Transceiver TH7122x Cookbook TH7122 and … Note Transceiver TH7122x Cookbook 39011 07122 01 Page 3...

Application Note Transceiver TH7122x

Cookbook

39011 07122 01 of 18 AN7122x-Cookbook Rev. 003 June/05

TH7122 and TH71221 Cookbook

1 General Description The TH7122 and TH71221 transceiver ICs are highly versatile ISM band RF components suitable for a wide range of applications. Both devices require external components for setting the operating frequency and IF bandwidth. The TH7122x, through proper component selection, may be used for any frequency between 27 and 950MHz. It performs well in wide and narrow band applications and with FM, FSK, and ASK (OOK) modulation. This application note is called a “Cookbook” because it is a tested collection of real applications and reference example circuits. The detailed descriptions of the procedures for selecting the appropriate components will help get your design up and running fast.

Please also consult the data sheets and evaluation board descriptions for detailed technical information. These can be found on the Melexis Web Site at www.melexis.com. Assistance and questions can be accessed using Melexis Knowledge Base Web Forum at www.melexis.com/forum.

Fig. 1: TH7122 and TH71221 IC block diagram

2 Important Features Usable in stand-alone or programmable user mode (via 3-wire bus serial control interface - SCI) The reference oscillator input (RO) can be a crystal oscillator or an input buffer from an external TCXO

or microprocessor reference. It drives a programmable R counter with a range of 4 to 1023. The input range is 1 to 16MHz.

The N counter has a very wide range of 64 to 131071, so very small VCO frequency steps are possible. The VCO is a negative resistance oscillator with a tuned circuit between pins 20 and 21. There is also

an internal varactor (not shown). This allows the VCO to be tuned to any desired frequency by selecting the appropriate inductor. An external varactor can be added for wider tuning ranges.

The loop filter for the VCO on pin 23 is external to allow optimizing the design for wide band or narrow band applications.

The LNA and IF sections are tuned by external elements. Usually a ceramic filter is used for the IF, but crystal filters can be used for narrow band applications.

The FM detector on pin 3 can be tuned with a ceramic discriminator or LC combination. OA1 is an operational amplifier to be configured as a data slicer OA2 is biased at Vcc/2 and can be used as an AFC amplifier. Both FSK and ASK (OOK) transmission and reception are possible with a switchable peak detector for

ASK reception. ASK and FSK operation is possible without changing any parts by just loading in the correct control data

via the SCI bus.

IN_LNALNA

GAIN

_LNA

OUT_

LNA

IN_M

IX

IF

MIX26

29 28 30

OUT_

MIX1

IN_IF

A

VEE_

IF32 31 1

VEE_

LNA27

IFA

VCC_

IF2 RSSI7

1.5pF

IN_DEM3

MIX

DemodulatorFSK

OUT_DTA

OA2bias

OUT_DEM

SW1

SW2

6

OA18

INT15

INT2/PDO

4

200k

VEE_PLL

OUT_PAFSK

ASK

LOIN

_DTA

ASK/

FSK

RE/S

CLK

TE/S

DTA

FS0/S

DEN25

PS_PA24 FSK_

SW

FS1/

LD

VEE_

RO

11 19 9 1716151312

RO RO

Rcounter

Ncounter

RO10

SCLKSDTASDEN

Control Logic SCI

PKDET

VCO

PA

VCC_PLL20TNK_LO 23 22LF21 18 VEE_

DIG

14 VCC_

DIG


Cookbook


Document Content

1 General Description ...................................................................................................1

2 Important Features.....................................................................................................1

3 VCO Design ................................................................................................................3 3.1 Standard FSK VCO .......................................................................................................... 3 3.2 Standard ASK VCO .......................................................................................................... 4 3.3 VCO with External Varactor.............................................................................................. 4 3.4 Direct VCO Modulation for Narrow Band......................................................................... 5 3.5 High Speed Data Communication .................................................................................... 5 3.6 FSK Crystal Modulation.................................................................................................... 6 3.7 Analog FM or FSK ............................................................................................................ 6 3.8 Multi-Band Switching ........................................................................................................ 7

4 IF Filtering...................................................................................................................7 4.1 IF Filter - Standard............................................................................................................ 8 4.2 IF Ceramic Filter - Narrow Band....................................................................................... 8 4.3 IF Crystal Filter - Narrow Band ......................................................................................... 8

5 FSK and FM Detectors ...............................................................................................9 5.1 FSK Detector - Standard .................................................................................................. 9 5.2 FSK Detector – LC Tank .................................................................................................. 9 5.3 High Speed FSK Detector .............................................................................................. 10 5.4 FSK Detector - External AFC (I) ..................................................................................... 10 5.5 FSK Detector - External AFC (II) .................................................................................... 11 5.6 FSK Squelch Circuit ....................................................................................................... 11 5.7 FM Detector .................................................................................................................... 12 5.8 Wide Band FSK / ASK Detector ..................................................................................... 12

6 ASK (OOK) Detectors and RSSI..............................................................................13 6.1 ASK Detector - Standard ................................................................................................ 14 6.2 ASK Detector - Peak Detector ........................................................................................ 14

7 RF Input Matching....................................................................................................15

