Application Note TH7122 and TH71221 used with Narrow-Band IF Circuit

Transcription

1 1 Why use the TH7122 or TH71221 with a narrow-band IF IC when fully integrated narrow-band transceiver ICs are available 1.1 Frequency Coverage TH7122 or TH to 960MHz 160 to 916MHz MHz MHz All the tuning components for the TH7122 (LQFP32) and TH71221 (32L QFN5x5) RF transceivers are external, so the operating frequency up to 950MHz depends only on the selection of the external tuning coils and capacitors. The tuning range can also be up to about a 2:1 ratio with the selection of the proper low cost external varactor diode. If a wider tuning range is desired, PIN diodes can be used for switching the VCO and any other tuned circuits. An RF transceiver circuit with a tuning range of 27 to 915MHz has been designed using this method. 1.2 Receive current consumption: TH7122 = 8mA + NBIF IC = 4mA 12mA 24mA 20mA 22mA Highly integrated transceivers have complex integrated IF filters which consume a significant amount of current. The IF filters must be centered at low frequency to keep current consumption reasonable, so this limits how far away the image frequency can be. 1.3 Image frequency TH7122 +/ or +/- 42.8MHz or 90MHz, depending on 1 st IF filter -200kHz kHz -125kHz Integrated transceivers have built-in image reject mixers with image rejection from 40dB at the high frequencies to 60dB at the lowest frequency. However, the image always falls inside the operating band where there is a high probability of receiving a strong signal which is above the image attenuation. The image frequency with the TH7122 can be selected to be above or below the desired receive frequency and so can be put in a band Page 1 of 7 AN7122x NBIF

2 less likely to cause interference. The most common 1 st IF frequencies for the TH7122 would be 10.7 or 21.4MHz, but any frequency is possible depending on the 1 st IF filter is used. 45MHz is another possibility for which crystal filters are available. The mixer output is relatively flat to 80MHz. 1.4 Simple Programming TH bit registers bit registers bit registers bit registers Since most of the TH7122 performance parameters are set by external components, the programming is very easy. For example, the VCO frequency setting is set by the reference and N counters so that N FVCO = FREF R Where F ref is the reference crystal frequency, R is the R counter setting and N is the N counter setting. 1.5 Any crystal from 3.58 MHz to 16MHz can be used for the TH7122 reference. In some cases, this allows the microprocessor crystal to be used for both the micro and the TH7122 reference. There is a Melexis application note about this: AN7122-external-RO.pdf 1.6 The TH7122 uses a simple integer-n synthesizer. All the newer highly integrated transceivers use a fractional-n synthesizer with a high frequency reference crystal. While fractional-n synthesizers allow for very small frequency steps to give more accurate tuning, it is almost always possible to find values for the TH7122 R and N counters which will give a desired tuning accuracy within a specified error. For example, suppose it was desired to tune to MHz using a 3.58MHz crystal. Assume the frequency error can be 100Hz. If R = 274 and N = 11871, frequency error = - 61Hz, PLL reference frequency = 13.1kHz. 1.7 The TH7122 can produce very clean ASK signals at data rates up to 10kbps at frequencies up to 433MHz. The only requirement is that the PLL reference frequency is high (1-3MHz). This can be done at non standard frequencies by using a custom crystal frequency such that R is small for the transmit frequency. For receive, R and N can be set to give the desired receive frequency. 1.8 TH7122 controlled bandwidth transmitted FSK signals can be produced by modulating the TH7122 VCO with transmitted data which has been low-pass filtered. This Page 2 of 7 AN7122x NBIF

3 is equivalent to the complex modulation methods used with fractional N synthesizer. In many cases, a simple RC low-pass filter is enough. 2 Measurement results on TH7122 NB receivers 868MHz parameter Value Condition sensitivity -121dBm FM deviation = 3kHz S/N = 12dB (max S/N = 34dB) selectivity, 25kHz 40dB with 2 generators selectivity, 50kHz 48dB with 2 generators image rejection 53dB with +100,+200kHz signals 433MHz parameter Value Condition sensitivity -121dBm FM deviation = 3kHz S/N = 12dB (max S/N = 40dB) selectivity, 25kHz 45dB with 2 generators selectivity, 50kHz 50dB with 2 generators image rejection 42dB with +50,+100kHz signals 150MHz parameter value Condition sensitivity -120dBm FM deviation = 2.5kHz S/N = 12dB selectivity, 12.5kHz 60dB with 2 generators selectivity, 25kHz 65dB with 2 generators image rejection 42dB with +50,+100kHz signals Page 3 of 7 AN7122x NBIF

