matching through a matching network

Figure 1. Testing of 75 Ohm systems requires impedance matching to improve level test accuracy

The true impedance matching should be as shown in Figure 1(b). No matter which reference plane is viewed from, the impedance is matched. At this time, high-precision level testing can be achieved. How to design this matching circuit?

What are the design considerations for matching circuits?

For the design of the matching circuit, the following three aspects need to be considered:

(1) Whether the bandwidth of the matching circuit is sufficient and whether it has a flat amplitude-frequency response;

(2) Whether the loss of the matching circuit can be as low as possible, which is especially important for weak signal testing;

(3) Whether complete matching can be achieved. Only complete matching can achieve high-precision level testing.

Radio and television usually work in the VHF and UHF frequency bands. The impedance conversion between 50 Ohm and 75 Ohm can be achieved by using inductors and capacitors to form an "L-shaped" matching network. Moreover, the loss of this reactive matching network is very low, but its bandwidth is relatively small. Narrow, and the flatness of the amplitude-frequency response is not very good unless a high-order matching circuit is used. However, this increases the design difficulty and loss of the matching circuit.

Is there a more appropriate matching circuit design method?

The answer is yes. By using high-frequency resistors to design the matching network, not only can complete matching be achieved, but also a flat broadband amplitude-frequency response can be achieved. However, the disadvantage is also obvious-large loss. This matching circuit can be understood as an attenuator with impedance transformation. The 50-to-75 Ohm impedance converters currently on the market basically adopt this design method.

Minimal loss resistor matching network - MLP

It is very particular to use high-frequency resistors to design a matching network. After all, resistors will lose signals. Too few resistors in the matching network will not play the role of impedance matching. Too many resistors will cause too much loss and will reduce the system test sensitivity. In fact, there is a minimum loss matching network, which can be realized by two resistors, usually called Minimum Loss PAD (MLP). The following will introduce the design of this resistor matching network and how to select the resistance value of the resistor.

Figure 2. Minimum Loss PAD constructed from resistors

The topology of the MLP impedance transformation network is relatively simple, which is an "L-shaped" network constructed of two resistors, as shown in Figure 2. If you want to obtain the desired impedance at the left and right reference planes, the two resistor values ​​should How to choose?

The determination of the resistance value needs to be calculated according to Ohm's law, which is the calculation process of series and parallel impedance. The specific derivation process will not be described in detail. Only the conclusion is given here.

Considering the situation of Z1 >> Z2, in order to achieve impedance matching at the two reference planes, the required R1 and R2 are respectively

How to determine the attenuation of MLP?

The determination of MLP attenuation still needs to be based on Ohm's law. Combined with the equivalent circuit diagram shown in Figure 3, the relationship between the input and output voltages can be determined, and then the relationship between the input and output power can be determined. After some derivation, the relationship between input and output power is as follows

This means that the attenuation of the MLP impedance transformation network is

Figure 3. Equivalent circuit diagram after introducing Minimum Loss PAD

For the impedance transformation design of 50-to-75 Ohm, according to the above formula, it can be determined that the resistors R1 and R2 are respectively

R1=43.3Ω

R2=86.6Ω

The corresponding loss is -5.72 dB. When actually using a spectrum analyzer for testing, you can directly write this loss value into the spectrum analyzer to automatically compensate for the tested level value.

It is worth mentioning that impedance transformation can also be achieved by using three or more resistors, but this higher-order circuit means greater losses. A resistor cannot achieve impedance transformation, so the network shown in Figure 2 is the minimum attenuation network that performs impedance transformation, which is why it is called Minimum Loss PAD.

In addition, when selecting a resistor, you should also pay attention to the frequency range of the resistor. It must be able to cover the frequency range of the signal to be tested, otherwise the designed impedance matching network will not achieve the expected effect.

What does the 75 Ohm system interface look like? Is the interface compatible with 50 Ohm systems?

Commonly used BNC, N, SMB, SMC and other interfaces in testing have characteristic impedances of 50 Ohm and 75 Ohm. In the field of radio and television, N-type interfaces with 75 Ohm characteristic impedance are more commonly used.

For coaxial connectors and cables, the characteristic impedance depends on the ratio of the radii of the inner and outer conductors. The outer conductor size of the 75 Ohm N-type interface and the 50 Ohm N-type interface is the same, but the higher the characteristic impedance, the thinner the inner conductor. The two interfaces are shown in Figure 4. Therefore, a 75 Ohm male head can be connected to a 50 Ohm female head, but a 75 Ohm female head cannot be directly connected to a 50 Ohm male head, otherwise the connector will be damaged.