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In recent weeks, I have been repairing and rebuilding the transmitting array of a fellow Part 15 AM broadcaster in Worcester, MA. Currently, there are three Part 15 AM transmitters in operation in this city. All three of them are Procasters and all three, to a greater or lesser degree, have the same problem.
When any of the three transmitters are operating in the clear, with no detectable interfering signals present (such as during daylight hours), they behave just like any other low power or full power transmitter. But as soon as distant stations invade the frequency, a terrible warbling sound is heard that completely distorts the audio coming from the Procaster.
This problem not only cuts down on the usable range of the Procaster in areas where its signal still dominates. It also gives the listener the impression that a pirate broadcaster with cheap, inferior equipment is operating on the frequency and creating a sonic mess. Needless to say, that is the last impression any Part 15 broadcaster wants to make.
At first, I thought that the transmitter was significantly off frequency, since it is a remote possiblility with PLL tuned models. But that turned out not to be the case. Then I thought that perhaps the build in audio processor in the Procaster was a cheap afterthought and it was responsible for the problem. So I bypassed the unit’s own audio processing, which improved the sound. But it did not change the warbling in fringe reception areas every time another station’s signal crept on to the frequency.
I was wondering, has anybody with experience setting up and operating a Procaster noticed this performance flaw? If you have, did you find any way to fix it? I am appalled that the second highest priced Part 15 AM transmitter on the market would allow such an obvious problem to go uncorrected. Right now I’m really missing the frugality, intelligence, and flawless operation of the SSTran AMT-5000, that’s for sure.
Total posts : 45366If the warbling sound is ~synced with program audio, then the carrier frequency of the Part 15 AM system is being affected by its modulation.
Maybe one or more of the d-c voltages or circuits in the frequency synthesizer of the transmitter needs better shielding and/or regulation. There could be other causes, too.
You might want to contact the manufacturer of the transmitter for his comments/advice.
Total posts : 45366Is it possible that it’s FM’ing? I found with the Talking House that I could listen using an amateur radio transceiver in the FM mode quite easily. I agree that it would be shocking given the cost of the transmitter.
Total posts : 45366Although I have no formal electronics training, previous experience with an antique radio restoration made me think of inadequate DC voltage somewhere in the circuit or mismatched stages of RF amplification. Poor shielding in the design is also an astute possibility and observation. Yes, the warbling may be in sync with program material, but I’d have to listen again just to be sure. Like I said, it affects the fringe reception areas, but how much of the “fringe” is effected depends upon the total strength of the interfering station or stations. Looks like I’m going to have to contact Gerry and give him the bad news. There is no excuse for this in a $700 transmitter.
Total posts : 45366Let us know what you find out from Procaster.
Total posts : 45366There was another case I found during my experimenting where the RF from the antenna was getting back into the PLL causing it to become unstable. Adding shielding around the PLL fixed this. This particular problem was seen with both my home brew design and with a popular kit transmitter.
Finding the exact cause of your problem will be difficult and time consuming since there may be more than one cause so contacting the manufacturer is likely your best option.
Neil
Total posts : 45366Neil,
How did you shield your pll? In some electronics, vulnerable components are covered with a thin piece of aluminum that is soldered into the ground trace of the circuit board.
Other variations of this shiled are just covering the components but are not a physical part of the ground in the circuit.
Maybe they need to shield their pll and see what happens??
Total posts : 45366Rock95seven says: “…thin piece of aluminum that is soldered into the ground trace.”
Can aluminum be soldered?
Total posts : 45366OK I will try to as clear as possible. I would take a manila looking file folder and cut it into a rectangular shape and cut at an angle at the four corners. Then I could bend the paper downward to make a cover over the circuit I wanted to shield. I should add that I covered the exterior of my paper circuit house with copper foil tape. Then using as short as possible a wire soldered to the foil covered house to a ground trace on the board. I used this technique many times when trying to shield a circuit from either radiating or to shield the circuit from radiation back in my EMI/EMC days. I hope this was clear enough.
Total posts : 45366Good suggestions have been offered but I think I had the same problem with one of my home brew transmitters. With mine, the warbling was present with and without audio modulation.
The cause was phase jitter in the PLL which caused the transmitter frequency to fluctuate and when the output is mixed via heterodyning with another station the jitter becomes audible.
