AM Transmitters – Engineering Radio

Locking AM station carriers to GPS

This is not a new idea, many people have discussed it in the past. The National Radio Systems Committee (NRSC) has a guidance paper, NRSC-G102 which gives a detailed explanation of why synchronized AM carriers are beneficial. There was even a move by some to have it included in the AM revitalization plan of a few years back. The NAB opposed this idea, saying it would be too expensive. That is unfortunate because out of all of the revitalization initiatives, GPS locked carriers had the best potential for an actual technical improvement. While it may be expensive for some very old tube type transmitters, for more modern solid state transmitters, GPS referenced carriers can be implemented as little as $200.00 US.

The FCC rule (73.1545(a)) for AM Carrier frequency specifies:

AM stations. The departure of the carrier frequency for monophonic transmissions or center frequency for stereophonic transmissions may not exceed ±20 Hz from the assigned frequency.

40 Hz is quite a bit of movement on a 20 KHz AM (18 KHz in ITU region II and III) channel. The reason for trying this is simple; there are many co-channel and first adjacent channel AM stations which at night, interfere with each other.

Above is a typical spectrum analysis of an AM station on 940 KHz. This was a 10 minute peak hold for an NRSC-2 spectrum mask measurement. The carrier is approximately 20 dB greater than the audio, which means that most of the interference between co-channel AM stations is created by the carriers beating against each other. By locking carriers to the same reference, that carrier interference will be greatly reduced. NRSC-G102 goes into great detail on the listenability of interfering stations with synchronous AM carriers (Page A-3).

Stations drifting off frequency also cause greater adjacent channel interference.

Almost all transmitters made in the last 30 years have an option to use an external frequency generator or 10 MHz reference. The required drive levels vary. The easiest way to implement this is by using a GPS locked programmable frequency source such as the Leo Bodnar LBE-1420. It can be programmed to any frequency from 1 Hz to 1.1 GHz, has a frequency stability of 0.000001 PPM (10^-12), and an output level of 3.3 V peak-to-peak. This drive level is not enough for some transmitters. For those situations an additional amplifier such as a Mini Circuits ZHL-3A+ is needed.

Here are a few AM transmitters that except an external RF source including a GPS disciplined oscillator.

Nautel J-1000

An external RF source can be plugged into the EXT RF IN connector (J-6) on the Remote Interface board. The source must be on the carrier frequency ± 5 Hz and have a peak-to-peak voltage of between 5 – 15 V (sine wave or square wave), 50 ohm impedance.

An external 10 MHz signal can be connected to the RF synthesizer board 10 MHz REF INPUT (J2). The external 10 MHz frequency reference must be precisely 10.00 MHz and have a peak-to-peak voltage of between 2.2 – 8.0 V (sine wave or square wave).

Jumpers on the Remote Interface Board and RF synthesizer board need to be configured appropriately for each source. Consult the manual pages 3-9 and 3-11.

Nautel ND series

An external RF source can be connected to ABA1J1 on the external interface board. The RF drive must be on the carrier frequency ±5Hz with level of between 5 – 12 V peak-to-peak (sine wave or square wave) and have a 50-ohm impedance.

Do not remove the crystal from the RF Drive board as the PDM frequency for the modulator is derived from it.

To select the RF drive source for the transmitter the links on the RF Drive board need to be changed. Consult the manual pages 3-3 and 3-14.

Nautel XL series

An external RF frequency source can be connected to the Exciter Interface board, J7. The external drive signal must be between 5 – 12 volts peak-to-peak (sine wave or square wave). Consult manual pages A1 and B4.

Nautel XR series

An external RF source can be connected to the remote interface board’s digital EXT RF IN (J6). This replaces the internal carrier frequency oscillator for one or both exciters (A/B). The external RF source must be the carrier frequency, within ± 5 Hz, have peak-to-peak voltage between 5 – 12 V (sine wave or square wave). Consult the manual pages 7-1.

Broadcast Electronics AM2.5 – AM10A, AM5E

The transmitter has an external RF input on the top of the unit (EXTERNAL RF INPUT). The input is designed for an external stereo generator or reference oscillator with a signal level from 5 to 15 volts peak-to-peak. To use this input, program jumper P7 on the exciter circuit board in position 1-2. Consult the manual page 2-19.

Broadcast Electronics AM500 – AM1A

The transmitter has an external RF input on the ECU rear-panel (EXTERNAL STEREO RF INPUT (J1). The input is designed for an external stereo generator or reference oscillator with a signal level from 5 to 15 volts peak-to-peak. To use this input, program jumper P7 on the exciter circuit board in position 1-2. Consult the manual page 2-20.