8 RF Output Matching.................................................................................................15

9 LNA Output and Mixer Input Matching ...................................................................16

10 Special Considerations............................................................................................16


Cookbook


3 VCO Design If the internal varactor diode of the TH7122x is used for tuning the VCO at Vcc = 3.0V, the tuning ratio is fmax / fmin ≤ 1.135. For Vcc = 5.0V, the ratio is approximately 1.191. This determines whether or not an external varactor is required. For example, at 27MHz operation with a 10.7MHz IF (intermediate frequency), the VCO frequency in the receiver will be 37.7MHz. This gives fmax / fmin = 1.40, so an external varactor will be required. Tuning ratios up to 2:1 are possible with hyper-abrupt tuning diodes. Another consideration when tuning the VCO is the tuned circuit impedance. The VCO is a negative resistance oscillator, and its resistance decreases with increasing frequency. Its noise can be improved by reducing the tank circuit losses to increase the Q of the tank. The loaded Q of the equivalent RLC parallel combination of the VCO tank is L/C/R . It is limited by the parasitic elements of the IC package and the PCB. Care must be taken not to make the tank inductor too small, the VCO may not start or may oscillate at a high parasitic frequency which is determined by the circuit traces and stray capacitance of the coil and/or varactor circuit. On the other hand, the coil inductance must be small in order to have a low tuned circuit impedance )/( CL . This means that the tuning capacitance should be large. In this case, an external varactor must be used even though the same frequency could be tuned with the internal varactor. With a tuning voltage range from 0.2V to Vcc - 0.2V, the equivalent capacitance Cint across pins 20 and 21 is approximately 3.37 to 4.34pF when Vcc = 3.0V. When Vcc = 5.0V, the minimum capacitance is approximately 3.06pF. The VCO directly generates the RF signal in transmit mode, so the VCO and transmit (carrier) frequency are the same. In receive mode, the VCO is also active and its frequency is offset from the receive frequency by the IF (because of the super-heterodyne receiver architecture). In this mode the VCO is also called LO (local oscillator). There is always a slight LO signal passing to the receiver input. This undesired leakage is lowest if the VCO current is set to the smallest value; this can be done by setting the VCOCUR register bits to 00.

VCC

IN_I

FA

FS0/

SD

EN

TNK_

LO

IN_D

EM

INT2

/PD

O

INT1

OU

T_D

EM

RSS

I

OU

T_D

TA

VEE_RO

RO

FSK_SW

IN_DTA

ASK/FSK

VCC_DIG

RE/SCLK

GAIN_LNA

VEE_IF

IN_MIX

OUT_MIX

OUT_PA

OUT_LNA

VEE_LNA

VC

C_P

LL

VE

E_D

IG

VEE_

PLLLF

FS1/

LD

9

10

11

12

13

14

15

16

IN_LNA

VCC

_IF32

31

30

29

28

27

26

25

TH7122x

1718192021222324

81 2 3 4 5 6 7

L0

CF2

C0RF33k

1nFCF1

CB6100pF

RPS

3.1 Standard FSK VCO RF, CF1 and CF2 are the values used on the standard evaluation boards (EVBs). C0 can be added to adjust the VCO tuning voltage that can be monitored on pin 23 so it is not too close to Vcc or GND. This way the VCO frequency can be centered for optimum performance in the particular operating range. This circuit is recommended for data rates up to about 20kbps NRZ (non-return to zero). The FSK modulation is achieved by switching the capacitive loading of the crystal oscillator, just as shown in the TH7122x data sheets.


Cookbook


CP

S

1nF

100pFCF1

RB0VCC

IN_I

FA

FS0/

SD

EN

TNK_

LO

IN_D

EM

INT2

/PD

O

INT1

OU

T_D

EM

RSS

I

OU

T_D

TA

VEE_RO

RO

FSK_SW

IN_DTA

ASK/FSK

VCC_DIG

RE/SCLK

GAIN_LNA

VEE_IF

IN_MIX

OUT_MIX

OUT_PA

OUT_LNA

VEE_LNA

VC

C_P

LL

VEE_

DIG

VEE

_PLLLF

FS1/

LD

9

10

11

12

13

14

15

16

IN_LNA

VCC

_IF32

31

30

29

28

27

26

25

TH7122x

1718192021222324

81 2 3 4 5 6 7

CF239pF

L0

RF C0 CB6100pF

RPS

10033

k

3.2 Standard ASK VCO RF, CF1 and CF2 are set to give a wider PLL bandwidth. This allows the VCO to correct faster for frequency disturbances caused by load pulling effects in the ASK-modulated PA. C0 is added to reduce the tuning range of the VCO. This results in smaller frequency disturbances (spurious FM) caused by switching the PA stage on and off. CPS also helps to minimize spurious FM by reducing the rise and fall time of the PA switching on and off.

L0

IN_I

FA

FS0/

SDE

N

TNK

_LO

IN_D

EM

INT2

/PD

O

INT1

OU

T_D

EM

RSS

I

OU

T_D

TA

VEE_RO

RO

FSK_SW

IN_DTA

ASK/FSK

VCC_DIG

RE/SCLK

GAIN_LNA

VEE_IF

IN_MIX

OUT_MIX

OUT_PA

OUT_LNA

VEE_LNA

VCC

_PLL

VEE

_DIG

VEE_

PLLLF

FS1/

LD

9

10

11

12

13

14

15

16

IN_LNA

VCC

_IF32

31

30

29

28

27

26

25

TH7122x

1718192021222324

81 2 3 4 5 6 7

VD1

CF2

RF1

C0

C01

CB6100pF

RPS

R012210k

CF1

RF

VCCRB0100

3.3 VCO with External Varactor This circuit can be used to extend the frequency range. The frequency extension can be even down to 27MHz. The VCO frequency fvco is given by:

⎟⎠⎞

⎜⎝⎛

+⋅

++=

C01CdC01CdCintC0L0

12π1fVCO

Where Cint = internal diode capacitance Cd = diode capacitance of VD1 RF1 connects the varactor to the tuning voltage and filters the RF signal present on the diode. R01 is used in low frequency applications to prevent parasitic oscillations at high frequencies. It is also possible to put R01 in series with pin 20. The combination of C0 and C01 set the tuning range. If VD1 has a large capacitance at 0V, C01 should not be made larger than about 100pF or the low impedance of the circuit may prevent start-up of the VCO.


