An AM transmitter which is mounted at the base of a loaded monopole is exposed to a high RF field intensity which has been observed to cause problems. How this presents problems and how to resolve these took quite a bit of effort to identify and fix.
Recently, I installed a modified Ramsey AM-25 transmitter at the base of an outdoor antenna and at first all seemed to work well. I did notice that the audio sounded a bit muddy and brought the transmitter in for a bench test. It sounded great so it was placed outside but the audio again didn't sound right.
Meanwhile, I modified my indoor "high efficiency" transmitter by adding a digital frequency synthesis circuit to replace the crystal. The same muddy audio problem appeared where with the crystal control the audio was fine. The effect heard was very subtle yet with some program material it was readily apparent. It was found that since both transmitters were installed near the loading coil at the antenna base the digital frequency synthesis circuit operation was being disrupted by the RF. Both were housed in plastic enclosures which provide no shielding from RF so as a test a metal shield, connected to the circuit ground, was installed in the indoor transmitter and the audio is now clear and clean. The shield is a 4" X 4" square of circuit board material placed between the circuitry and the loading coil as shown in the photos without and with the shield in place. The shield will be attached to the inside of the cover plate in the final version.
Someone (MRAM perhaps?) has previously posted about the problem of FM in AM transmitters and it could be that this is caused by the RF getting into unshielded digital circuitry. The synthesizer loses lock during high audio peaks and within milliseconds reacquires lock and this momentary unlocked condition causes the transmit frequency to shift. This not only distorts the audio but causes splatter and spurious emission in the band so it needs to be corrected if present.
Neil
That is a big finding, Neil, regarding a transmitter generating its own interference once tuned to resonance.
So many circuits are sold to us in plastic cases. It's amazing that equipment isn't much less reliable than much of it is.
I think for the part 15 station with serious coil/antennas, metal enclosures are the way to go.
There could never be a full power transmitter in anything other than a metal enclosure.
Perhaps a temporary "cheat" while awaiting a metal case, would be to move the transmitter a few feet away from the loading coil by using an "illegal" coaxial transmission line.
Regarding "FM'ing" of the Talking House, I had this problem early on with my first Talking House.
It was most noticed when the received signal was low to weak. The received audio had a "warble" quality to it.
It was found that peaking a coil in the PLL section in conjucntion with netting the frequency eliminated the "FM'ing" at least to the point where it was no longer apparent.
I pointed this out to someone in private conversation that a transmitter's circuitry can be disrupted when sufficient levels of RF are dancing across the transmitter's circuits, either due to lack of a proper return path at the output, or from external sources or close proximity to the coupled radiator.
Great info Neil!
RFB
Years ago I worked in a lab where we had a mini-computers (DEC PDP 11/20 and 11/40). These were used for real time monitoring and control of infant incubators in two NICUs so it was fairly important that they operated reliably. On one occasion I was in the lab and keyed down my 2.5 watt 2 meter handheld and the computers halted. This happened several times and established the vulnerability of this hardware to RF. This was before the FCC set up the Class A and B computing device standards. Our newer DEC PDP-11/23 with these standards in place was not affected. At the time I served on the Medical Center Electrical Safety Committee and we received a request from a patient who wanted to use a CB radio in his room. Upon hearing my story about the computers the Committee's answer was "Negatory, Good Buddy!"
RFB, yes, unboxed transmitters can be a problem. I encountered this several times when cabinets were open for maintenance and adjustment. Often is was necessary to disconnect the antenna and use a dummy load. I presently have trouble adjusting the power input with the part 15 AM transmitters connected to the antenna because the RF interferes with the V and I measurements (they are very unsteady on the DVM). A good ole analog VOM is a nice way around this.
Carl, I "cheated" with a transmission line and this was a major clue to identifying what was happening.
From now on, it is metal cases for all my transmitters.
Neil
The Big Talker Shortwave transmitter designed by the good work of Neil, MRAM, PhilB, DRS2, Ermi Roos and a long list of contributors, is awaiting permanent installation in a metal cabinet.