Harris Gates AM series

An external RF source can be plugged into J-1 on the Oscillator board. The source must be on the carrier frequency ± 20 Hz and have a peak-to-peak voltage of 5 volts. Frequency source selector P-6 must be set to external. Consult the manual, Oscillator Board Schematic.

Harris DX series

An external frequency generator can be connected to J2 on board A3. Jumper P5 should be set to either 20K ohms or 50 ohms depending on the source impedance. Jumper P6 can be set to either external source or automatic source selection. The drive level needs to be 4 to 4.5 volts peak-to-peak square wave for high impedance inputs or 0 to +25 dBm for 50 ohm impedance sources. Consult the manual, page A-2.

DX-50 oscillator board, A-17 external source connected

Newer DX series oscillator boards which have automatic source selection will fail over to the internal oscillator if anything happens to the externally generated RF signal.

Harris DAX

An externally generated carrier frequency or 10 MHz reference signal can be connected to connector J11 for the external carrier or J10 for the 10 MHz reference on the External I/O board. External carrier or 10 MHz reference must then be enabled via the VT100 screen. The external carrier frequency or 10 MHz reference must be above 2.0 volts peak-to-peak. Consult the manual page 3-15.

Harris 3DX

An externally generated carrier frequency can be connected to J12 (RF CARRIER) jack on the external IO board. The drive levels need to be 4 to 5 volts peak-to-peak, square or sine wave. On the carrier frequency +/- 5 Hz. The input is impedance is selectable for either 50 ohms or 10 K ohms.

An externally generated 10 MHz reference frequency can be connected to J10 (10 MHz REFERENCE). 10 MHz reference level needs to be 1 to 5 volts RMS, square or sine wave. The input impedance is selectable for either 50 ohms or 10 K ohms.

Programming for these options is done on the exciter setup page. Consult manual page 2-43.

Harris SX series

SX series transmitter have either an oscillator board or a frequency synthesizer board. Both will accept an external frequency source. The oscillator board is A16J1 and it needs a 5 volt peak-to-peak carrier frequency signal. Frequency source selector P-6 must be set to external.

The frequency synthesizer board external frequency input is also J1, however, it requires a 10 volt peak-to-peak signal. Frequency source selector P-6 must be set to external.

Conversion table for various RF power levels into a 50 ohm impedance

Volts, Peak-Peak	Volts, RMS	dBm	mW
2.2	0.77	10.8	12
3.3	1.23	14.35	27
5	1.76	17.9	62
10	3.5	23.9	250
12	4.24	25.5	360
15	5.3	27.5	562
20	7.07	30	1000

A 10 MHz reference input is preferred over direct carrier frequency generation simply for the ease of implementation. With direct carrier frequency generation, the frequency output of the GPSDO needs to be double checked. One misplaced digit and severe damage to the transmitter can result.

AM Tube type transmitters, plus early solid state transmitters such as the Harris MW1A may have instructions for implementing AM stereo. Since the AM stereo exciters generated the carrier frequency, those instructions would be a good guide on how to connect an external frequency generation source. However extensive modifications may be needed to the oscillator section depending on the transmitter.

Honestly, this is cheap enough that I think all new AM transmitters should come with this from the factory.

Audio Processing

Any radio station’s on air signal is its biggest marketing tool.

What sounds bad:

Over use of compression (gain reduction)
Over use of high frequency EQ
Over “equalization” on all frequencies
Over modulation
Overly aggressive composite clipping
Improper use of FM pre-emphasis
Poorly tuned transmitters (tube type)
Poorly matched antenna systems (all types)
Poor quality audio input
Over use of bit reduction on the STL
Analog STL’s that are off frequency
Playback of bad audio recordings

What sounds good:

Moderate use of compression to bring up audio levels for in car listening
Using equalization that suites format (e.g. more mid-range for all talk, more bass for urban, etc.)
Properly adjusted processor output levels for the correct modulation levels
Setting the pre-emphasis correctly
Tuning tube type transmitters for minimum distortion
Tuning antennas for adequate impedance and bandwidth
Making sure that audio input levels are correct, the audio is properly distributed and terminated with the correct impedance
Using STLs that have enough throughput that either no bit reduction or minimum bit reduction is used
Regularly check analog STL frequencies and re-adjust as necessary
Get rid of all bad audio recordings in the automation/playback system. Make sure that new files are from good sources and/or are re-recorded correctly

I took a little road trip between Christmas and New Years (Happy New Year!). I cannot help myself, I ended up tuning around the radio to see what was on. Suffice to say, I found the usual formats and a few locally focused stations. What struck me was the sound of some of the stations. While most sounded acceptable, if not somewhat generic, there were a few that had ear splitting, headache inducing audio. These stations were often over modulating and way over processed. It would have been better if there were no processing at all.

