Broadcast Warehouse PLL+ 1 Watt Exciter

(PDF Version)

The Broadcast Warehouse PLL+ 1 Watt Exciter is a compact FM broadcast exciter with specifications that put many commercial exciters to shame.  The modern innovative design allows audio and RF performance never before seen in kit or module exciters.  The 'virtual VFO' dual-loop system allows perfect audio flatness to below 10 Hz.  AFC bounce and modulator overshoot are a thing of the past.  You can now pass that low bass without distortion and get that perfect stereo separation that you have been demanding from your exciter.  Broadband 'no tune operation' allows for ease of use.  The only adjustment required is of the direct-reading decimal dial switches for frequency selection.  RF power is muted during PLL out-of-lock conditions and the built-in harmonic filter keeps your signal clean.  The expansion connector allows for external modules to be connected to the board, such as the Broadcast Warehouse PLL+ LCD.



RF Power Output 1,000 mW (+/- 100 mW), 50 ohms
DC Power Requirements 13.3 - 16 VDC, 300 mA max.
Harmonic Output -60 dBc
Spurious Output -85 dBc
Frequency Steps 100 kHz steps
Out-of-Lock Power Down -50 dBc
Frequency Stability +/- 200 Hz
Audio Input Level adjustable
Audio Frequency Response 10 Hz - 100 kHz
S.N.R >80 dB
Distortion <0.05%
Pre-Emphasis None, 50 µs, or 75 µs (switchable)

Principles of Phase Locked Loop Systems

The Voltage Controlled Oscillator (VCO) feeds a portion of its RF into one side of a Phase Locked Loop (PLL) chip.  The other side of the PLL chip is fed with a reference frequency, usually derived from a quartz crystal, which is very stable.  The phase locked loop chip outputs a high or low voltage.  High or low is subject to whether the reference frequency input is lagging in phase or leading in phase compared to the RF input from the VCO.  In other words, high if the reference frequency is higher in frequency than the VCO frequency and low if the reference is lower.  The reference frequency is usually in the range of 10 kHz to 100 kHz and also forms the step size of the VCO.  A reference frequency of 100 kHz can not have a lower step size than 100 kHz.  Crystals are physically very large at these frequencies so we tend to use a higher frequency crystal and divide it down to the reference frequency.  The 100 MHz signal from our VCO needs to also be divided down to the reference frequency and to do this we need a 'divide-by-N' counter.  "N" is any number which can divide our frequency to the reference.

The phase locked loop system will comprise of:

  1. The Voltage Controlled Oscillator (VCO).
  2. The 'divide-by-N' (100 MHz to reference frequency).
  3. A stable crystal for the reference.
  4. A fixed divider (to divide the crystal to the reference frequency).
  5. A phase comparator.
  6. The loop filter (voltage smoother).

In the example below we will use a 8 MHz crystal, a reference of 100 kHz and the RF frequency we will lock to is 99.9 MHz  The reference divider is 80 and the RF divider is 999.


The 8 MHz crystal is divided by 80 down to 100 kHz.  This stable signal is fed into one of the inputs of the PLL chip.  The RF signal from the VCO is fed into the 'divide-by-N' counter.  This counter will need to have "N" set to 999 to achieve a divide-down from 99.9 MHz to 100 kHz.  When the VCO has a frequency of 99.9 MHz, both the inputs to the phase locked loop chip will have the same frequency and phase.  The output pulses from the phase locked loop chip are feed into a loop filter circuit.  This low-pass filter circuit smoothes and averages the phase locked loop pulses and produces a DC voltage which is applied to the frequency determining element of the VCO, which is usually a varicap diode.  This slightly changes the frequency of the VCO, and the process is repeated.  This is why the name 'loop' is used.  The frequency is checked against the reference, the voltage is changed in respect of any frequency error, the voltage is applied to the oscillator, the frequency moves.  This process is happening continually within the PLL chip.  Adjusting the VCO until it is on frequency and will keep readjusting to keep it there.  If we changed the 'divide-by-N' number to 997 then the PLL would adjust the VCO until both inputs to the phase comparator were equal in phase and frequency.  This would force the VCO to now have an output of 99.7 MHz.

The Broadcast Warehouse phase locked loop system employs a modern chip that contains an oscillator for a quartz crystal, a divider for the reference, a 'divide-by-N' counter and a phase locked loop section (phase comparator).  All of these sections are configurable by serial control.  This control is fed from a Broadcast Warehouse software program contained in a microcontroller.  The loop filter is built around a standard op-amp.  Some exciters still use many logic chips for the various dividers and associated functions but the Broadcast Warehouse system uses only two, if we do not count the loop filter section.