The long wave transmitter, Deep Voice, will also go metal.
Now, based on this life changing discussion, EVERY transmitter in the house is destined for metal, even the Wholehouse.
Neil,
Your conclusion that the RF field is high surrounding a part 15 transmitter located at the bottom of a vertical antenna is valid.
Your conclusion that muddiness in the audio is due to the RF field interacting with the transmitter circuits is a stretch unless you absolutely eliminate other possible causes. One possibility is a long audio run to your outdoor transmitter vs. a short run to an indoor installation. Another, is the long power run to the outdoor unit vs. the indoor unit.
Audio "muddiness" = distortion. Unfortunately, the term "muddiness" is associated with a subjective description of audio quality favored by the audiophile community, and is highly controversial.
Have you verified that there is actually distortion being introduced in your outdoor installation? I know you have the technical expertise and instruments to measure this. Have you verified that the outdoor unit is experiencing FMing of the synthesizer output or any other distortions of the synthesizer or audio waveform at any point? Distortions can be discovered with a scope while comparing the sample points in the circuit with RF output enabled and disabled.
Your post raised a big flag because I have never seen any reports of a similar problem with other popular Part 15 transmitters. I can personally stand by my observations that this is not a problem with the AMT3000 or AMT5000 transmitters. I have never seen a report of a similar problem with the Procaster or Rangemaster. The ongoing discussions of audio quality of the TH transmitters are legendary, but the RF field has never been a suspect in causing any TH audio degradation.
Your problem appears to be unique to your Ramsey and experimental transmitters. With the possible exception of the Ramsey AM-25, I think all the other popular transmitters have been carefully designed, with proper attention to good bypassing and circuit board layout practices. You might take a look at such areas as IC power bypass, proper buffering between the PLL chip and the RF driver, PCB ground plane, and RF roll off capacitors on the audio inputs. A metal shield around the synthesizer should not be necessary.
Phil, thanks for the input.
It would be improper to extrapolate what I observed with two particular transmitters and antenna systems to all the others available and I don't believe I did so, but if what I wrote conveyed this impression it was not intentional. This experience was offered in case others were experiencing the same problem with a transmitter in order to give them something to try.
The Ramsey AM-25 I use has been modified for higher efficiency but the synthesizer and VFO are "stock". The "muddiness" I experienced is indeed subjective but there was a definite perceived difference in audio quality between the transmitter installed outdoors with the antenna and when it was moved indoors on a dummy load. Having done a lot of semi-pro audio recording in the past I have an acute ear for distortion and though not as reliable as THD analysis I knew something was amiss. The same receiver and headphones were used for both tests. The audio line was ruled out when I moved the outdoor transmitter 15 feet away from the antenna and used a coax to feed the antenna and the distortion was gone.
The indoor tx. (pictured) has a ground plane under the circuitry and I added one to the back of the add-on synthesizer (the paper in the photo behind the blue perf board is an insulator). For each IC, VCC to VEE is bypassed with .01 uF caps and the board rails are bypassed with 0.1 uF caps at two locations. You might notice the resistor connecting the synthesizer to the main board. This conveys the operating frequency square wave output to drive the final and would probably best be redone with RG174 coax for this signal (I tried it this way but it didn't eliminate the problem). I observed the PLL VCO control voltage and saw it change with modulation peaks. I also monitored the phase det. output from the IC and observed erratic signals with the modulation peaks. The audio input line to the "indoor" tx. includes an on board bypass cap. Nothing unusual was seen via scope at the modulator output of either transmitter.
The observation that the distortion is gone with the shielding installed strongly suggests that RF was directly affecting the PLL circuit. I experimented with a small shield covering only the PLL circuit and this cleared the problem. It is hard to imagine that with all the bypass caps this would be the case but the loop filter circuit is relatively high impedance and could be vulnerable. To address this I added two 0.1 uf caps directly from the VCO input to ground at the chip and this greatly reduced the problem but not completely. Though this adds a pole to the filter the system still acquires and maintains lock on a dummy load test.
