FM PLL controlled VCO

  
The main oscillator is printed in blue and is voltage controlled. In this construction the VCO range is 88 to 108 MHz. As you can see from the blue arrows, some energy goes to an amplifier and some energy goes to the PLL unit. You can also see that the PLL can control the frequency of the VCO. What the PLL do is that it compare the VCO frequency with the reference frequency (which is very stable) and then regulated the VCO voltage to lock the oscillator at desired frequency. The last part that will affect the VCO is the audio input. The amplitude of the audio will make the VCO change in frequnency FM (Frequency Modulation). I will explain it all in detail under section Hardware and schematic.
FM PLL controlled VCO - schematic

The main oscillator is based around the transistor Q1. This oscillator is called Colpitts oscillator and it is voltage controlled to achieve FM (frequency modulation) and PLL control. Q1 should be a HF transistor to work well, but in this case I have used a cheap and common BC817 transistor which works great. The oscillator needs a LC tank to oscillate properly. In this case the LC tank consist of L1 with the varicap D1 and the two capacitor (C4, C5) at the base-emitter of the transistor. The value of C1 will set the VCO range. The large value of C1 the wider will the VCO range be. Since the capacitance of the varicap (D1) is dependent of the voltage over it, the capacitance will change with changed voltage. When the voltage change, so will the oscillating frequency. In this way you achieve a VCO function. You can use many different varicap diod to get it working. In my case I use a varicap (SMV1251) which has a wide range 3-55pF to secure the VCO range (88 to 108MHz). Inside the dashed blue box you will find the audio modulation unit. This unit also include a second varicap (D2). This varicap is biased with a DC voltage about 3-4 volt DC. This varcap is also included in the LC tank by a capacitor (C2) of 3.3pF. The input audio will passes the capacitor (C15) and be added to the DC voltage. Since the input audio voltage change in amplitude, the total voltage over the varicap (D2) will also change. As an effect of this the capacitance will change and so will the LC tank frequency. You have a Frequency Modulation of the carrier signal. The modulation depth is set by the input amplitude. The signal should be around 1Vpp. Just connect the audio to negative side of C15. Now you wonder why I don't use the first varicap (D1) to modulate the signal? I could do that if the frequency would be fixed, but in this project the frequency range is 88 to 108MHz. If you look at the varicap curve to the left of the schematic. You can easily see that the relative capacitance change more at lower voltage than it does at higher voltage. Imagine I use an audio signal with constant amplitude. If I would modulated the (D1) varicap with this amplitude the modulation depth would differ depending on the voltage over the varicap (D1). Remember that the voltage over varicap (D1) is about 0V at 88MHz and +5V at 108MHz. By use two varicap (D1) and (D2) I get the same modulation depth from 88 to 108MHz. Now, look at the right of the LMX2322 circuit and you find the reference frequency oscillator VCTCXO. This oscillator is based on a very accurate VCTCXO (Voltage Controlled Temperature controlled Crystal Oscillator) at 16.8MHz. Pin 1 is the calibration input. The voltage here should be 2.5 Volt. The performance of the VCTCXO crystal in this construction is so good that you do not need to make any reference tuning. A small portion of the VCO energy is feed back to the PLL circuit through resistor (R4) and (C16). The PLL will then use the VCO frequency to regulate the tuning voltage. At pin 5 of LMX2322 you will find a PLL filter to form the (Vtune) which is the regulating voltage of the VCO. The PLL try to regulate the (Vtune) so the VCO oscillator frequency is locked to desired frequency. You will also find the TP (test Point) here. The last part we haven't discussed is the RF power amplifier (Q2). Some energy from the VCO is taped by (C6) to the base of the (Q2). Q2 should be a RF transistor to obtain best RF amplification. To use a BC817 here will work, but not good. The emitter resistor (R12 and R16) set the current through this transistor and with R12, R16 = 100 ohm and +9V power supply you will easy have 150mW of output power into 50 ohm load. You can lower the resistors (R12, R16) to get high power, but please don't overload this poor transistor, it will be hot and burn up Current consumption of VCO unit = 60 mA @ 9V.



 

Recommended videos


  • Phase Locked Loop Tutorial | PLL Basics
    Duration: 6:34.
  • Popular FM transmitter
     
  • How to build a professional surveillance UHF Transmitter.
    Duration: 40:38.

News


  • Radio magazine
    Any transmitter setting can also be scheduled from program sources to power. WorldCast will offer the new audio processor software feature on new units and as an upgrade to many Ecreso currently in the field. WorldCast Systems will ...
     21 April 2017
  • RF Globalnet (press release)
    The Bridgeport site is one of several locations that will see a complete replacement of systems, including a modernized building to house all transmission, auxiliary and cooling systems. All will be outfitted with the HD Radio
     31 March 2017
  • Radio magazine
    In many cases, the GatesAir transmitters will replace aging tube models, which will help with Cumulus' goal of reducing operational expense and energy use in its plants as part of the oadcaster's sustainability efforts. The order includes air <
     29 March 2017
  • Radio World
    Over the last few years, we have added smaller sites to our network and therefore needed to source some lower-power (up to 2 kW) for these locations. Many of these sites are unmanned, thus it was essential to have a high level of
     16 March 2017
  • Radio magazine
    11, all NRK, P4 and Radio Norway were shut down in that region. All local stations in Nordland continue to transmit on FM, as will the majority of all local radio stations in Norway. NRK also states that there has been no dramatic drop
     23 February 2017
  • Radio magazine
    Radio magazine One notable exception where FM antenna models are specified is in cases when radiation exposure as calculated through FM model becomes a problem. Since the make and model of translator antennas are specified at the constructio
     20 January 2017
  • Radio World
    KCRW is a six-station cluster consisting of KCRW, Santa Monica, KDRW, Santa Barbara, KCRU, Oxnard, KCRY, Mojave & KCRI, Indio, and KERW, Los Osos-Baywood Park. Additionally it operates seven low power translator stations in the communities o
     20 December 2016
  • Radio World
    Eric Pere: Generally, operate far more efficiently than DTV or DAB transmitters so, the advantages of water cooling aren't as significant in the FM sphere. If a manufacturer ..... Fewer engineers entering the market today do so as
     23 November 2016
  • EE Times
    EE Times Power AmpsThese amplifiers are sold as components of , which are FCC certified for sale in the U.S. The issue, according to the citation, is that although the containing the amplifiers are FCC certified, the power amplifiers do
     27 October 2016

.