Cookbook


L0

IN_I

FA

FS0/

SDE

N

TNK

_LO

IN_D

EM

INT2

/PD

O

INT1

OU

T_D

EM

RSS

I

OU

T_D

TA

VEE_RO

RO

FSK_SW

IN_DTA

ASK/FSK

VCC_DIG

RE/SCLK

GAIN_LNA

VEE_IF

IN_MIX

OUT_MIX

OUT_PA

OUT_LNA

VEE_LNA

VCC

_PLL

VEE

_DIG

VEE_

PLLLF

FS1/

LD

9

10

11

12

13

14

15

16

IN_LNA

VCC

_IF32

31

30

29

28

27

26

25

TH7122x

1718192021222324

81 2 3 4 5 6 7

VCCRB0

CF2CF1

RF

C0

CM2CM1

RM1

RM2

RM3

mod

CB6100pF

1M

10k

VD1RF1

C0122

R01

100

RPS10

k

3.4 Direct VCO Modulation for Narrow Band

This circuit is usually used in narrow band applications. Rather than switching the crystal oscillator’s capacitive loading for FSK generation, this circuit employs so-called direct VCO modulation. This means data is directly injected into the VCO control line through the loop filter. CF1 is usually 1.0µF or larger and CF2 is usually 100nF or larger. RF is usually around 1.5 to 3.3kΩ. To get flat modulation response, it is necessary that CM1·RM1 = CF2·RF. Since the VCO is very sensitive, the modulation signal must be attenuated. RM1 = 1MΩ and RM3 = 10kΩ for convenience, and the modulation sensitivity is adjusted by changing RM2. CM2 can be added to reduce the rise time of the digital modulating signal to reduce the occupied bandwidth. This arrangement can also be used with only the internal varactor. The design equations for the loop filter are in the Melexis application note: TH7122 and TH71221 Used In Narrow Band FSK Applications

L0

IN_I

FA

FS0/

SDE

N

TNK

_LO

IN_D

EM

INT2

/PD

O

INT1

OU

T_D

EM

RSS

I

OU

T_D

TA

VEE_RO

RO

FSK_SW

IN_DTA

ASK/FSK

VCC_DIG

RE/SCLK

GAIN_LNA

VEE_IF

IN_MIX

OUT_MIX

OUT_PA

OUT_LNA

VEE_LNA

VC

C_P

LL

VEE

_DIG

VEE_

PLLLF

FS1/

LD

9

10

11

12

13

14

15

16

IN_LNA

VCC

_IF32

31

30

29

28

27

26

25

TH7122x

1718192021222324

81 2 3 4 5 6 7

RPSCB6

100pF

VCC

CF2

CF1

RFCM1

RM1mod

3.5 High Speed Data Communication This circuit is usually used in high speed applications for data rates of up to about 115kbps NRZ. It is similar to the preceding one and also employs direct VCO modulation. But here no external varactor is needed. The design equations for the loop filter are in the Melexis application note TH7122 and TH71221 High Speed Data Communication.


Cookbook


FS0/

SDE

N

INT1

OU

T_D

EM

RS

SI

OU

T_D

TA

VEE_RO

RO

FSK_SW

IN_DTA

ASK/FSK

VCC_DIG

RE/SCLK

VCC

_PLL

VEE

_DIG

FS1/

LD

9

10

11

12

13

14

15

16

17181920

85 6 7

XTAL

CX1CX2

3.6 FSK Crystal Modulation This is the standard approach to generate an FSK signal. The crystal frequency, that provides the reference to the internal PLL synthesizer, is pulled by two external capacitors CX1 and CX2. An internal switch at pin FSK_SW is either open (if the signal at pin IN_DTA is logic high) or connected to ground (if the signal at pin IN_DTA is logic low). This way either CX1 or the combination of CX1 + CX2 determines the crystal frequency. So the FSK signal is generated at the crystal frequency and then up-converted in the PLL. The polarity of the signal at IN_DTA can also be inverted in programmable mode. This circuit is recommended for data rates ranging from DC to about 20kbps NRZ.

FS0/

SDE

N

INT1

OU

T_D

EM

RS

SI

OU

T_D

TA

VEE_RO

RO

FSK_SW

IN_DTA

ASK/FSK

VCC_DIG

RE/SCLK

VCC

_PLL

VEE

_DIG

FS1/

LD

9

10

11

12

13

14

15

16

17181920

85 6 7

XTAL

CX1 VD2

A1R R C

CX

IN_FM

R CX

IN_FM

100k

3.7 Analog FM or FSK In this circuit a varactor diode is used to modulate the crystal. An external 2nd order Sallen-Key low-pass filter can be added to shape the modulating signal. The easiest Sallen-Key setup is to use equal filter resistors and equal capacitors. This makes the filter cut-off frequency and the Q independent of one another. Direct feedback from the A1 output to the (-) input creates a unity-gain characteristic. If the requirements for filtering are low, a very simple passive RC filter can be used instead. The filter cut-off frequency is given by:

RC2π1f C ⋅

=

The circuit can be used, for example, to transmit audio signals.