The Problems of Phase Locked Loop Systems

The loop filter is the most crucial part of the phase locked loop system and plays the biggest part in achieving a high-quality exciter.  The design goal is to have the PLL system get the VCO to the correct frequency fast and to appear transparent.  When we FM modulate the VCO, we are moving the frequency of the VCO in proportion to the audio signal we apply.  The PLL circuit's job is to correct any frequency errors.  Hmmm...  Audio introduces frequency shifts and the PLL is trying to correct it.  You can see that the two do not go hand-in-hand.  If we design the loop filter too well, the quick response will strip the audio and not allow any deviation and hence no or minimum audio.  If we relax the requirement to allow better audio to pass uncorrected, then we introduce other problems, such as long PLL lock time (the time is takes the PLL to correct any frequency or get the VCO to frequency).  The ideal PLL system would allow us to get to frequency fast and then somehow relax itself and change the loop filter characteristics to improve the audio.  We need the PLL circuit to not correct the audio (modulation) as much as when the VCO is genuinely off frequency.

Multispeed Loop Systems

Multispeed loop systems can be designed in many ways.  We have seen and tested systems from complex to the very complex.  We have chosen a system that has a minimum component count and still retains excellent performance.  We have managed to keep the component count down by putting the intelligence of the system into software.  The dual speed loop system we use is only one extra component above our standard single loop system.  This component is an analog switch which has two of its switches placed across two of the resistors in the loop filter.  When out-of-lock, the switch shorts out the resistors enabling more current to be dumped into the capacitors of the loop filter and hence, quicker charge time and faster lock up.  When on-channel, the switches are opened.  The hard part is knowing when to switch.  Some other exciters use the lock detect signals from the phase comparator chip to determine when the VCO is in lock.  We have found this to be far from perfect as high level, low frequency content in the audio (heavy bass) can make the lock signal from the phase comparator read wrong.  This could cause the transmitter to switch to fast lock when heavy bass is applied and then we would be back to square one, distortion.

Broadcast Warehouse has taken these lock detect signals from the phase comparator and connected them to a microcontroller where they are analyzed by a propriority software routine to determine whether the VCO is really on frequency or off frequency.  The software can detect that the VCO is still on frequency even if we deviate the carrier with audio by 1 MHz.  This enables us to obtain very, very low bass response with very, very low distortion figures and still have an accurate lock detect system and fast lockup time.

Circuit Description

The frequency determing element is formed by coil L1 and varicap VD1 together with capacitors C17 - C20.  These components are used as part of a cascade oscillator whose output is then buffered by transistor T3.  The RF output from T3 is impedance matched to the base of transistor T5 by RFT1, a 4-to-1 matching transformer.  The high-power output from T5 is impedance matched by coils L2 and L3 and associated capacitors to the 50 ohm output socket CON2.  These components also provide harmonic filtering.

The PLL circuit is primarily IC2 which is a serially-programmed PLL chip.  The microcontroller IC3 reads the dial switches at power on and outputs a serial code to the PLL chip in a format that determines the output frequency that the PLL will try to lock the transmitter to.  The PLL chip outputs control pulses to the loop filter built around op-amp IC4.  The loop filter takes the sharp pulses from the PLL chip and converts them into a 'smoothed' signal ready to apply to the frequency determining component, varicap diode VD1.  IC1 is an analog switch that shorts out two of the resistors in the loop filter which enables the transmitter to get on frequency faster.  When the oscillator is on frequency, the analog switch switches out, which greatly improves the audio response of the transmitter.  The microcontroller IC3 determines when to switch the analog switch in and out by reading the lock detect signals from the PLL chip.  The microcontroller can also use this information to switch off transistor T3 with open collector configured T4.  This mutes the RF output when the transmitter is in an 'out-of-lock' condition.  LED1 provides visual indication of the PLL locked condition.

Audio is fed into the modulation input connector CON1.  It is passed through a high-frequency low-pass filter formed by C37, C38, and a ferrite bead to keep any RF from feeding back into the modulation circuitry.  From here the signal passes to variable resistor VR1 where modulation levels can be set.  From the output of the variable resistor the audio signal passes through resistor R30 and jumper J1.  This jumper allows either capacitor C1 or C2 to be put in parallel with R30 forming a pre-emphasis filter.  0, 50, or 75 microseconds are selectable depending on jumper selection.  From here, audio is fed via a resistive potential divider to the varicap diode VD1.  The audio imposed onto VD1 causes the frequency of the transmitter to shift and modulation is achieved.