The solution was a careful redesign of the PLL feedback filter circuit which completely eliminated the problem.
It would seem that you should contact Procaster and run this by them.
Neil
Total posts : 45366Thanks for the feedback. We had no idea this was happening. When we do the final test, there are no other interferring signals – so all appears just fine. The PLL design is conventional and seems OK – it might need tweaking to make things less sensitive. It could be PLL phase jitter being caused by other signals entering via the antenna. Maybe another stage of buffering is required between the PLL and final stage. Perhaps the PLL needs a separated filtered power feed. It is doubtful that it is audio causing this as you observe that when no fringe stations are present, there is no warbling. This will be tricky to figure out. We have to create this problem to do some evaluation of the situation – how to do this – can anyone provide suggestions? Any input from forum posters would be useful in helping to narrow down what is causing this and how to fix. Will keep you posted as to what we find.
Total posts : 45366The Procaster range is typically 1 mile during the day with a good install and good location – it was never meant for long range reach being that its ERP is basically microwatts.At night, all bets are off due to the Skywave effect. This is what you are hearing and refer to as ‘warbling’. I see nothing wrong with the performance.If a crystal is used instead of a PLL then stability would be better and the ‘warbling’ sound might be reduced, but the user would have little choice over which frequency to broadcast on.Other low power transmitters may have a higher Q in the tuned circuit but as the Q increases, the audio bandwidth decreases and the audio quality suffers. The Procaster design isa compromise between ease of setup, good audio quality and reasonable range. Those who want better range should consider obtaining a broadcast license and use a more powerful transmitter.Normal broadcast stations use crystal oscillators for precise frequency control, but the Procaster was fitted with a PLL which gives people the freedom to choose their own frequency.The aim of the Procaster design was to allow a non-technical person to easily setup a short range AM broadcast transmitter without having knowledge of radio workings.The original intent of the FCC allowing unlicensed radio broadcasts did not include the ranges we see here but rather broadcasting around ones own property. So we have exceeded that already.‘Warbling’ recording located on our website here: https://www.chezradio.com/misc
Total posts : 45366Just to note: The FCC considers that a carrier ratio of about 20 dB (100:1) is needed between desired and undesired AM broadcast co-channel stations to produce acceptable S/N for the stronger station in the audio output of an AM receiver tuned to that carrier frequency.
The carrier frequency tolerance for licensed AM broadcast stations is ±20 Hz. Most of them operate within a tolerance of ±5 Hz, with very low phase noise/jitter.
But when co-channel stations are present with nearly the same net field strength at the receive antenna as the desired station but on a slightly different carrier frequency, the AGC circuit of the receiver will see the net signal it is sampling to be changing in amplitude. The various carrier signals constantly add to and reduce the amplitude of the signal used by the AM receiver AGC (automatic gain control) circuit.
That changing r-f level produces an AGC change in the r-f amplification of the receiver, which results in a similar change in its audio output level. The audio result of that has been described here as “warbling.”
Probably its a bit more than should be expected for a network including a “Part 15 AM” transmitter to perform as well under these conditions (or even nearly as well) as a network using only licensed AM broadcast transmitters.
Even so, if the received carrier of the Part 15 AM station exceeds that of the interfering signals sufficiently, it can produce useful nighttime service — typically just closer to its transmit antenna.
Total posts : 45366Great post! I operate a Procaster and am absolutely thrilled with it, BUT, I don’t have any competition in the area so the only time I experience heterodyning is in the afternoon then evening when a station on the same frequency in Toronto comes in and steps on my signal. I agree with the other suggestions, hope you contact Gerry at Chez Radio and please do share his response.
Total posts : 45366If two stations or more are trying to come in on one frequency and they are ever so slightly out of sync. you will get that. Like one station is on 1050 and another is ever so slightly off at say 1052 you will get that between them. Same as playing two notes on a guitar at the same time and instead of them both being dead on frequency they are ever so slightly out of tune with each other you will get a beating between them the same way.
Total posts : 45366Barry, I used a copper clad printed circuit board as a shield. It is placed on top of the PLL components and is connected to circuit ground with a short length of hookup wire.
Neil
Total posts : 45366Welcome to the forum.