Cookbook


470

470

IN_I

FA

FS0/

SD

EN

TNK_

LO

IN_D

EM

INT2

/PD

O

INT1

OU

T_D

EM

RSS

I

OU

T_D

TA

VEE_RO

RO

FSK_SW

IN_DTA

ASK/FSK

VCC_DIG

RE/SCLK

GAIN_LNA

VEE_IF

IN_MIX

OUT_MIX

OUT_PA

OUT_LNA

VEE_LNA

VC

C_P

LL

VE

E_D

IG

VEE

_PLLLF

FS1/

LD

9

10

11

12

13

14

15

16

IN_LNA

VCC

_IF32

31

30

29

28

27

26

25

TH7122x

1718192021222324

81 2 3 4 5 6 7

C02

RS1

RS2

C01

L03

L01L02

C03

RF1CF2

CF1

RF

PD1

VD1

PD2

CB6100pF

330pF

BAR64

BAR64

BBY65

1nF

100pF2.2nH

27nH

1 Hµ

10k

R0322

VCCRB0

22

RPS

BS1

BS2

3.8 Multi-Band Switching This circuit shows one method for switching the VCO bands to cover a wider tuning range. In this case, the lowest frequency is around 27MHz and the highest is 950MHz.

PD1 and PD2 are PIN diodes with a low capacitance at 0V. VD1 is a wide range tuning diode. BS1 and BS2 are the band switching inputs. R03 prevents parasitic oscillations when the low frequencies are selected.

When BS1 and BS2 = Vcc, the lowest frequency is set by L02 and L03, VD1 and the TH7122x internal varactor.

When BS1 = 0V, and BS2 = Vcc, the low frequency circuit is shorted by PD1, and the frequency is determined by LO2 and the TH7122x internal varactor. This setting is for medium bands.

When BS1 = Vcc and BS2 = 0V, the frequency is determined by LO1 and the TH7122x internal varactor. This is the highest frequency band setting.

Remember that the off capacitance of the PIN diodes is in parallel with the coils and the series inductance of the PIN diode and C01 adds to the coil inductance and becomes significant. Also, the drive to BS1, and BS2 must be able to sink approximately 10mA at Vcc = 5V.

4 IF Filtering LC, ceramic or crystal filters can be used for the IF filter. The most common frequency for this type of application is 10.7MHz. 21.4MHz IF filters can also be used. 455 or 450kHz is not practical because the internal capacitor coupling the IF to the demodulator is only 1.5pF. Also, with single conversion and a low IF, the receiver has virtually no image rejection. The output resistance of pin 32, OUT_MIX, is about 330Ω to match most ceramic filters. In order to match other filters, a passive matching network can be used. A simple PI or L matching network can be used, but a PI network with a higher Q has the advantage of reducing the spurious responses of the filter far from the center frequency. A PI network can also be added to the normal ceramic wide band filter to suppress spurious responses. The input resistance of pin 1, IN_IFA, is about 2kΩ in parallel with a few pF of capacitance. This is very convenient because this is about the required termination for 10.7MHz crystal filters. Just adding a resistor between pins 1 and 2 can terminate filters which require a smaller load resistance of for example 330Ω. In the normal application, the IF filter is the same as the ones used in most FM radio receivers. Its bandwidth is usually around 180kHz, but the FSK deviation in most ISM applications is usually around 30kHzpk-pk. Therefore, it is not necessary to terminate the filter with 330Ω to keep the response flat, so the resistor at IN_IFA can be omitted.


Cookbook


The IF amplifier 3dB low and high end frequencies are about 400kHz and 30MHz, respectively and the –3dB sensitivity at 10.7MHz is about 150µV with a ceramic discriminator.

IN_D

EM

VEE_IF

IN_MIX

OUT_MIX

1 2 3

VC

C_I

F31

30

32

IN_I

FA

CB5100nF

CER

FIL

VCC

4.1 IF Filter - Standard This filter circuit can be used with all types of 330Ω filters. SMD type ceramic filters from Murata are for example:

SFECV10.7MJA00 @ BIF = 150 kHz (size 7x3mm) SFECV10.7MHA00 @ BIF = 180 kHz (size 3.5x3.1mm)

IN_D

EM

VEE_IF

IN_MIX

OUT_MIX

1 2 3

VCC

_IF

31

30

32

IN_I

FA

CIF

1C

IF2

LIF1

CER

FIL

RL0VCC

CB5100nF

4.2 IF Ceramic Filter - Narrow Band If a NB ceramic filter with 600Ω is used then some additional components are recommended: LIF1 = 2.2µH, CIF1 = 180pF, CIF2 = 220pF, RL0 = 820Ω A leaded type ceramic filter from Murata is for example:

SFKLA10M7NL00 @ BIF = 30 kHz

IN_D

EM

VEE_IF

IN_MIX

OUT_MIX

1 2 3

VC

C_I

F

31

30

32

FIL1

LIF1

CIF1 CIF2

10 Hµ

22pF

3.9p

F

FIL2 IN_I

FA

CB5100nF

VCC

4.3 IF Crystal Filter - Narrow Band If a crystal filter with 3kΩ termination is used then the additional components should be: LIF1 = 10µH, CIF1 = 22pF, CIF2 = 3.9pF Remember that a single 2-pole crystal filter has a maximum attenuation of only 25dB, so a 4-pole filter should be used for best results. One half (2 poles) of 4-pole crystal filter from ECS is for example:

ECS-10.7-7.5B @ BIF = 7 kHz


Cookbook


5 FSK and FM Detectors The FM detector is an analog detector. The detector output signal can be observed on pin 8. If the TH7122x is used in a tone or voice application, this signal would go to a tone decoder or audio amplifier.

