Single-to-differential Conversion in
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High-frequency Applications
Introduction
The aim of this application note is to provide the ur with different techniques for sin-gle-to-differential conversions in high frequency applications.
The first part of this document gives a few techniques to be ud in applications where a single-to-differential conversion is needed.thief的复数
The cond part of the document applies the same techniques to Atmel broadband data conversion devices, taking into account the configuration of the converters’ input buffers.
This document does not give an exhaustive panel of techniques but should help most urs find a convenient method to convert a single-ended signal source to a differen-
tial signal.
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Conversion Techniques in HF Applications
5359A–BDC–01/04
Single-to-differential Conversion Techniques
Note:
All lines are 50Ω lines unless otherwi specified.
Technique 1: Direct Conversion Using a 1:√ 2 Balun The following implementation is the simplest one in theory but not necessarily the easi-est to implement in practice due to the limited availability of 1:√ 2 baluns.The typical configuration of this technique is the following:
Figure 1. Single-to-differential Conversion Using a 1:√ 2 balun
The disadvantage of this method is that it can be difficult to find a 1:√
2 balun on the mar-ket since the number of turns on the condary has to be 2√ 2 times the number of turns on the primary.
For example, if the primary has 10 turns, then the condary should have 2 x 7 turns,which could be of some difficulty (the total number of wires is 24 in this example, which is a huge number for an RF transformer). However, power hybrid junctions exist that have the same properties and may be found more easily.
The advantage of this configuration is that there is no inrtion loss during the transfor-mation from single to differential (power from the primary to each condary is conrved, P 1 = P 2 global power).
Furthermore, no additional discrete components are required for the matching between the source and the receiver.
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Conversion Techniques in HF Applications
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Technique 2: Conversion Using a 1:1 Balun
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In the following configuration, a standard 1:1 balun is ud.Figure 2. Single-to-differential Conversion Using a 1:1 Balun
The drawbacks of this solution is that a 100Ω (2 x 50Ω) resistor is required for the match-ing (50Ω at the source and 100Ω in parallel to 2 x 50Ω at the receiver input), and that
eta是什么意思while P 1 is supplied at the source, only half the power is transmitted to the receiver (the loss is due to the 100Ω resistor): P 2 = P 1/2 in W (or P 1 - 3dB in dBm). Extra components are also required to provide biasing.
The advantage of this configuration is that it us a standard 1:1 transformer that is easy to find on the market.
Notes:
1.The 100Ω resistor has to be placed as clo as possible to the load (input buffer).
2.25Ω lines have to be ud at the output of the balun.
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Conversion Techniques in HF Applications
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Technique 3: Conversion Using a 1:1 Balun with Double Secondary
In the following figure, a standard 1:1 double coil balun is ud.Figure 3. Single-to-differential Conversion Using a 1:1 Double Coil Balun正时链条
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Again, this configuration has one main disadvantage, which is that two 50Ω resistors arepancake
required for the matching (50Ω at the source and 2 x 50Ω in parallel at the receiver input), and that as in the preceding technique, while P 1 is supplied at the source, only half the power is transmitted to the receiver (the loss is due to the 100Ω resistor): P 2 =P 1/2 in W (or P 1 - 3dB in dBm). In addition, 100Ω lines are required to keep the imped-ance matching.
The advantage of this configuration is that the middle point can be easily ud for biasing.
Notes:
1.The 50Ω resistors have to be placed as clo as possible to the load (input buffer).
哥伦比亚自行车2.25Ω lines have to be ud at the output of the balun.
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Conversion Techniques in HF Applications
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Technique 4: Conversion Using a 1:1 Balun with
Twisted Cable
This last configuration us a 1:1 balun but in a totally different way: it makes u of the
fact that each coil has the same potential drop. In this configuration, however, the pri-mary and condary are well-isolated from one another. Figure 4. Single-to-differential Conversion Using a 1:1 Twisted Pair Balun
The drawback of this configuration is that there is a dissymmetry at low frequencies (the
threshold depends on the manufacturer’s specifications): what is transmitted in BF on the primary branch is not on the condary since the latter is grounded. A simple way to recover a symmetry at low frequency is to add a third whorl in parallel to the primary and connected to ground (e Figure 5 on page 6).
The other drawback is that only half the power is transmitted from the source to the receiver.
However, the advantage of this configuration is that the primary and condary are well-isolated from one another.
When using this kind of transformer, special care has to be taken with regard to the specifications of the twisted pair, in particular for which impedance environment the transformer was built.
Notes:
1.The AC coupling capacitors may be removed if the common mode is ground.
2.The AC coupling capacitors have to be placed as clo as possible to the load (input
buffer).南美栗鼠
3.The two 50Ω external resistors have to be placed as clo as possible to the load
(input buffer).
4.25Ω lines have to be ud at the output of the balun.
