Our 1640 AM transmitter is fed by an FM signal at 85.5 MHz coming from a C.Crane FM transmitter, but the stereo sub-carriers are putting “birdies” on the AM signal.
Our 1640 AM transmitter is fed by an FM signal at 85.5 MHz coming from a C.Crane FM transmitter, but the stereo sub-carriers are putting “birdies” on the AM signal.
Is there a known way to shut off the stereo on the C.Crane and transmit in mono?
birdies
How can it do that? I assume you are using a receiver to feed the AM transmitter. It filters all that out and should provide clean audio to the AM. I doubt the transmitter is the problem,try a different reciever. To disable stereo in the CCrane,open the circuit to the 38mhz crystal,no pilot and mono output.
You Are Right
Yes Lee, and my “birdie” problem turned out to be something else, but I didn’t make that correction right away because I still wanted to know how to mute the stereo, which you’ve provided. Weak FM signals go farther without the stereo baggage, which is a benefit.
What I found was that the audio level to the C.Crane had gotten set low, so I was severely under modulating. At the receiver end a lot of background hash was getting amplified. Now the FM modulation is corrected and no more “birdies.”
Thank you, sir!
level settings
Properly set levels are very important and can make a huge difference. My boss says it is what separates the men from the boys…
I really like the CCrane transmitter,it has the best audio of any I have tested. The range is limited but legal,so mount it high and in the clear,it will do well.
What clear and solid range
What clear and solid range are you getting from the C.Crane FM TX?
TIA …
CCrane Range
Hi Ken,well,about 750 feet for full quieting to a car or portable reciever. 500 feet to a friends house with a simple indoor antenna,about a mile to a friend with an outdoor amplified yagi. It can be heard choppy beyond these distances and quickly fades away.
My transmitter is mounted 20 feet above my roof,about 35 feet above ground housed in a piece of PVC pipe and strung up a tree by a rope with the wires trailing straight down.
RE: CCrane Range
Cool. That’s more distance than I expected.
The reason I ask is that I’m contemplating building a translator system as you’ve apparently done, but all in one cabinet. For example, I could take the board out of my little Part 15 Spitfire AM TX, and stick it together with a custom built FM receiver and a good directional FM receiving antenna (a loop?). Then send the signal to it from the CCrane.
How would I isolate the TX and the RCVR from each other in the same box?
TIA …
translator
They should get along ok if you use a metal shielded box and divide it inside to keep them from each other. You may also want to put a couple of rf chokes on the reciever and even some bypass caps to keep the power feed clean. I have feedthru caps for just this type of application if you need some,cheap!
I am not using a translator,it is direct wire feed here for all four transmitters mounted outside. I may deploy a translator some day tho…
I don’t understand …
I’m
I don’t understand …
I’m late getting back to this, but why do you have a wire feed from an FM transmitter to an AM transmitter? How does this work? IOW, what is the purpose of using an FM transmitter unless the intent is to feed the AM transmitter wirelessly (the AM TX would need a rcvr)? … why don’t you just send the content directly to the AM TX?
I like pvc
it’s so useful for experimenting.
Ive been thinking about the same thing for a small certified fm transmitter i have. (the scosche fmt-4) But powering that little bugger is a pain so I may try a different tx and place it into a capped off pvc pipe then mount it on a piece of mast pipe.
I can mount it on the porch here as long as it stays three feet above my home. (rules here at the park) Yeah pvc is awesome lol.
Kens Question re IC circuits
Ken,
I faced the same issues you raised with your question: “So here’s my question: How and what type of IC circuit would be used to get back the ground at either end of an audio cable? And how would it prevent lightning damage?” and I solved the problem. The trouble is I don’t think I can adequately explain it but here goes:
The situation was to get signals from DC to 4 kHz through about 4 miles of leased dry metallic pairs provided by the TELCO. I designed two circuits.
The transmitters took a single ended signal and converted it to a differential signal. A simple op amp inverter with its output connected to another inverter provided the differential signal…one line from the first amp output, the other to the second amp output.
At the receiving end, one line went through an inverter and this output was then “added” to the other wire’s signal through a summing op amp circuit. Two op amps on each end.
Lightning and spikes were a problem so I used two Zener diodes to establish a +/- clamping voltage and diodes to connect the lines to the clamp rails. Any spikes were dumped through the diodes into the Zeners. I also put resistors in series with the lines so the clamp network didn’t get pounded too hard. This system worked for over ten years with no failures.