That got me thinking, what is or rather what should be the point of audio processing? Way back in the day, there were loudness wars. These were often program director ego induced efforts to sound louder than the competition because if you were louder, it meant you had more power. As listeners tuned their analog car radios from station to station, the signal that “jumped out” was mostly likely to attract more listeners. At least that was the way it was explained to me in the by a program director in the late 1980s.

We are no longer living in a listening environment where loudness is of huge importance. The number of audio sources has increased greatly; iTunes, Amazon Alexa, Spotify, Tune in, Pandora, YouTube Music, Sirius XM, iHeart, and AM/FM radio. Audio levels can be anywhere and listeners have gotten into the habit of raising or lower the volume as needed. Outside of program directors (or whatever they are called these days) offices, loudness means next to nothing. If you asked an average audio consumer how loud their program sounded, they would not likely know how to answer you.

I believe what most people are looking for is an enjoyable listening experience. The most important quality of any type of audio processing is that the product sounds good. The problem is “sounds good” is very subjective. Perhaps a better term would be technically sounds good. The audio should be free from distortion and artifacts of CODEC bit reduction. Overdone AAC or HE-AAC has this strange background swoshy platform behind everything which is headache inducing. Instruments should sound as they do when heard live. In other words, Susan Vega’s voice in the original Tom’s Diner should sound like Susan Vega.

Next would be compensating for difference levels in program material. A bit of gain reduction so that those in mobile listening environments can hear all of the program material. Finally, some format specific equalization can be useful. That is it. Moderate use of various audio processing tools can certainly accomplish those things. Like everything else, too much of a good thing is bad.

Repairing an RF module for a DX-50

I like these types of posts. Many people are intimidated by component-level repairs. I write this to show that it is possible, with minimal test equipment and easy-to-understand directions, to make repairs and get things back in working order.

Every year, we lose two or three RF amplifier modules from the DX-50s. Normally, this happens after a thunderstorm. Sometimes it is a spontaneous failure.

The project starts here; faulted RF module

These are fairly simple medium-wave RF amplifiers. There are no adjustments or tuning circuits on the amplifier board itself. They use eight IRFP-350 RF devices. There are two fuses to protect the transmitter power supply against device short circuits. If a fuse blows; Section C, RF amplifier Modules, of the DX-50 manual has a good troubleshooting guide. There is a very good chance that one or more of the RF devices is bad. Unfortunately, they have to be removed to be tested.

Gates Air recommends that if one device is failed, all four devices from that section are replaced at the same time even if they test good. The IRFP350s are inexpensive devices and it is easier to replace everything at once. The Mouser Part number is 844-IRFP350PBF and they are $3.81 each as of this writing. The PBF suffix means it is lead-free.

The other part that can be bad, but it is unlikely, is the TVSS diode across the gate and source (CR1-4). These are inexpensive as well. Mouser part number is 576-P6KE20CA and they are $0.38 each. It is good to have a few of these on hand.

The most difficult part of this is dealing with the heat sinks. The devices get stuck to the heat sink pads after so many years, so it takes a little effort to get them separated. The manual recommends gently prying the device away from the heat sink with a small screwdriver. They can be reused if you are careful and do not rip the insulator pad. However, if the insulator pad rips, it needs to be replaced. Mouser part number 739-A15038108 ($0.86 each).

To test each IRFP350 after it has been removed, use either a DVM with greater than 3 VDC on the resistance setting or a Simpson 260 on the Rx10,0000 range. Connect the positive lead to the Drain, the negative lead to the Source, and then use a jumper connected to the Gate to turn the device on or off. Alternatively, you can use a 9-volt battery to turn the device on and off.

If the device does not turn on or off, it is defective.

The TVSS diode should measure open (>2M) in both directions. Anything other than that, the unit is defective and needs to be replaced.

The first step is to remove the heat sink. I used a small screwdriver under the leads to gently pry the MOSFETS off of the heat pads. If you are careful, all of the heat pads will survive. Once the heat sink is off, I remove all four of the suspected MOSFETS. The leads are heavy gauge, so it takes a little bit of work with the solder pump and solder wick. I tested each MOSFET and found one shorted unit, the other three test okay. However, since these are inexpensive devices, I replaced all four.

Good device:

Bad device:

Assembly is in reverse order. Make sure that none of the insulating pads were torn during disassembly. I like to get everything attached to the heat sink before soldering the leads. It helps with lining everything up. Take care and make sure that the ferrite beads on the drain leads of Q3,10 and Q4,11 are re-installed with the new devices. These are necessary to prevent high-frequency oscillations.

