I have been reading with interest the ongoing discussion about AM radios in Electric Vehicles. Rather than rehash the what, I thought it would be nice to dig into why it is happening.
My first thought is that many of the electronics use PDM or PWM to control various stages of charging, converting, or discharging the storage system. I quick review of a typical EV basic diagram shows that there are several systems involved
Searching through various chip makers’ data sheets on Li-ion battery chargers, DC voltage to voltage converters, regenerative braking systems, traction motor inverters, and so on shows that all of those systems use PWM. Some of those PWM frequencies are right in the AM band, while others are not. That explains why different manufacturers have different takes on AM radios in EVs.
Basic Electric Vehicle
All of those electrical components are controlled by an electronic system that handles battery charging,
This basic diagram shows several sections that rely on PWM to function. The traction inverter is very complicated, with sensors running to each motor and each wheel for traction control, etc.
I imagine the average EV driving down the road in a cloud of PWM-based electrical noise. Whether or not that creates interference with AM reception depends solely on the PWM frequency the chip manufacturer chooses. That is not all, even when sitting in the garage charging, the Li-ion battery chargers use PWM.
It seems a monumental task to attempt to mitigate the noise issue. The real question is; does the general public and more specifically, those who want to own an EV care about AM broadcasting?
There are many alternative entertainment options these days. I would say the average Tesla driver listens to iTunes.
It would be interesting to test MA-3 reception in a Tesla. That would be a real-world test to see how the HD Radio codec stands up to electrical noise. I would say the same about DRM, but you would need to find a receiver first.
One small RF project that I am working on; a 770 KHz notch filter. I always figure if I am having this problem, then others may be having it too. This is a relatively simple idea, a resonant LC circuit (AKA a tank circuit) tuned to the carrier frequency. It should have a bandwidth of +/- 15 KHz of the design frequency. Another requirement; use the parts I have available. Finally, the environment in which this is to be used is a high-noise room; with lots of computers, LED lights, etc therefore it needs to have excellent RF shielding.
Something like this would work well for anyone that lives around an AM transmitter site and is having problems with receiver sensitivity or transmitter intermodulation.
The basic design looks like this:
Parallel LC tank circuit
Time for a trip to the local storage facility known as “The Barn.” In my backyard, there is a small agricultural structure that is used for storage of just about everything. In The Barn, I found several parts salvaged from an old Energy Onyx Pulsar AM transmitter. As such, they are more than capable of receiver operation and could likely handle a fair amount of RF power in the transmit mode.
CDM F2B 0.01 uF capacitor with back of N connector inputs
Finding a type F2B 0.01 uF capacitor, rated at 2000 volts and 11 amps, the value of the inductor was calculated. For the inductor, a 20 uH coil with taps will work great. For receive-only applications, much smaller-sized components can be chosen. Also, there are many bandstop filters with multiple poles. Those are great, but I like the simplicity of the parallel resonant LC circuit.
20 uH inductor salvaged from Energy Onyx transmitter
The N connectors were salvaged from I don’t know where and the enclosure used to house a power supply for a Radio Systems console.
N connectors for input and output.
For shielding, I sanded the paint off of the enclosure where the lid is attached and tacked some brass screen down with gorilla glue. This will make a good RF contact surface. The outer of the N connectors are bonded to a piece of copper ground strap which also has a grounding lug on it.
Enclosure lid with brass screen to make contact
I used the Libra VNA to tune it up:
S12 shows return loss, S21 shows Phase
The scan shows it is -31 dB on the carrier frequency. It is -17 dB on 760 KHz and -20 dB on 780 KHz. This is good, because I may still want to listen to the station on the remote receiver. According to the smith chart, it is actually resonant on 771.5 KHz, but that is close enough for this application. I think the resonance went up slightly when I put the cover on after the tune-up.
There are several tank circuit calculators online. It is best to have more capacitance and less inductance to keep the Q of the circuit low and suppress the sidebands as well as the carrier.
While cleaning out a closet at home, I found a 3.5-inch disk with some interesting memos. When I left WGY in the spring of 1996, I made a backup copy of all the items in my documents folder. I figure it was an intelligent thing to do since I was still working for the same company in the role of Director of Engineering.
In those days, management wanted a precise accounting of all off-the-air incidents. The studio was staffed with a board operator who monitored the air signal at all times. Anytime the carrier dropped, there would be a note in the transmitter log. Those 5 second interruptions are likely due to thunderstorms. Lightning would strike somewhere nearby inducing an EMP on the tower. The venerable MW-50B would kill the PDM for a brief period as protection from VSWR. If I were at the transmitter site, the insulators in the guy wires would start crackling anytime a storm was within 10 miles of the site.
The helium balloon incident involved one of those metallic helium party balloons which escaped and ended up tangled in the 240-ohm open wire transmission line. This caused multiple VSWR trips for both the main and backup transmitters. I remember pulling up to the site and having a bit of a chuckle. By the time I got there, the balloon had mostly been burned into oblivion by the RF and the station was back on the air.
Another interesting item is our standard reception report form letter:
WGY reception report letter; name and address redacted because of the internet
These were printed out on WGY letterhead and mailed. I sent out several of them every week. I think the furthest away was Cape Town, South Africa.