There is an expansion connector on the board to allow connection of other Broadcast Warehouse products, such as a LCD frequency selector.  Connection details are provided with the relevant expansion product.

Assembly Instructions

This kit is not really a first-time kit builders project.  If you have not soldered before, we recommend you get some soldering experience from a simpler project or get this kit assembled by someone who has previous experience in electronic construction and soldering.

  1. Empty the contents of the kit and proceed to check all the components off against the component list.  It is a good idea to tick off each component as you go through.  When you have double checked all the parts, proceed.
  2. We always start will the lowest height components first, which are the resistors.  Insert each resistor and solder one at a time taking care to make a good solder joint and not to short across other pads/holes.  Double check the component is the correct one before soldering.
  3. Now insert and solder diodes D1 - D13 observing the polarity (SEE DIAGRAM).  Do the same for varicap VD1 and inductor L4.  Ferrite bead (marked FB) is next.
  4. Next, it is time to insert the ceramic capacitors C1, C4, C8, C9, C10, C11, C14, C15, C18, C19, C20, C24, C25, C27, C28, C29, C30, C31, C32, C33, C34, C35, C37 and C38.  These are non-polarized and can be inserted and soldered either way around.
  5. Switches 1 to 3 should be next and these can be followed by the chip holders for IC1 - IC4.  Make sure you line the notch on the chip holder with the notch on the ident on the printed circuit board (PCB).  This will help you in making sure you insert the chip the correct way around in the socket.  (See Diagram)
  6. Variable resistor VR1 should be put in next followed by voltage regulators REG1 and REG2, and then transistors T1 - T4.  LED LED1 should be next, marking sure the flat on the LED aligns with the flat on the silkscreen ident on the PCB.  Transistor T5 can be inserted and soldered next.  Leave the heatsink for T5 off for now.
  7. Now insert the polarized electrolytic capacitors C2, C3, C6, C10, C13, C16 and C26 MAKING 100% SURE they are soldered in correctly.  (See Diagram).  The board has a positive symbol next to the positive hole of each polarized capacitor.  Insert the negative stripe side away from the positive (+) marking.  Now insert ceramic capacitor C17.
  8. Insert and solder jumper J1.  You may, if you wish, put the jumper tab on, but we recommend you wait until the end when we will configure the settings of the board.  The pre-emphasis capacitors C22 and C23 can be put in next.  Connectors 1 to 4 can be soldered in if you wish to use them.  Variable capacitor VC1 is next.
  9. Inductors L1 (metal can) and plastic type L2 and L3 can be inserted next, followed by crystal X1.  The push on heatsink for T5 can now be pushed on, taking care to avoid twisting and damage to the transistor.
  10. Make the RF transformer from the toroid core (blue/yellow ring) and twisted enameled wire as shown in the diagram.
  11. Oh!  You can now insert all the chips into thier correct chip holders.

It is advisable that you check your work and all the components are where they should be and that there are no solder splashes or shorts underneath the circuit board.  It is better to spend five minutes double checking everything, rather than risk damage at switch on due to a mistake during assembly.

If you are sure everything is OK, you can proceed to the setup and testing section.

Construction Diagrams

Setup and Testing

Make sure you have the Broadcast Warehouse PLL+ 1 Watt Exciter assembled before proceeding, consult the assembly instructions for more info.  Once constructed the PLL+ Exciter should not need any adjustments.

Power Supply

OK!  Now that the unit is assembled, and you have double-checked for construction errors, we can get ready to switch on the unit.  For correct full-band operation, you will need a regulated power supply that is capable of giving out between 13.8 and 15 volts.  13.4 volts is the minimum needed to allow the PLL to cover the full 87.5 to 108 MHz.  15 volts is a safe maximum voltage.  Any more and the components may run too hot.  If you do not supply a minimum of 13.4 volts then we cannot guarantee that the PLL will work correctly at the top of the band.  12 volts may only allow the unit to lock to 105 MHz or so.  With the correct supply, connect a 50 ohm load to the PLL's RF output connector.  A dummy load is preferred over an antenna.

Frequency Selection

Before you turn the power on, you must select your frequency.

The FIRST switch represents units of 10 MHz, where "8" would mean 80 MHz.  (0 = 10 = 100 MHz)

The SECOND switch represents units of 1 MHz, where "9" would mean 9 MHz.

The THIRD switch represents units of 0.1 MHz (100 kHz), where "7" would mean 700 kHz.