CB4

RPCERDIS

IN_I

FA

FS0/

SDE

N

TNK

_LO

IN_D

EM

INT2

/PD

O

INT1

OU

T_D

EM

RS

SI

OU

T_D

TA

VEE_RO

RO

FSK_SW

IN_DTA

ASK/FSK

VCC_DIG

RE/SCLK

GAIN_LNA

VEE_IF

IN_MIX

OUT_MIX

OUT_PA

OUT_LNA

VEE_LNA

VCC

_PLL

VEE

_DIG

VE

E_P

LLLF

FS1/

LD

9

10

11

12

13

14

15

16

IN_LNA

VCC

_IF32

31

30

29

28

27

26

25

TH7122x

1718192021222324

81 2 3 4 5 6 7

CB5100nF

10nF

C4

OUT_DTA

OUT_DEM330p

F

1.5nFC5

C3

10nF

VCC

RSSI

5.1 FSK Detector - Standard The normal ceramic discriminator FM detector acts like a high Q coil. CP is used to tune it, and RP is used to set the detector bandwidth. An adjustable coil or fixed coil with a trimmer capacitor can be substituted for the ceramic discriminator. In this case, RP may not be needed because the coil Q will be low. Any ceramic discriminator can be used with the TH7122x by adjusting CP. The range is usually about 9 to 15pF. The detector bandwidth is set by the demodulator output resistance of 270kΩ and the external capacitor C4. The TH7122x features an internal AFC circuit. It is disabled by default in stand-alone user mode and can be activated on programmable user mode. The AFC allows the receiver to track on frequency variations of the transmitter.

CB4 CP

LDIS

IN_I

FA

FS0/

SDE

N

TNK

_LO

IN_D

EM

INT2

/PD

O

INT1

OU

T_D

EM

RS

SI

OU

T_D

TA

VEE_RO

RO

FSK_SW

IN_DTA

ASK/FSK

VCC_DIG

RE/SCLK

GAIN_LNA

VEE_IF

IN_MIX

OUT_MIX

OUT_PA

OUT_LNA

VEE_LNA

VCC

_PLL

VEE

_DIG

VE

E_P

LLLF

FS1/

LD

9

10

11

12

13

14

15

16

IN_LNA

VCC

_IF32

31

30

29

28

27

26

25

TH7122x

1718192021222324

81 2 3 4 5 6 7

CB5100nF

33pF10nF

C4

OUT_DTA

6.8 Hµ

330p

F

1.5nFC5

C3

10nF

VCC

RSSI

OUT_DEM

5.2 FSK Detector – LC Tank This circuit shows how the ceramic discriminator can be replaced by an LC tank. CP and LDIS are forming a parallel resonant circuit and so the formula to calculate the desired IF follows this equation:

CP⋅=

LDIS1

2π1fIF

An LC discriminator can be used to setup the IF circuit for any desired frequency within the range of 0.4 to 22MHz if no standard ceramic discriminator is available.


Cookbook


CB4

RPCERDIS

IN_I

FA

FS0/

SDE

N

TNK

_LO

IN_D

EM

INT2

/PD

O

INT1

OU

T_D

EM

RS

SI

OU

T_D

TA

VEE_RO

RO

FSK_SW

IN_DTA

ASK/FSK

VCC_DIG

RE/SCLK

GAIN_LNA

VEE_IF

IN_MIX

OUT_MIX

OUT_PA

OUT_LNA

VEE_LNA

VCC

_PLL

VEE

_DIG

VE

E_P

LLLF

FS1/

LD

9

10

11

12

13

14

15

16

IN_LNA

VCC

_IF32

31

30

29

28

27

26

25

TH7122x1718192021222324

81 2 3 4 5 6 7

CB5100nF

10nF

OUT_DTA

RL1

RL2

C4

OUT_DEM

C51.5nF10

0k

100k

C3

10nF

VCC

RSSI

5.3 High Speed FSK Detector If the transmit part of the receiver is setup for high speed, according to para. 3.5, then the receiver should be as well. This can be done by adding to resistors RL1 and RL2 at the output of the demodulator. They are reducing the IC’s output impedance (which is about 270kΩ) and so the cut-off frequency is increased. Note that RL1 and RL2 are AC-wise put in parallel. The design equations for the detector are in the Melexis application note TH7122 and TH71221 High Speed Data Communication.

CB4 RP

CP

CE

RD

IS

IN_I

FA

FS0/

SDE

N

TNK

_LO

IN_D

EM

INT2

/PD

O

INT1

OU

T_D

EM

RS

SI

OU

T_D

TA

VEE_RO

RO

FSK_SW

IN_DTA

ASK/FSK

VCC_DIG

RE/SCLK

GAIN_LNA

VEE_IF

IN_MIX

OUT_MIX

OUT_PA

OUT_LNA

VEE_LNA

VCC

_PLL

VEE

_DIG

VE

E_P

LLLF

FS1/

LD

9

10

11

12

13

14

15

16

IN_LNA

VCC

_IF32

31

30

29

28

27

26

25

TH7122x

1718192021222324

81 2 3 4 5 6 7

CB5100nF

100pF

10nF VD2

C3OUT_DTA

C4

330p

F

100k

10nF

1.5nFC5CS

VCC

R3 RSSI

OUT_DEM

5.4 FSK Detector - External AFC (I) An external AFC circuit can be added to the ceramic discriminator or a coil to further increase the detection range. This is useful when working with SAW transmitters which may have a frequency tolerance up to 200kHz. The AFC time constant is determined by the demodulator output resistance of 270kΩ times C3. Be sure to turn on OA2 in the TH7122x when using this circuit. The capacitance across CERDISC is:

CdCSCdCSCP+⋅

+

Where Cd is the diode capacitance.