The model for Radio Engineering these days is such that one engineer is covering multiple stations in various locations. At the very least, this person has a full (if not overflowing) plate. Thus, when something breaks, the procedure very often is; to pull the suspected module or board, call the manufacturer and order a replacement. That works as long as the manufacturer supports the model in question or has parts. As we all have learned by now, replacement parts are subject to the global supply chain, which is tenuous.
Then there is the question of AM transmitters. Is it worth it to replace an AM transmitter these days? I suggest it would depend on the market and revenue. In some cases, yes. In other cases, keeping the older equipment running makes more sense.
Troubleshooting is becoming a bit of a lost art. In addition to the time it takes, we tend to be unfocused and obsessed with rapid gratification, ready for the next social media post. What is lacking is the ability to take apart the layers of a problem, accept our initial analysis may be flawed, move beyond those assumptions, and work until the issue is solved. Troubleshooting is often like a crime scene investigation. There are several logical steps;
Assess the current situation; take steps to ensure it is safe to proceed. Remove all power from the transmitter and don’t work on failed transmission equipment during thunderstorms
Gather evidence; look for fault indicators, alarms, automated log entries, burned components, abnormal meter readings, etc
Check external factors; power failures, lightning or storm damage, excessive heat, moisture, etc
Check internal factors; aged components, bad cables or connectors, improperly seated boards or components, and obvious signs of damage
Work from one side of the issue to the other
Check the maintenance logs (if there are any) to see if this problem has occurred before and what was the fix
Use available resources; troubleshooting guides provided in equipment manuals, factory support, and available test equipment
If a failed component is found, make sure that it is the problem and not a symptom of something else
Here is a good example of a recent troubleshooting evolution; I went to change over to transmitter #2 and these fault lights appeared:
DX-50 transmitter, faulted, no power output
The conversion error on the A/D converter indicates why the transmitter power output is zero.
The first step; secure the transmitter, remove all power, etc. Next, consult the book!
The Harris DX-50 manual gives good troubleshooting guidance. This transmitter was manufactured on March 22, 1990. It has been a reliable unit, to date. Section K.4 Analog to Digital Converter (A34) of the manual suggests loss of audio clock frequency sample due to the following;
Loose connection with the carrier frequency sample cable coming from the RF drive splitter (A15)
Bad or missing jumper connections on P-10, frequency divider section
Bad U-29 (74HC161, 4-bit binary counter, only in use if the carrier frequency is above 820 KHz, Not Applicable)
Bad U-12 (74HC14, Schmitt trigger)
Bad CR13 or 14 (1N914)
Fortunately, there was a working DX-50 about 15 feet away, so I was able to make some measurements at various places on the A/D converter board.
On the working transmitter (DX-50-1), at the RF sample input (input of R83) on the A/D converter board, I see a nice strong sine wave, on frequency:
WABC carrier from RF drive splitter to A/D converter boardWABC carrier frequency
Second, I measured the logic pulses on TP-6, as described in the manual. Those look good.
On the non-working transmitter, I made the same measurements and found a fuzzy sine wave way off frequency on the input of R83. The logic pulses on TP-6 was normal.
Definitely lost the RF sample. Since the transmitter is 32 years old, I suspected the cable (#92, RG-188 coax) between the RF drive splitter and the A/D converter had gone bad. Perhaps rubbed through on a rough metal edge or something like that. Several checks with a Fluke DVM showed that there were no shorts to ground or internal conductor shorts. End-to-end checks on both the shield and inner conductor proved good. So, not the cable…
I then went on a bit of a wild goose chase suspecting the output from the oscillator to be low or the drive regulator power supply was defective. The drive level going into the PA was close to normal but slightly lower than the previous maintenance log entry. Also, drivers 8A and 8B were both on, which is not normal and made me suspect the drive regulator.
I made a call to GatesAir and spoke with a factory rep, who had me swap out the A/D converter, oscillator, driver power supply regulator board, and the buffer amp/pre-driver module between the working and non-working transmitter (while the low-power aux was on the air). With the working transmitter close by, I was able to confirm that these boards or modules were not the cause.
Finally, I went back to the RF drive splitter and use my camera to take a picture:
DX-50 RF drive splitter (A15) J-17, board side
There is a 6-pin connector on the underside of the board (J-17). Pin 2 (from the right) is the center conductor and pin 1 is the shield of the cable going to the A/D converter board. Upon closer examination, the solder joint on pin 2 is suspect. I re-heated this connection with a soldering iron and viola, the transmitter started working again.
WABC DX-50-2, returned to service
The extenuating circumstances; the air conditioning at this site was slowly failing and that part of the transmitter was subjected to heat cycling several times. More recently the HVAC system was in the process of being replaced, of course, on one of the hottest days of the year. This pulled a lot of warm, humid air into the room. Also, as this is transmitter #2, it was not in regular use until recently (we began a procedure for operating on alternating transmitters for two-week periods).
All of this work took place over the course of two and a half days or so. That would be a lot of time for the module swap guys who tend to move on to the next outage quickly. On the other hand, buying a new 50 KW AM transmitter is an expensive proposition these days and there are very long lead times on some of these units. Being persistent and focused paid off in the end.
