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高频情况下的单端信号和差分信号的转换


Single-to-differential Conversion in High-frequency Applications
Introduction
The aim of this application note is to provide the user with different techniques for single-to-differential conversions in high frequency applications. The first part of this document gives a few techniques to be used in applications where a single-to-differential conversion is needed. The second 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 users find a convenient method to convert a single-ended signal source to a differential signal.

Conversion Techniques in High-frequency Applications Application Note

Rev 5359A-BDC-01/04

Single-to-differential Conversion Techniques
Note: All lines are 50? lines unless otherwise specified.

Technique 1: Direct Conversion Using a 1:√ 2 Balun

The following implementation is the simplest one in theory but not necessarily the easiest 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

P1 (W) or P1 (dBm) 50?
1:sqrt(2)

P2 (W) or P2 (dBm)

Middle point used for biasing 1 1 x √2

50?

50?

√(Rout/Rin) = √(100/50) =√2
The disadvantage of this method is that it can be difficult to find a 1:√2 balun on the market since the number of turns on the secondary has to be 2√2 times the number of turns on the primary. For example, if the primary has 10 turns, then the secondary 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 insertion loss during the transformation from single to differential (power from the primary to each secondary is conserved, P1 = P2 global power). Furthermore, no additional discrete components are required for the matching between the source and the receiver.

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Technique 2: Conversion In the following configuration, a standard 1:1 balun is used. Using a 1:1 Balun
Figure 2. Single-to-differential Conversion Using a 1:1 Balun

P1 (W) or P1 (dBm) 50?
1:1

P2 = P1/2 (W) or P2 =P1 – 3dB (dBm) 25? Line 50? 100? 50 ? 25? Line Equivalent to 50 ?

1

1

The drawbacks of this solution is that a 100? (2 x 50?) resistor is required for the matching (50? at the source and 100? in parallel to 2 x 50? at the receiver input), and that while P1 is supplied at the source, only half the power is transmitted to the receiver (the loss is due to the 100? resistor): P2 = P1/2 in W (or P1 - 3dB in dBm). Extra components are also required to provide biasing. The advantage of this configuration is that it uses a standard 1:1 transformer that is easy to find on the market.
Notes: 1. The 100? resistor has to be placed as close as possible to the load (input buffer). 2. 25? lines have to be used at the output of the balun.

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Technique 3: Conversion In the following figure, a standard 1:1 double coil balun is used. Using a 1:1 Balun with Double Secondary
Figure 3. Single-to-differential Conversion Using a 1:1 Double Coil Balun

Must see 100 ? at each coil P1 (W) or P1 (dBm) 50? 100? Line
1:1

P2 = P1/2 (W) or P2 = P1 – 3dB (dBm) 50?

100? 50? Biasing 50? 1 100? 1 100? Line 50? Equivalent to 100 ?

Again, this configuration has one main disadvantage, which is that two 50? resistors are required for the matching (50? at the source and 2 x 50? in parallel at the receiver input), and that as in the preceding technique, while P1 is supplied at the source, only half the power is transmitted to the receiver (the loss is due to the 100? resistor): P2 = P1/2 in W (or P1 - 3dB in dBm). In addition, 100? lines are required to keep the impedance matching. The advantage of this configuration is that the middle point can be easily used for biasing.
Notes: 1. The 50? resistors have to be placed as close as possible to the load (input buffer). 2. 25? lines have to be used at the output of the balun.

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Conversion Techniques in HF Applications
Technique 4: Conversion This last configuration uses a 1:1 balun but in a totally different way: it makes use of the fact that each coil has the same potential drop. In this configuration, however, the priUsing a 1:1 Balun with mary and secondary are well-isolated from one another. Twisted Cable
Figure 4. Single-to-differential Conversion Using a 1:1 Twisted Pair Balun

Must see 50? P1 (W) Or P1 (dBm) 50?
1:1

P2 = P1/2 (W) or P2 = P1 – 3dB (dBm) AC coupling capacitor

25? Line 50?

50? Biasing 50?

50?

50?

25? Line AC coupling capacitor Equivalent to 50?
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 secondary 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 (see 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 secondary are wellisolated 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 close as possible to the load (input buffer). 3. The two 50? external resistors have to be placed as close as possible to the load (input buffer). 4. 25? lines have to be used at the output of the balun.