Cookbook


CB4

IN_I

FA

FS0/

SDE

N

TNK

_LO

IN_D

EM

INT2

/PD

O

INT1

OU

T_D

EM

RS

SI

OU

T_D

TA

VEE_RO

RO

FSK_SW

IN_DTA

ASK/FSK

VCC_DIG

RE/SCLK

GAIN_LNA

VEE_IF

IN_MIX

OUT_MIX

OUT_PA

OUT_LNA

VEE_LNA

VCC

_PLL

VEE

_DIG

VE

E_P

LLLF

FS1/

LD

9

10

11

12

13

14

15

16

IN_LNA

VCC

_IF32

31

30

29

28

27

26

25

TH7122x1718192021222324

81 2 3 4 5 6 7

CB5100nF

10nF VD2

C3OUT_DTA

C4

330p

F

100k

10nF

R3

1.5nF

LDIS6.8 Hµ

C5

VCC

100pFCS

RSSI

OUT_DEM

5.5 FSK Detector - External AFC (II) For really wide detection ranges, a coil tuned discriminator can be used together with the external AFC. In this circuit, if VD2 is a BB639 diode, it will tune the 6.8µH coil. The tuning can be adjusted by changing C7 and/or by adding another CP in parallel with LDIS.

CB4

RP

RSQ1

RSQ2CERDIS

IN_I

FA

FS0/

SDE

N

TNK

_LO

IN_D

EM

INT2

/PD

O

INT1

OU

T_D

EM

RS

SI

OU

T_D

TA

VEE_RO

RO

FSK_SW

IN_DTA

ASK/FSK

VCC_DIG

RE/SCLK

GAIN_LNA

VEE_IF

IN_MIX

OUT_MIX

OUT_PA

OUT_LNA

VEE_LNA

VCC

_PLL

VEE

_DIG

VE

E_P

LLLF

FS1/

LD

9

10

11

12

13

14

15

16

IN_LNA

VCC

_IF32

31

30

29

28

27

26

25

TH7122x

1718192021222324

81 2 3 4 5 6 7

CB5100nF

10nF

C4

OUT_DTA

OUT_DEM330p

F

10nF

C5 1.5nF

C3

15k220k

VCC

RSSI

5.6 FSK Squelch Circuit The circuit diagram shows how squelch functionality can be added to the standard FSK application circuit. The RSSI output is used to detect an RF signal at the receiver input IN_LNA. In case an RF signal is available, the RSSI signal goes up to a certain voltage level. The absolute voltage level depends on the actual RF input level. This DC voltage level can be adjusted with the potentiometer RSQ1 before it passes the resistor RSQ2. Now this voltage is used to set the threshold of the comparator OA1 by feeding it to pin INT1.The impedance of RSQ1 and RSQ2 must be lower than the output impedance at OUT_DEM plus the internal 200kΩ resistor at the (-) input of OA1 to “overwrite” the DC content from the demodulator output.


Cookbook


A2

CB4RP

R

CER

DIS

IN_I

FA

FS0/

SDE

N

TNK

_LO

IN_D

EM

INT2

/PD

O

INT1

OU

T_D

EM

RS

SI

OU

T_D

TA

VEE_RO

RO

FSK_SW

IN_DTA

ASK/FSK

VCC_DIG

RE/SCLK

GAIN_LNA

VEE_IF

IN_MIX

OUT_MIX

OUT_PA

OUT_LNA

VEE_LNA

VCC

_PLL

VEE

_DIG

VE

E_P

LLLF

FS1/

LD

9

10

11

12

13

14

15

16

IN_LNA

VCC

_IF32

31

30

29

28

27

26

25

TH7122x1718192021222324

81 2 3 4 5 6 7

CB5100nF

10nF

OU

T_FM

VCC

C

C

RSSIR 1.5nF

C5

5.7 FM Detector This circuit can be applied to process FM signals at the demodulator output. An external Sallen-Key low-pass filter is used to band-limit the output signal. The simplest setup is to use equal filter resistors and equal capacitors. This makes the filter cut-off frequency and the Q independent of one another. Direct feedback from the A2 output to the (-) input creates a unity-gain characteristic. The filter cut-off frequency is given by:

RC2π1f C ⋅

=

The circuit is useful, for example, to receive audio signals. It is complementary to the FM transmit circuit shown in para. 3.7.

