TH7122

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Summary

Figure 1: Schematic for the TH7122 Test Board.
Figure 2: PCB Layout for the TH7122 Test Board.

The TH7122 is a single-chip FM/FSK/ASK transceiver designed for SRDs operating in the range of 300-930MHz.[1] The device can operate down to 27MHz using an external varactor diode. Like the ADF7012, the device includes a built in VCO and PLL, circuitry to control the RF chain via an SPI interface, and the addition of a receive chain including LNA, mixer, and limiting IF amplifier. Using few external components, the device can also generate and receive analog FM, which will be used in this application. The device datasheet and application notes give a wide variety of example circuits and theory of operation, including examples to extend the frequency range to 144MHz, and examples to generate and receive analog FM.

Testing

IF Filter

The Murata SFVLF10M7MF00-B0 10.7MHz IF filter was tested using an Anritsu MS4622B vector network analyzer. The insertion loss and 3dB bandwidth were measured as follows (datasheet specifications in parenthesis):

  • Center Frequency = 10.7MHz (10.7MHz)
  • Insertion Loss = 4.19dB (5dB)
  • 3dB Bandwidth = 51kHz (50kHz)
S21 response of the Murata SFVLF10M7MF00-B0 IF filter.

Since the filter is a 330 ohm device, 280 ohms of resistance was used in series of either side of the device in order for the device to "see" a 330 ohm system impedance. A correction factor of +16.3909dB was added to the raw data to remove the losses due to the addition of the extra resistors. This technique was verified in LTspiceIV with a 330 ohm Chebyshev filter as an example.

Simulated response of a 330 ohm filter as measured in a 50 ohm system.

Bill of Materials

Part Quantity Unit Cost Total Cost Source
TH7122 1 $13.40 $13.40 Digi-Key
0603 Capacitor 19 $0.0096 $0.18 eBay - mib_instruments
1206 Tantalum Capacitor 1 $0.44 $0.44 Digi-Key
0603 Inductor (Taiyo Yuden HK Series) 5 $0.064 $0.32 Digi-Key
0603 Resistor 7 $0.00294 $0.02 eBay - mib_instruments
100k SMD Potentiometer 1 $0.25 $0.25 Digi-Key
SMA Male Connector 2/5 $2.51 $1.00 Dealextreme
MAV27V2000 Varactor 1 $0.36 $0.36 Digi-Key
BB833 Varactor 1 $0.49 $0.49 Digi-Key
5MHz Crystal 1 $0.77 $0.77 Digi-Key
10.7MHz Ceramic Filter 1 $1.83 $1.83 Digi-Key
10.7MHz Discriminator 1 $1.07 $1.07 Digi-Key
Male Header 10 $0.01 $0.10 eBay
PCB 1.55"x1.05"/3 $5.00 $2.71 Laen's PCB
Total Unit Cost $22.94

PCB/Construction Photos

References

  1. Melexis Microelectronic Systems, "TH7122 27 to 930MHz FSK/FM/ASK Transceiver," Feb. 2009. [Online]. Available: http://www.melexis.com/Wireless-ICs-Sensing/RFID-Sensor-Tag-ICs-433MHz/TH7122-121.aspx. [Accessed 11 March 2012].