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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 : 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 : 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 : 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 : 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 : 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.
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