CB4 RP

R8R9

CP

CE

RD

IS

IN_I

FA

FS0/

SDE

N

TNK

_LO

IN_D

EM

INT2

/PD

O

INT1

OU

T_D

EM

RS

SI

OU

T_D

TA

VEE_RO

RO

FSK_SW

IN_DTA

ASK/FSK

VCC_DIG

RE/SCLK

GAIN_LNA

VEE_IF

IN_MIX

OUT_MIX

OUT_PA

OUT_LNA

VEE_LNA

VCC

_PLL

VEE

_DIG

VE

E_P

LLLF

FS1/

LD

9

10

11

12

13

14

15

16

IN_LNA

VCC

_IF32

31

30

29

28

27

26

25

TH7122x

1718192021222324

81 2 3 4 5 6 7

CB5100nF

100nF

100pF

10nF VD2

OUT_DTA

OUT_DEM

VCC

C4330pF

100n

F

1.5nFC5

CS

C10

C6

R2

R1

33k

100k

680k

1M

5.8 Wide Band FSK / ASK Detector If the ASK peak detector and the FSK detector with AFC are required in one application, the AFC circuit can be driven from the OUT_DEM like this. R8 and C10 filter out the audio or data signal from OUT_DEM. R9 is to decouple the RF signal on the diode from CS. The output swing on pin 8 is from about 0.7V to Vcc -0.25V so the diode should be set up so CERDIS is tuned to the center frequency at:

20.45Vcc +


Cookbook


6 ASK (OOK) Detectors and RSSI The logarithmic RSSI signal is used for ASK detection. When the TH7122x is switched to ASK mode, the RSSI pin 7 is internally connected to the OUT_DEM pin 6 in order to feed the RSSI signal directly to the data slicer which is setup by OA1. Therefore only one capacitor is needed to set the detector bandwidth on either pin 7 or 6. Below is a typical RSSI graph. The curves show the voltage at pin RSSI versus RF input power for both settings: LNA high and low gain.

Fig. 2: RSSI output voltage vs. RF input level

Note the following:

• There is a variation in the absolute value of the RSSI voltage vs RF signal level. Therefore, the absolute value for the RSSI voltage is not an accurate indication of the signal level.

• The slope of all the curves is relatively constant. • The usable RSSI range is about 70dB

When the RSSI signal is used for ASK detection, the absolute value of the voltage is not important.

RS

SI /

V 1.0

1.2

1.4

1.6

1.8

0.8

0.6

0.4

0.2

0.0-130 -120 -110 -100 -90 -80 -70 -60 -50 -40 -30 -20

RF input / dBm

Typical RSSI curve

low LNA gainhigh LNA gain


Cookbook


ASK (OOK) data detection can be done two ways:

IN_I

FA

FS0/

SDE

N

TNK

_LO

IN_D

EM

INT2

/PD

O

INT1

OU

T_D

EM

RS

SI

OU

T_D

TA

VEE_RO

RO

FSK_SW

IN_DTA

ASK/FSK

VCC_DIG

RE/SCLK

GAIN_LNA

VEE_IF

IN_MIX

OUT_MIX

OUT_PA

OUT_LNA

VEE_LNA

VCC

_PLL

VEE

_DIG

VEE

_PLLLF

FS1/

LD

9

10

11

12

13

14

15

16

IN_LNA

VC

C_I

F32

31

30

29

28

27

26

25

TH7122x

1718192021222324

81 2 3 4 5 6 7

OUT_DTA

1.5nFC5

C310nF

6.1 ASK Detector - Standard This bit slicer is very simple. The time constant is approximately 200kΩ · C3. It works for both ASK and FSK reception. For NRZ data, the ratio of 1’s to 0’s should not be greater than about 5:1 because this method operates by filtering the average voltage of the data signal to the data comparator. If the data is RZ like Manchester, this is not a problem. The detector bandwidth is set by the RSSI output impedance of 33kΩ and C5.

IN_I

FA

FS0/

SDE

N

TNK

_LO

IN_D

EM

INT2

/PD

O

INT1

OU

T_D

EM

RS

SI

OU

T_D

TA

VEE_RO

RO

FSK_SW

IN_DTA

ASK/FSK

VCC_DIG

RE/SCLK

GAIN_LNA

VEE_IF

IN_MIX

OUT_MIX

OUT_PA

OUT_LNA

VEE_LNA

VCC

_PLL

VEE

_DIG

VEE

_PLLLF

FS1/

LD

9

10

11

12

13

14

15

16

IN_LNA

VCC

_IF32

31

30

29

28

27

26

25

TH7122x

1718192021222324

81 2 3 4 5 6 7

OUT_DTA

100nF

R2

C6

R1

100k

680k

1.5nFC5

6.2 ASK Detector - Peak Detector This circuit can be used for ASK if the DC component of the data is not constant. This is usually the case for NRZ (non-return-to-zero) codes. C6 is charged by the peak detector. The discharge time constant is C6 · (R1 + R2). Pins 6 and 7 are connected together, so one capacitor, C5 sets the detector and RSSI frequency response. R1 is selected to be 100kΩ, and then R2 is set to give the desired offset to the (-) input of the internal OA1 (the output data comparator). If the system is designed to operate at short ranges, R2 can be reduced to lower noise and interference.


Cookbook


7 RF Input Matching The LNA input pin IN_LNA can be considered as a parallel circuit of a capacitance Cin and a resistance Rin. Cin is relatively frequency independent and at Cin = 2pF while Rin ranges from about 600Ω at 27MHz to 200Ω at 900MHz. When designing a matching network, it should end with a series inductor. A capacitor from IN_LNA to ground may cause parasitic oscillations of the LNA at high frequencies above 1GHz. At high frequencies, an inductor in series with the input can resonate with Cin. Rin is close to the required load resistance for the TH7122x power amplifier (PA), so it can be connected to the PA output pin OUT_PA as is done in the evaluation boards. The LNA noise figure is about 2.3dB while its IIP3 is about –18dBm. During transmit, the LNA input is shunted to ground. The shunt resistance is approximately 33Ω. This is to protect the LNA input and to prevent the PA output from nonlinear distortions that could otherwise be caused by the PN junction of LNA input transistor. The shunt can be turned off by setting bit 20 of the ‘B’ word to ‘0’.