4 2.1 TH NBIF IC test board Fig. 1: TH NBIF IC circuit Page 4 of 7 AN7122x NBIF

5 Fig MHz CW output spectrum Fig. 3: 460 MHz CW output spectrum Page 5 of 7 AN7122x NBIF

6 2.2 Component list for different frequencies Part Size 27MHz 150MHz 460MHz 930MHz Description C pF 33pF 33pF 33pF 2nd mixer crystal tuning C nF 1nF 1nF 1nF RSSI filtering C pF 56pF 56pF 56pF 2nd mixer crystal tuning C nF nF nF nF data slicer depends on data rate C nF 10nF 10nF 10nF coupling to NBIF C pF 91pF 91pF 91pF 455kHz discriminator tuning CB0 A 10uF 10uF 10uF 10uF power supply filter if needed CB nF 1nF 330pF 330pF LNA tank bypass CB nF 100nF 100nF 100nF NBIF bypass CB nF 1nF 330pF 330pF RF output bypass CB nF 10nF 10nF 10nF VCC bypass CB nF 100nF 100nF 100nF VCC bypass CB nF 1nF 100pF 100pF VCO supply bypass CB nF 100nF 100nF 100nF VCC bypass CB nF 100nF 100nF 100nF NBIF bypass CF uF 1uF 1uF 1uF PLL loop filter CF nF 100nF 100nF 100nF PLL loop filter CLP nF 10nF 10nF 10nF de-emphasis filter CM nF 330pF 100pF 100pF VCO modulation coupling CM pF 68pF 68pF 68pF MIX output to 1st IF filter matching CM pF 100pF 100pF 100pF MIX output to 1st IF filter matching CMI pF 1.5pF 1pF.5pF coupling from LNA to mixer CO pF 3.9pF 4.0pF capacitor in parallel with LO to reduce tuning range CO nF 39pF 3.9pF 0 external varactor tuning CPS nF 1nF 1nF 1nF PA rise time control CRX nF 1nF 100pf 100pF LNA input decoupling CRX pF 15pF 4.0pF 3.0pF LNA tank tuning CTX nF 1nF 100pF 100pF PA output decoupling CTX pF 12pF 6pF 4pF Output matching CTX pF 22pF 10pF 4pF Output matching CX pF 20pF 20pF 20pf Reference crystal tuning DISCR1 JTC JTC455C24 JTC455C24 JTC455C24 JTC455C24 455kHz discriminator FIL1 thru-hole LT10.7MFP LT10.7MFP LT10.7MFP LT10.7MFP 1st IF filter MHz ceramic FIL2 CFW LT455FW LT455FW LT455FW LT455FW 2 nd IF 455KHz filter for 12.5kHz channels LM uH 4.7uH 4.7uH 4.7uH MIX output to 1st IF filter matching LO 1008/ uH 56nH 10nH 2.7nH VCO tuning LRX1 1008/ uH 470nH 56nH 10nH LNA input matching Page 6 of 7 AN7122x NBIF

7 Part Size 27MHz 150MHz 460MHz 930MHz Description LRX2 1008/ uH 56nH 15nH 3.9nH LNA tank tuning LTX0 1008/ uH 100nH 15nH 4.1nH PA output decoupling LTX1 1008/ nH 100nH 27nH 7.8nH Output matching R K 1.5K 1.5K 1.5K ceramic discriminator load R LNA decoupling R LFIF decoupling R K 10K 10K 10K VCO tuning coupling RF K 2.7K 1.8K 2.7K PLL loop filter RL st If filter load if needed RLP K 10K 10K 10K de-emphasis filter RM K 100K 100K 100K VCO modulation input RM K 100K 100K 100K VCO modulation level adjust RO K 10K 10K 10K PLL tuning coupling 22 Ohms for low frequencies to RO prevent HF oscillation RPS K 15K 33K 47K sets max output power RS K 10K 10K 10K RS K 10K 10K 10K SPI input filter - filters noise from micro SPI input filter - filters noise from micro RS K 10K 10K 10K SPI input filter - filters noise from micro U1 LQFP32 TH7122 TH7122 TH7122 TH7122 or TH71221 for QFN5x5 U2 SSOP16 TA31136 TA31136 TA31136 TA31136 NBIF IC (Toshiba) VD1 SC-79 SMV1249 SMV1249 SMV1249 none VCO tuning diode 2nd mixer crystal or X XREF 3-4MHz 3-16MHz 3-16MHz 3-16MHz PLL reference crystal For the latest version of this document, go to our website at: Or for additional information contact Melexis Direct: Europe, Africa: Americas: Asia: Phone: Phone: Phone: sales_europe@melexis.com sales_usa@melexis.com sales_asia@melexis.com ISO/TS and ISO14001 Certified Page 7 of 7 AN7122x NBIF