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Technique 5
Figure 5. Single-to-differential Conversion Using a 1:1 Twisted Pair Balun

AC coupling capacitor
1:1

50?

25? Line 50? Biasing 50? 50? 50?

25? Line AC coupling capacitor Short-circuit at DC

Like the previous configuration, the LF which is not transmitted by the secondary is not by the primary either.
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 close as possible to the load (input buffer). 3. The two 50? external resistors have to be placed as close as possible to the load (input buffer). 4. 25? lines have to be used at the output of the balun.

Single-to-differential Conversion Applied to Atmel Broadband Data Conversion Devices
Notes: 1. All lines are 50? lines unless specified otherwise. 2. The external capacitors and resistors have to be placed as close as possible to the load.

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Conversion Techniques in HF Applications
Figure 6. 2 x 50? to Ground Internal Receiver Termination (Ground Common Mode)

50?

1:sqrt(2)

1 T1

1 x √2

50?

1:1

25? Line Receiver (converter input buffer) 1 100? 25? Line 50?

1 T2

1:1

100? Line 50? 1

50?

50? T3 1

1 100? Line 50?

Applies to: TS8308500 8-bit 500 Msps ADC in CBGA 68 (analog and clock input) 50?
-

50?

1:1

25? Line

T4

TS8388B 8-bit 1 Gsps ADC in CBGA 68 (analog and clock input) TS83102G0B 10-bit 2 Gsps ADC (analog input)

25? Line

50?

-

50?
1:1

25? Line

50?

T5

25? Line

50?

Possible configurations (to be connected directly to the receiver)

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Figure 7. 2 x 50? to Ground External Receiver Termination (Ground Common Mode)

50?

1:sqrt(2)

1 T1

1 x √2

50?

1:1

25? Line Receiver (converter input buffer) 1 100? 25? Line 50? High Z

1 T2

1:1

100? Line 50? 1

50?

50? T3 1

1 100? Line 50?

Applies to:
-

TS8388B 8-bit 1 Gsps ADC in CQFP 68 (analog and clock input) AT84AD001B dual 8-bit 1 Gsps ADC (analog input)

50?

1:1

25? Line

-

50?

T4

25? Line

50?

50?
1:1

25? Line

50?

T5

25? Line

50?

Possible configurations (to be connected directly to the receiver)

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Conversion Techniques in HF Applications
Figure 8. 2 x 50? to Ground via a Capacitor Receiver Termination

50?

1:sqrt(2)

1 T1

1 x √2

50?

1:1

25? Line Receiver (converter input buffer) 1 100? 25? Line 50? 10 or 40 pF

1 T2

1:1

100? Line 50? 1

50?

50? T3 1

1 100? Line 50?

Applies to:
-

TS83102G0B 10-bit 2 Gsps ADC (clock input) TS81102G0 8-/10-bit 2 Gsps DMUX (data and clock input) TS86101G2 10-bit 1.2 Gsps MUXDAC (data input)

-

50?

1:1

25? Line

50?
-

T4

25? Line

50?

50?
1:1

25? Line

50?

T5

25? Line

50?

Possible configurations (to be connected directly to the receiver)

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Figure 9. 2 x 50? to Ground with Biased Common Mode Receiver Termination

50?

1:sqrt(2)

1 T1

1 x √2

AC coupling

50? 1 T2

1:1

1

25? Line 100? 25? Line

10 or 100 nF

Receiver (converter input buffer)

50? VCCA/2

1:1

100? Line 50? 1

10 or 100 nF

50?

50? T3 1

1 100? Line 50?

Applies to: AT76CL610 Dual 6-bit 1 Gsps ADC (clock input)
-

50?

1:1

AT84AD001B Dual 8-bit 1 Gsps ADC (clock input)

25? Line

50?

T4 50? 25? Line

50?
1:1

25? Line

50?

T5 25? Line

50?

Possible configurations (to be connected directly to the receiver)

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Conversion Techniques in HF Applications
Figure 10. External 2 x 50? to Ground with Internally Biased Common Mode Receiver Termination

50?

1:sqrt(2)

1 T1

1 x √2

AC coupling

50? 1 T2

1:1

1

25? Line 100? 25? Line

10 or 100 nF

Receiver (converter input buffer)

2 K? 100? 0.662 x VCCA 2 K?

1:1

100? Line 1

50?

10 or 100 nF

50? T3 1

1 100? Line 50?