Taking the above as an example; if we set SWITCH1 to 8, SWITCH2 to 7, and SWITCH3 to 9, we would set the PLL to a frequency of 87.9 MHz.  Example: [(8 x 10) + (9 x 1) + (7 x 0.1)] = 87.9 MHz

If you select an invalid frequency then the PLL lock LED will flash repeatably and no RF output will occur on any frequency.

To reset a new frequency, you must turn power to the unit off then back on again.

If you have a frequency meter you can also fine tune the frequency by the adjustment of VC1.  For example, 99.200001 instead of 99.201341 MHz.  Disconnect the audio before trying to adjust VC1.  You will obviously need the unit on and powered up first before this adjustment can be made.  If you don't have a frequency meter, don't worry.  The unit will still be in spec.

Audio Input and Pre-Emphasis

Audio is fed in via the RCA/PHONO connector CON1.  If you have an external stereo encoder, then remove the jumper J1.  If you have an audio limiter with pre-emphasis capability then also remove the jumper J1.  Otherwise, if no stero encoder or limiter with pre-emphasis is in-line with the PLL you should configure jumper J1 to suit the pre-emphasis requirement for your region.  75 microseconds is used for the USA and Japan and 50 microseconds for the rest of the world.  With your audio applied at the desired level to the PLL+, adjust variable resistor VR1 for 100% modulation (which is a maximum peak deviation of +/- 75 kHz).

RF Output

The RF output can be connected to CON2 or you can solder to the pads on the top or bottom of the board.  The RF output power from the PLL+ is fixed at about 1 watt and can not be adjusted.  Please do not alter the coils L2 and L3.  They form part of the harmonic filtering and should not be adjusted.  If you require less RF output power, then use a resistive attenuator formed from three resistors.  Details are in any good radio handbook, such as the ARRL Handbook for Radio Amateurs.  Always connect a good 50 ohm load on the RF output to avoid damage to T5.

Component List

Component Value Marking / Identification
R1, R2, R3, R4, R6, R7 10k BROWN, BLACK, ORANGE, GOLD
R12, R16, R17, R22, R23 1.2k BROWN, RED, RED, GOLD
R13, R20, R24 4.7k YELLOW, PURPLE, RED, GOLD
R15, R26 220 RED, RED, BROWN, GOLD
VR1 1k potentiometer Small, yellow pot marked "102"
C1 39 pF 39 pF
C2, C3, C6, C35 2.2 µF 2.2 µF
C4, C8, C11, C14 100 nF 104 or 100N
C5, C7 470 µF 470 µF
C9, C24, C27 82 pF 82 pF
C10, C15, C29 10 nF 103 or 10N
C12, C13, C16, C26 100 µF 100 µF
C17 220 pF 220 pF
C18 4.7 pF 4P7 or 4.7 pF
C19, C30 27 pF 27 pF
C20, C33 56 pF 56 pF
C21 Not Used
C23 4.7 nF 4700
C22 6.8 nF 6800
C25, C28, C32, C38 1 nF 102 or 1N
C31 12 pF 12 pF
C34, C36, C37 33 pF 33 pF
IC1 4066 4066
IC2 MC145170 MC145170
IC4 LF351 LF351
T1, T2, T3, T4 MPSH10 MPSH10
T5 2N4427 2N4427
L1 5-1/2 MC120 Metal can 00754
L2 2-1/2 S18 Red coil
L3 3-1/2 S18 Orange coil
L4 0.15 µH inductor Yellow axial - µH15
REG1 78L05 78L05
REG2 78L10 78L10
X1 8 MHz crystal 8.000
VC1 5 - 65 pF trimmer Yellow adjustable trimmer
SWITCH1, SWITCH2, SWITCH3 Decimal rotary switch Black switch marked 0 - 9 in circle
D1 - 13 1N4148 diode 1N4148
VD1 BB909A varicap Black axial with yellow stripe
CON1, CON2 RCA/PHONO connector RCA/PHONO connector
CON3 2-pin Molex socket 2-pin Molex socket connector
CON4 10-way IDC connector 10-way ribbon socket
J1 3-pin jumper header 3-pin header
HEATSINK Clip on heatsink Black finned heatsink
RFT1 Toroid and wire Blue/yellow ring with enameled wire
8-pin IC socket 8-pin IC socket 8-pin IC socket
14-pin IC socket 14-pin IC socket 14-pin IC socket
16-pin IC socket 16-pin IC socket 16-pin IC socket
18-pin IC socket 18-pin IC socket 18-pin IC socket
PCB Black board You are joking!


[broadcast warehouse]