Engineering Radio – One Hundred percent Human Generated Content for your Reading Enjoyment

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.

WNPI-DT gets a new GatesAir transmitter

The public TV viewers in the St. Lawrence Valley get an updated transmission system. I recently finished installing this GatesAir ULXTE-6 transmitter in South Colton, NY. That is way up on the very northern fringes of the Adirondack Mountains. It replaces the Thales CCT-U-TDU2 8KW UHF TV transmitter, which was installed around 2004, early on during the analog/digital TV conversion.

South Colton, NY is home of Sunday Rock. According the the historical marker:

This glacial bolder (erratic), twice preserved by local citizens, marks the gateway to the “Great South Woods.” In frontier days it was said that there was no law or Sunday beyond this point. May all who pass this way continue to enjoy the beauty of the mountains.

This in the northern end of the lake effect snow belt, where the average yearly snowfall tops 6 to 10 feet. Sometimes there is continuous snowfall starting in November and ending when Lake Ontario finally freezes in late January or February.

Back to the business at hand; the new transmitter placed:

The 25 KW heat exchanger was placed where the old analog heat exchanger was:

This system uses 1 1/4 inch flexible tubing, which is easier to work with than the 1 1/2 inch steel reenforced tubing.

Measuring the Comtech mask filter with the network analyzer:

Old Dielectric dual mask filter for Thales transmitter

New mask filter mounted on the old dielectric mask filter stand:

This arrangement allows for the reuse of the two coax patch panels, on for antenna/dummy load, the other for main antenna/backup antenna.

3 inch rigid line between the transmitter and filter

Post mask filter RF signal analysis using a Rohde Schwarz ETL:

The GatesAir equipment always has good performance parameters.

The carrier frequency is measured using the pilot. In this case, both exciters have the GPS reference connected and working. I think the ETL OCXO might be 11 Hz low.

Anyway, this was a fun project.

One of These Days

The name of a song on Pink Floyd’s album, Meddle, released in 1971. The only lyrics in the song are drummer Nick Mason, who says “One of these days I am going to cut you into little pieces.” This was recorded at double speed with Mason speaking in falsetto, then played back at normal speed. Anyway, a good song from one of my favorite Pink Floyd albums.

And so it was for this Thales UHF TV transmitter. Installed in 2005 or so during the early transition to digital TV for PBS affiliate WNPI-DT.

Decommissioning a liquid cooled transmitter requires a few extra steps. First and foremost, as much as possible the antifreeze needs to be captured and collected for proper disposal. In this case, approximately 110 gallons (417 liters) of Dowtherm heat transfer fluid was drained into barrels.

Next, all of the RF modules and power supplies were removed from the transmitter. Both needed to be drained of HTF.

The outdoor heat exchanger presented a new problem:

It was attached to the concrete pad with hammer fixed anchors which needed to be ground off with a hand grinder.

It was a little bit chilly on a 10 F (-12 C) day, laying on the concrete pad, in the snow, under the heat exchanger with a hand grinder grinding the top of of eight little round bolts. After that was done, I managed to pry the legs loose and tip it slightly to get the rest of the HTF out into a bucket. I think the HE had about 15 gallons (57 liters) of HTF.

Next, all of the smaller sub assemblies were removed; the upper and lower RF module and power supply frames, the two control module frames, the rails that held the control modules, the AC power input and distribution frame and the controller frame and all the circuit boards. The RF module and power supply frames had HTF tubes and pipes that needed to be drained. The circuit boards are disposed of as E-waste.

The wiring harness was removed.

Finally, the stainless steel main cabinet frame was cut into manageable pieces with the battery powered sawzall (reciprocating saw) so that it could be carried out of the building.

All in all, it was a fun project.