I also noticed that if the transmitters were moved about a foot away from the base of the loading coil/antenna the problem resolved. This was a clue that the RF was causing the problem.
If I have missed something your advice will be appreciated.
Since I found a solution for my particular problem I don't deem it a priority to perform an audio distortion measurement to confirm what I heard, but this would certainly yield useful information to confirm that what I now perceive as "clean and clear" audio actually is. Maybe I'll put it on the table as a winter project.
Neil
Forgot to mention that my theory on why only the modulation peaks unlocked the PLL is that with these peaks the RF field is higher than that of the carrier. A marginally stable circuit could be "tipped over the edge" with this.
Neil
Another possible factor relating to the audio distortion is that the output power of this modified transmitter could be 2-3X more than the unmodified transmitter.
Rich, I had considered this but lacking any data about the output power of the various transmitters I couldn't make a comparison.
For the record, the modified Ramsey AM-25 delivers 82 mW to the feedpoint before the loading coil and the "high efficiency" home brew delivers 86 mW to the feedpoint before the loading coil (both with 100 mW DC input).
The voltage from the junction of the top of the coil and the radiator can be about 150 to 200 volts to ground at this power level and more so on the modulation peaks. Discounting the radiated signal, this voltage being capacitively coupled at this frequency to the circuitry could also be a factor. The shield, acting as a Faraday shield, placed between the coil and the circuitry would isolate the circuitry from this.
Neil
Having tinkered with tube RF and then FM transistor RF, building experiments from time to time, I noticed that when RF reaches a certain intensity, it can flow into nearby circuits and paralyze whatever they are doing.
I was very surprised that in building the Big Talker shortwave transmitter on open circuit cards without ground shielding between stages, reaching as much at 1/2 Watt when fully turned up, there was no unwanted RF "back flow", which I actually anticipated.
The signal from the shortwave dipole, hanging in the room, did get into my wireless mic receiver and cause a buzzy image that needed to be suppressed by adding a clip-on toroid to the receiver's power supply wire.
That RFI isn't more common with part 15 equipment is a tribute to the good design, and also might suggest that the part 15 power levels are "just in the safety margin" above which RFI might set in.
OK, Neil. Thanks for the clarifications. I think there are many readers that probably took your report as a red flag for ALL synthesizer transmitters.
I took another look at your "without" shield picture. I think the main culprit is the perf-board implementation of the PLL circuit. It probably wouldn't be susceptible to the RF if it were implemented on a PC board with plenty of copper for ground, a good voltage distribution, and adequate voltage bypass caps. But, it's OK for testing as long as the shield works.
The only problem I ever had with part 15 AM RF was when I tried to use a cheap $29 "walkman" CD player years ago. Had it a few feet from the transmitter with a 3 ft audio cable. The play would stop working and display "Error" when the transmitter was turned on. Another portable CD player did work and showed no susceptibility to RF. I have seen reports on the web from others who had similar problems with portable CD players.
Phil,
Perf board (aka Vector Board) is not a good choice for RF circuits however for digital circuits it can offer advantages, one being that the rather random runs of the wiring reduces coupling among the signal paths and another being that vias are not needed for complex wiring. It is difficult to achieve good bypassed power distribution though and this may be a factor in what I reported.
If this transmitter was to be offered as a product a redesign would be needed. It started out as a breadboard prototype to determine if high efficiency could be achieved and used a simple crystal oscillator. When high efficiency was realized it became my main transmitter and I wanted the frequency agility of a synthesizer so this was added. This "add on" and patching doesn't yield the best physical layout but such is the nature of prototyping.
Analog and digital circuits don't co-exist well and it takes careful design to avoid interference.
Neil
"Perf board (aka Vector Board) is not a good choice for RF circuits...
...Analog and digital circuits don't co-exist well....."
Now what.
The Big Talker shortwave transmitter is built on perf board (Vector Board), and the forthcoming Deep Voice long wave transmitter will be a modified version on the same type of perf board with the addition of digital PLL.
Here's the plan, unless other advice arrives... we will erect a metal cage around the PLL, within the metal transmitter enclosure.