8 RF Output Matching The internal PA provides an open-collector output at pin OUT_PA. In order to provide bias to the PA, this pin is usually connected to positive supply by an inductor (LTX0). The saturation voltage of the PA output is about 0.7V. In order to avoid saturation of the output stage the peak output voltage swing should be fewer than VCC - 0.7V. The maximum available output power (TXPOWER = ‘11’) for different values of the power select resistor RPS on pin 24 is given in the data sheet. Since the open-collector output transistor can be considered as a current source, the only parameters needed to design the output matching network are the output capacitance, the peak voltage swing and the power which should be delivered to the load. The equation for the optimum load resistance is given in the data sheet. An example is given for a 3V supply and to deliver 10mW:

( ) ( )Ω≈

⋅−

=⋅

−= 260

mW102V7.0V3

P2VVR

2

O

2CESATCC

L

According to the output power vs. RPS curve given in the data sheet, the RPS value must be approx. 30kΩ for 434MHz and 50kΩ for 868MHz applications. The internal capacitance at OUT_PA is typically 3pF. This can be part of the PI matching network or can be tuned in a parallel tank circuit together with the inductor LTX0. If the transceiver must be designed for a wide frequency range, matching is more difficult. A wide band transformer and/or multiple stage PI network can be used. If a PI network is used, LTX0 is usually made large enough to act as an open circuit for the RF signal.


Cookbook


9 LNA Output and Mixer Input Matching The LNA output is also an open collector. Normally, it is tuned with an LC circuit and coupled to the mixer with a capacitor. However, a SAW filter can be added between the LNA output and the mixer input for better image rejection. At low frequencies, a double-tuned LC filter can also be used. When the SAW filter is placed between the LNA and mixer, the receiver sensitivity is improved slightly, but the image rejection is limited by leakage between the LNA and mixer pins. Maximum image rejection can be achieved if the SAW filter is put on the LNA input. But then the system noise figure is degraded by the loss of the filter, and an RF switch is usually required for switching between transmit and receive. Similar to the LNA input the mixer input IN_MIX can also be considered as a parallel circuit of a capacitance Cin and a resistance Rin. Again Cin is relatively frequency independent but now at Cin = 1.5pF. The mixer input resistance Rin also doesn’t vary much over frequency and is approx. Rin = 200Ω.

10 Special Considerations

• VCOCUR should always be set to the lowest possible setting (‘00’) on receive. This is important at frequencies above 500MHz to prevent excessive VCO signal levels on the antenna (LO leakage). It usually gives the best receive sensitivity at frequencies below 500MHz.

• The high VCOCUR setting (‘11’) is usually used for transmit. • The largest possible capacitor should be added in parallel with RPS when transmitting ASK signals.

This reduces the rise time of the PA turn on and improves the ASK spectrum. A 1nF capacitor is used in the evaluation boards.

• Pin 27 is the LNA ground and the PA ground. This pin should be connected to the ground plane and a ground trace on the top layer connecting it to the output connector ground.

• Resistors between a microcontroller and the SCI pins 15,16,17 will reduce interference caused by the microcontroller. 10kΩ can be used on pins 15 and 16 and a larger value up to 100kΩ can be used on pin 17 because this pin does not have a 120kΩ internal resistor to ground.

• Always use the highest possible PLL reference frequency for transmitting ASK. It should be 200kHz or higher. For frequencies like 315MHz, a 1MHz reference frequency can be used for transmit and 100kHz for receive.

• The high CPCUR setting (‘1’) should be used for ASK. • When operating at 868 or 915MHz into a mismatched nearby antenna or SAW filter, signal

reflections may cause instability caused by coupling to the VCO which operates at the same frequency as the output signal. This can be improved by adding a 3dB/90-degree directional coupler between the TH7122x output matching network and the antenna. Such a coupler can be purchased as a thin film circuit or constructed with a few passive components as shown in Fig. 3.

Fig. 3: 3dB/90° directional coupler setup with LC components for 868/915MHz

L2

L1

50

C3

C2

C1

5.1pF

2.7pF

2.7pF

12nH

12nH

100

50

R1

R2

antenna(load)

TH7122xoutput

matchingnetwork


Cookbook


• When programming a TH7122x operating at 5V from a PC using the parallel port, the voltage output from the PC is sometimes not large enough to reach 0.7*VCC (3.5V), and the TH7122x will not program. This can be fixed by lowering VCC to 4.5V for programming or adding 10k pull-up resistors on the SCI lines or by adding a non-inverting buffer IC with open collector outputs pulled up to Vcc with resistors.

• The TH7122x sensitivity can be improved by adding an LNA stage before the TH7122x. In this case, a SAW filter should be placed between the LNA and IN_LNA of the TH7122x so the IM performance of the TH7122x is not degraded by the extra gain of the LNA. And, it may be necessary to put a T/R switch on the antenna to switch between transmit and receive.


Cookbook


Your Notes

For the latest version of this document. Go to our website at

www.melexis.com

Or for additional information contact Melexis Direct:

Europe and Japan: All other locations: Phone: +32 1367 0495 Phone: +1 603 223 2362

E-mail: [email protected] E-mail: [email protected]

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