I am not familiar with the circuitry of the Procaster so my experience is limited to the PLL circuit which I used. The first indication I observed that suggested a problem was obtained by viewing the the spectrum of my transmitter output FFT using a Rigol oscilloscope where I noted a spur at one half the transmitter frequency. I then attached the scope to the PLL chip VCO output and to the 10 kHz reference divider output triggered on either signal. The phase jitter was obvious.
As I mentioned earlier, the fix involved changing the PLL filter between the phase comparator output and the VCO input which eliminated the jitter and the half frequency spur.
Though the reference is 10kHz, the jitter appeared somewhat random at a lower frequency which would explain the “warble” heard when heterodyining the transmitter ouput with the signal from a function generator as heard on a receiver.
Though there might be another cause, it may be useful to begin by viewing these two signals on a scope or by heterodyning the transmitter output with a clean sine wave from a function generator and listening on a receiver..
If you want more information feel free to email me using the contact feature by clicking on my user name and selecting Contact.
Neil
Total posts : 45366In a private email to Gerry at Chez Radio, I suggested he use a good strong dummy load to absorb most of the RF coming from a Procaster that is tuned to a “graveyard” frequency with no strong local present. That should simulate the condition inside the same building. Or he can try two dummy loaded Procasters tuned to the same frequency. That should also produce the same effect. I also suggested he tell us how to assemble our own dummy load for the Procaster since it comes in handy for troubleshooting problems like this, as well as adjusting the frequency fine tuning to be in sync with weak interfering stations.
Total posts : 45366Using an adjustment on board the transmitter, a fine tuning oscilator trimmer that could vary the frequency by maybe 2klz +or – just to eliminate the beating with the out of sync interfering stations. I hear this on AM sometimes when 6 stations are fighting to come in on one frequency and there’s the wavering because some of the commercial stations are not completely in sync. with each other from different places.
Total posts : 45366Several years ago PhilB, a member here, proposed a dummy load for transmitters which simulates the antenna and ground system.
He wrote “A “part15.us standard” antenna simulation circuit can be defined by forum consensus (I suggest a 30pF 500V 5% mica capacitor and a 30 ohm 1/4 watt 5% carbon composition resistor).”
These would be connected in series from the transmitter output to transmitter ground. Original post is here: http://www.part15.us/comment/19419#comment-19419
Neil
Total posts : 45366I built one of those PhilB “Dummy Loads” as just described and use it for testing the transmitters “on the bench”.
Using it to evaluate the “sync” with far aways stations is a good idea that I never thought of before, but now I will do it.
Total posts : 45366I want to just clarify that the warbling problem discussed in this thread is not a normal, acceptable phenonomon that the user can tweak. Signals that are 1-5 Hz off from each other are well within the realm of normal and can be minimized through the frequency fine tuning capacitor. Interfering signals that are off by a few Hz are not going to cause warbling in the audio in fringe areas, but inadequate shielding or buffering can. I used the SSTran recommended dummy load on the Procaster. It weakens the signal enough to produce nulls on a walkman with a directional loopstick two or three feet from the box.
Additional prediction: If Gerry can find a solution to this problem, it may also improve the audio clarity of the Procaster even when other signals are not present.
Total posts : 45366Information on progress made looking into this problem has been posted on our website here:
https://www.chezradio.com/misc
A slight improvement will be made on the next run of boards although we don’t know if this is contributing to this problem. Could it be a bad/damaged main board?
Total posts : 45366Thanks for posting.
Total posts : 45366Add me to the growing list of readers who have been following the response to this illusive hard-to-replicate problem by Chez Radio.
The video clearly showed a stable and professional AM signal being subjected to music one might expect to hear on the radio.
We are thankful for your contribution the part 15 experience.
Total posts : 45366Gerry thanks for the great support of this great product. While I am not experiencing this “warbling” problem with my Procaster, it’s great to know that you are not just another “one and done” manufacturer.
Total posts : 45366ChezRadio That Sounded Great Playing Pink Floyd
you made a Believer out of me that it is possible to have album rock on AM and sound good. I’ve got to try and save up for this transmitter for my station. I think it would make it sound great while still being legal.
I’ve got another idea for you you should come up with a AM stereo transmitter and call it the procaster album rocker. That version could be in C-Quam AM Stereo. Even though that thing was mono it did sound good from what I heard. Did you use any special audio processor other than the processor that was built into it?