Applies to: AT76CL610 Dual 6-bit 1 Gsps ADC (analog input)

50?

1:1

25? Line

50?

T4 50? 25? Line

50?
1:1

25? Line

50?

T5 25? Line

50?

Possible configurations (to be connected directly to the receiver)

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Figure 11. Internal 2 x 50? to Ground with Internal Bias Receiver Termination

50?

1:sqrt(2)

1 T1

1 x √2

50?

1:1

25? Line 1 50? 50? -5V AC coupling

1 T2

25? Line

Receiver (converter input buffer)
1:1

100? Line 1

50?

10 nF 50?

50? T3 1

274?

-5V

1 100? Line

10 nF 50?

50?

50?

1:1

25? Line

50?

T4 25? Line

50?

Applies to: TS86101G2 10-bit 1.2 Gsps MUXDAC (input master clock)

50?
1:1

25? Line

50?

50? T5 25? Line

Possible configurations (to be connected directly to the receiver)
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Conversion Techniques in HF Applications
Single-to-differential Transformers - References
This section gives some examples of transformers available on the market. They are provided for information only and are not exhaustive.

Wideband Transformer 4 to 2000 MHz GLSW4M202 from Sprague-Goodman

Table 1. GLSW4M202 Guaranteed Specification (from -40°C to 125°C)
Impedance (?) 50:50 Turns Ratio 11 3 dB Band Limits (MHz) 4-2000 Loss at 20 MHz (dB) Max 0.5 Model Number GLSW4M202

Figure 12. GLSW4M202 Pin Configuration

Figure 13. GLSW4M202 Typical Insertion Loss

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Wideband Transformer 4.5 to 1000 MHz GLSB4R5M102 from Sprague-Goodman

Table 2. GLSB4R5M102 Guaranteed Specification (from -40° C to 125° C)
Turns Ratio 1:1:1 3 dB Band Limits (MHz) 4.5-1000 Loss at 20 MHz (dB) Max 0.7 Model Number

GLSB4R5M102

Figure 14. GLSB4R5M102 Pin Configuration

Figure 15. GLSB4R5M Typical Insertion Loss

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Conversion Techniques in HF Applications
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Conversion Techniques in HF Applications
RF Wideband Transformer 0.5 to 1500 MHz CX2039 from Pulse

Table 3. GLSW4M202 Guaranteed Specification (from -40° C to 85° C)
Impedance (?) 50:50 Turns Ratio 11 2 dB Band Limits (MHz) Up to 1500 Primary Pins 4-6 Model Number GCX2039

Figure 16. CX2039 Pin Configuration

Figure 17. CX2039 Typical Insertion Loss

RF Pulse Transformer 500 kHZ/1.5 GHz TP-101 from Macom

The RF pulse transformer features 50? of either unbalanced or balanced impedance along with a fast rise time of 0.18 ns. Additionally, it features a low insertion loss of 0.4 dB (typical) and the TP-101 pin model is available in a flatpack package. Tables 4 and 5 provide the guaranteed specifications and operating characteristics.

Table 4. TP101 Guaranteed Specification (from -55° C to 85° C)
Feature Frequency range (1 dB bandwidth) Input impedance Output impedance Value 500 kHZ/1.5 GHz 50? unbalanced 50? balanced

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Table 4. TP101 Guaranteed Specification (from -55° C to 85° C) (Continued)
Feature Insertion loss 10/50 MHz VSWR 1 MHz/1 GHz VSWR 750 kHZ/1.5 GHz Value 0.5 dB maximum 1.4:1 maximum 1.8:1 maximum

Table 5. TP101 Operating Characteristics
Feature 750 kHz/1 MHz Input power 1 MHz/5 MHz 5 MHz/1.5GHz Rise time (10-90%) Droop (10%) Environmental Value 1.0 watt maximum 1.5 watts maximum 3.0 watts maximum 0.18 ns typical 300 ns typical MIL-STD-202 screening available

Figure 18. RF Pulse Transformer TP-101 Pin Configuration

Note:

Pins 1, 3 and 5 are grounded to case.

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Conversion Techniques in HF Applications
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Conversion Techniques in HF Applications
Hybrid Junction 2 MHz to 2 GHzH-9 from Macom
Table 6. H-9 Guaranteed Specification (from -55° C to 85° C)

Figure 19. Hybrid Junction H-9 Functional Diagram

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5359A–BDC–01/04


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