RF Turner Electronic Circuits

In a search for a RF module which would do everything and be programmed with a MCU I visited several PLL based electronics designs just to find out that nowadays you can get also the MCU included in that single chip RF solution. For example Silicon Labs and Analog Devices have very modern circuits which include PLL, VCO and MCU in a single IC. Lets view some less integrated interesting designs now.


How a modern (20 years modern) RF tuner circuit works

Here is a block diagram of a French design Synthe_BB209 and Synthe_POS150 (see REF /1/). Both are very similar but the latter uses POS150 module as the VCO where as the first one uses discrete components.

20 years modern RF module includes PLL and VCO which are controlled using a MCU.

Most modern RF (Radio frequency) turner designs include PLL (Phase-locked loop) and VCO (Voltage-controlled oscillator) which work together and the PLL is connected to the MCU (Microcontroller unit). After the VCO there is what ever you need to do with that RF signal you generated using those circuits (transmitter, receiver, different types of modulators, etc). So basically everything can be done with these building blocks and controlled with your most favorite MCU, usually using some serial interface.

MC145170 PLL IC from Motorola in DIL16 Package

PLL receives the frequency (F_OUT) that the VCO is currently generationg and using its counters and circuits decides if the frequency is too low or too high and adjusts its output voltage (V_VCO) according to that. So it is a normal closed loop control and when working as expected it keeps the F_OUT very stable as required. Usually the PLL gets its reference frequency from a stable crystal source (in this case 12 MHz crystal).

POS150 VCO IC from MiniCircuits in a Shielded Can

This transmitter BTW generates low power 200 mW standard FM modulated FM radio signal. To the VCO control voltage is added the modulation signal so that the VCO generates FM modulated output. If AM was required then the power amplifier (PA) power feed could be modulated. There is also the very important band pass filter in the PA section of the module since the VCO generates many fundamental frequencies or harmonic frequencies which have to be filtered out before outputting the signal.

LM317 in TO220 package can be programmed using the resistors R1 and R2. Vout is about 1.25*(1+R2/R1).

The green block is the power regulators part which just generates stable power using standard 78XX regulators and LM317 variable regulator for the PA (RF Power amplifier). The PA power is controlled in two ways: 1) The power supply voltage to the PA is adjusted with a trimmer resistor so that the wanted output power is generated and 2) the V_PA can be switched OFF (or 1.25 V) to mute the transmitter durinq LOCK OUT periods when the PLL is tuning to another frequency. There is a small delay circuit to accomplish that. The LOCK signal is also fed to the MCU connector so that the MCU knows when the module is locked to its frequency.



Synthe_BB209 and Synthe_POS150

These two French designs are new (2014) but use older components which are usually easy to find and cheap. Both designs are based around MC145170 PLL which can handle up to 150 MHz or so without external prescaling. Here are the original links for the Synthe_BB209 and Synthe_POS150 designs.

MV2109 is a direct replacement for BB109 or BB209 varactor diode.

Synthe_BB209 uses varactor diodes and transistor circuit in its VCO. Synthe_POS150 uses an integrated circuit POS150 as its VCO. The latter is more simple to build but the first one is cheaper.

Synthe_BB209

Synthe_POS150

Here are the original schematics of the modules.

Synthe_BB209

Synthe_POS150

And here are the more detailed schematics of the circuits.

Synthe_BB209 more detailed

Synthe_POS150 more detailed

A rather important part of the circuit is the band pass filter that is inside the PA stage. The original developer may be able to supply kits and evolution modules. Visit the French radio amateur site for more.


RESOURCES

/1/ http://radioinitiation.chez-alice.fr/
/2/ Synthe_BB209 PCB and parts list
/3/ Synthe_POS150 PCB and parts list
/4/ Synthe_BB209 Schematic
/5/ Synthe_Pos150 Schematic

DATASHEETS

MC145170-2 PLL Frequency Synthesizer with Serial Interface
POS-150 Voltage Controlled Oscillator 75-150 MHz
OP27 Low Noise, Precision Operational Amplifier
NE5532 Dual Low-Noise Operational Amplifiers
LM317 3-Terminal Positive Adjustable Regulator
78XX 3-Terminal Positive Regulators 
2N2222 Low Power Bipolar Transistor
BFG135 NPN 7GHz wideband transistor
BFR96 NPN Silicon HF Transistor
J310 N-Channel RF Amplifier
BB209 Silicon Tuning Diode
MSA1105 Cascadable Silicon Bipolar MMIC Amplifier

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