Total posts : 45366Below is a plot of field intensity vs. distance for a Part 15 AM station using about the best legal hardware permitted under FCC §15.219, and better-than-average soil conductivity.
The nighttime skywave of licensed AM stations can range from maybe 0.5 mV/m to 3 or 4 mV/m. If the skywave interference is 0.5 mV/m, then the required signal of a Part 15 AM station that needs to be just 10X greater would be 5 mV/m.
Notice in the plot that the radius to the 5 mV/m signal of that installation is less than 50 meters (50 meters = 164 feet).
Other conditions can be examined by referring to the chart.
Total posts : 45366Here is a link to a 13 1/2 minute narrated audio tour of the night time coverage area of Expresso 1620, one of the Part 15 stations in Worcester, MA that uses a Procaster transmitter. It was recorded on Sunday, 3/25/2018 when signal levels from interfering stations were more than usual. The warbling is quite evident:
[audio src="http://www.t1700.net/Expresso_Procaster_at_night.mp3" /]
Total posts : 45366Nighttime Dreams
During many years in hobby radio I have wished, along with many others, for a legal AM nighttime signal that would match the daytime signal.
Knowing that the rules don’t allow the small power increase that would make such equalibrium possible, I have wondered purely academically how much power it would take to do it.
Post # 28 gives a ballpark estimate, which may vary:
“The nighttime skywave of licensed AM stations can range from maybe 0.5 mV/m to 3 or 4 mV/m. If the skywave interference is 0.5 mV/m, then the required signal of a Part 15 AM station that needs to be just 10X greater would be 5 mV/m.” – Rich
10X greater is a convenient number of about 1 Watt.
Somebody does make a 1 Watt AM transmitter, but this is only a dream and we don’t recommend it.
Total posts : 45366Was it just me or did I actually read or hear about that that transmitter actually did not have very good range for what it was? From what I heard it didn’t do as well as they talking house transmitter does on a.m. with the wire antenna.
Total posts : 45366I listened to your demo using headphones and noted two things, both of which sound to me like what I hear at night at home using my outdoor antenna with the receiver tuned to a frequency where there are two or more weak stations on the same frequency. As we begin to hear your station there is a noticable beat frequency at about 1.5 to 2 Hz. This is caused by the carriers being off frequency of each other and alternating between reinforcement and cancellation which causes the receiver AVC to change the receiver sensitivity in step with the beat frequency. Note how the audio goes up and down at this beat frequency.
As you approach your transmitter and its signal becomes stronger this beating diminishes because your signal is now stronger than the other signal and the reinforcement and cancellation is reduced. But this is where you mentioned distortion which can be explained by the distant station’s audio being added to the audio of your station which the receiver cannot separate. The addition of the audio from the distant station will distort the desired audio of your station since what you hear is the sum of the two (or more) signals. As you get closer your signal audio becomes stronger than the distant audio and the unwanted audio becomes a smaller part of the summed received audio eventually becoming inaudible leaving only your station’s audio.
During daytime the distant stations’ signals are not present so there is no beating or distortion due to audio summing. What I heard sounds like normal nightime interaction of several signals of about the same strength.
It is possible that I do not hear all that you do but you should consider that what you hear is normal and not produced by a problem with your transmitter.
Neil
Total posts : 45366From Reply #30 of this thread: During many years in hobby radio I have wished, along with many others, for a legal AM nighttime signal that would match the daytime signal. … <snip, then this conclusion> 10X greater is a convenient number of about 1 Watt.
Unfortunately, increasing field intensity by 10X at a given location requires the radiated power to increase by 100X (other things equal).
Doing so for this scenario would require a 10 watt transmitter (again, other things equal).
Total posts : 45366Rich clarifies: “Increasing field intensity by 10X at a given location (seeking nightime equality compared to daytime) requires the radiated power to increase by 100X (other things equal).”
WAY beyond part 15!
Again… how many times do I have to repeat myself?.. this problem has NOTHING to do with carrier frequency and how far apart two or more signals are in frequency. From what I can see, the Procaster has great frequency stability that rivals licensed transmitters. There is no “beating” or “wavering” problem caused by two or more signals being a few HZ off from each other. The problem is PURELY AN AUDIO ONE but it only occurs when other stations (or even other sources of interference) are present on the operating frequency. If you don’t understand the nature of the problem, please refrain from commenting. Thank you.
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