Of course, the final test is in the transmitter. Generally speaking, I test the standby transmitter into the test load every two weeks. This is done in conjunction with the generator load test so as not to spin up the demand meter.

Excel spreadsheet formulas for Broadcast Engineers

There are many times when some mathematics is needed in this profession. For one-off situations, the calculator applications found on most smartphones will work just fine. However, sometimes the calculation is complex or is needed to be repeated many times. Excel Spreadsheets have many mathematical functions built in. Plugging a formula into an Excel spreadsheet is a handy tool.

I recently acquired this rather nice precision power meter:

It has an input power range of -60 to +20 dBm with a stern warning not to exceed +23 dBm. Since we will be using this for a variety of applications, I thought it might be useful to know approximately how much power will be presented to the instrument in any given situation. For example, we are installing a 30 KW FM transmitter soon. The directional coupler that will be used has a coupling factor of -48.5 dBm. The TPO is 28,000 watts.

The formula to convert Watts to dBm is dBm=10 X Log10(Pw) + 30, where Pw is power in Watts. Thus dBm=10 X log10(28000) + 30 or 74.4715 dBm minus the 48.5 dBm coupling factor which is 25.9715 dBm. That is too much input for this power meter. A 20 dB attenuator will need to be used.

Since I will be using this meter in other places, rather than doing that calculation over and over again, why not build an Excel spreadsheet? That would make it easy to check.

A simple Watts to dBm calculator in Excel looks like this:

=(10*LOG(C6))+30

This is copied into cell C11. C6 is the cell in which the Transmitter output power in watts is entered. The other cells contain the coupling factor (C5) and external attenuation (C7) In application, it looks something like this:

Excel spreadsheet power meter calculations

You can arrange these any way you like, just change the cell numbers to suit your needs.

I like to make the data entry cells green. You can lock the formula cells so that the formulas don’t get changed accidentally. Below the Approximate port power cell, is the IF statement that will return either a “LOW”, “HIGH”, or “OK” depending on the result value in C11. That looks like this:

=IF(C11>C9,"HIGH",IF(C11<C8,"LOW","OK"))

The spreadsheet itself is downloadable: Power meter port calculator

It would be very easy to make a system gain/loss calculator for using the licensed ERP to calculate the proper TPO.

Other examples of useful Excel spreadsheet formulas:

To convert from dBm to watts:

=10^((B22-30)/10)

B22 is the cell in which the power in dBm is entered. These can be any place you want on the spreadsheet.

Radio Frequency to Wavelength in Meters:

=299792458/B10

Where B10 is the cell in which the frequency in Hz is entered. 299792458 is the speed of light (Meters per second) in a vacuum. If you wanted the input frequency to be in kHz, simply move the decimal point for the speed of light three places to the left, e.g. 299792.458. For MHz move the decimal four places to the left, GHz five places, etc.

Convert electrical degrees to Meters:

=(299792.458/B10)/360*B11

Where B10 is the frequency in kHz and B11 is the number of electrical degrees in question.

An example of that in an Excel Spreadsheet can be downloaded: Frequency to Wavelength converter

Audio Frequency to Wavelength in Meters:

=(20.05*(273.16+B11)^0.5)/B12

Where B11 is the air temperature in degrees Celsius and B12 is the frequency in Hz. Room temperature is normally about 21 degrees Celsius (about 70 degrees Fahrenheit). Humidity and altitude can also affect the sound wave velocity, which will affect the wavelength.

Base (or common point) current calculator using base impedance and licensed power:

=SQRT(B12/B11)

Where B12 is the License power in watts and B11 is the measured base impedance of the tower (or common point impedance of the phasor).

Convert meters to feet:

=B11/0.3048

Where B11 is the length in meters

Convert feet to meters:

=B12*0.3048

Where B12 is the length in feet.

Convert degrees F to degrees C:

=(B11-32)/1.8

Where B11 is the degrees Fahrenheit

Convert degrees C to degrees F:

=(B12*1.8)+32

Where B12 is the degrees Celsius. In this case, the order of operations will work without the prentices but I kept them in place for uniformity.

Convert BTU to KW:

=B11/3412.142

Where B11 is the BTU/hr

Example of an Air Conditioner load estimation:

=(B11*B12-B11)*3412.142

Where B11 is the TPO, B12 is the transmitter AC to RF efficiency. The output is in BTU.

There is an entire list of Excel functions here: Excel Functions (alphabetic order)

You get the idea. Yes, there are smartphone applications as well as online calculators for most of these functions. However, I have found smartphone apps are becoming more painful to deal with as time goes on, mostly due to the ads. App developers need to make money, and you can buy apps for things that are often used. However, it is nice to have these types of calculators available offline. Besides, it is fun to play around with Excel formulas.