Now for some fun, and if any are not interested in war stories this can be skipped. I and a nurse were presenting a lecture to the American Academy of Pediatrics meeting in Cincinnati years ago. Our topic was electrical safety in the Neonatal Intensive Care Unit and the nurse was a co-presenter so it would have a medical connection. I explained the importance of proper equipotential grounding, good quality caps and receptacles, how to spot problems, etc. , and as an example I mentioned that the microphones we were using were cabled which ran about 1000 feet to the Communications Dept. and were not grounded and they were not grounded because if they were there would be hum from a ground loop and therefore could present a hazard. To demonstrate, I touched the microphone to the incubator we had for illustration and 60 cycle hum warmed the room. Well, the nurse probably wasn’t paying attention because when she took the microphone to make her remarks she backed into the metal base of the incubator, looked at me, and said “don’t do that!” I knew she had received a bit of a jolt on her backside so I said “I didn’t do anything.” Then it happened again. The audience of 400 plus seemed to be wise as to what what was happening but the nurse (a real cutie by the way) seemed to think I was putting my hand where it didn’t belong. We sorted that out later. You can’t make this stuff up!
Neil
Why I Use FM for STL
Hi Ken Norris!
I am happy to explain why I am using an FM relay to get audio to an AM transmitter. This comes from practices in “big time professional” radio, where I had the advantage of learning a few things even though I was only an announcer.
Fact: to send audio over wire for more than 50-feet the signal must (should) be converted from “unbalanced” to “balanced,” which means from 2-wire, signal wire surrounded by a shield, to 3-wire with plus/minus wires above ground and the shield making the 3rd wire. The reason is the 2-wire approach begins rolling-off high frequencies the longer it is. It is the reason telephone lines are 3-wire balanced lines.
Fact: Making the conversion from 2-to-3-wire requires transformers or IC circuits at both ends of the line, a design problem and expense in itself.
Fact: To run a wire 100-feet from front to back of the house requires drilling holes somewhere to run cables out of the way and then back up at the end of the line, which can be a sticky job.
Fact:The quick fix is wireless, and you are right that a receiver is needed at the other end, and for Part 15ers it gives an actual (2nd) reason to having an FM transmitter, which can also be counted as an FM radio station in its own right.
If the AM transmitter was outdoors you’d need to have an FM receiver out there protected from the weather, but you wouldn’t need to bury audio cable, and could avoid all wiring by using solar power, which is another project on our “to-do list!”
I hope I covered everything.
Balanced Audio vs. Single Ended vs. Radio Link
Hi Folks,
In his previous post Carl outlined some of the factors involved in sending audio signals down lines for hundreds of feet. His comments are correct, but there are other things to consider (as is always the case in technical stuff).
I have worked in instrumentation and signal transmission for most of my career and have seen and done most of it. Here’s a mini-lecture:
Let’s consider two types of wired signal transmission, Single Ended and Differential. Single ended refers to one wire carrying the signal and the ground (usually the cable shield) providing the return path. How far will this go? As far as you like except for a nasty problem in that the ground at the signal source may, and most likely will be, at a different potential than that of the load. Even within a dwelling this difference can be enough to cause hum problems. A shielded wire running across your desk or home entertainment system is probably not an issue but feeding a remote unit which may be at a different power system ground point certainly can be.
Given the opportunity, the best way to prevent “ground loops” is to run the system differential. This is a TWO wire system and does not require shielding, and as Carl explained, it requires either a transformer or IC circuits. For over 100 years the telcos have used differential feeds and drops where only the difference in potential between the ungrounded wires is the audio signal. They sometimes use the “third wire” which is most likely the earth ground for the ringing signal but that is not done much currently.
It was recognized very early in the technology that frequency response is a problem and the answer was to place loading coils along the differential signal route, usually every ten miles or so. The coil inductance counteracted the wire capacitance to provide for adequate frequency response. (One of my physics professors talked about the trans-atlantic cables as being equivalent to putting sand through a garden hose). These effects will not be seen on cable runs of a thousand feet or so so this is not an issue with us. The issue we face is the difference in ground potential.
One system I worked on used four miles of leased direct metallic unloaded differential pairs and using op amps I was able to maintain a bandwidth of 5 kHz with a S/N ratio exceeding 80 dB.
Bottom line for me is if I am sending an audio signal from here to there and there is a ground potential difference then I go differential. The other advantage of using a RF link is lightning protection. I can tell some stories about 4 miles of wire and lightning.
Neil
wireless solutions
Hi Carl & Neil,
FWIW I tried to address these issues in other threads, using the same sort of logic, i.e., that there is no such thing as a perfect ground, and so you not only have some bit of left over energy, but worse, that it may be different between outlets, and the difference potential can cause the ground to become a kind of transmitter, injecting a signal equivalent to the Hz of the AC, or a multiple of it (phasing) into the audio.
This is why electric guitars plugged through an instrument cable into a DI box can produce hum, and so all DI boxes I know of have a ‘ground lift’. It takes out the ground and passes only signal. That can pose a danger of course, because stray voltage is going to seek ground, and that ground connection might become your body! Thousands of instances where electric guitars and vocal mics bit their users.
So here’s my question: How and what type of IC circuit would be used to get back the ground at either end of an audio cable? And how would it prevent lightning damage?
The idea of using an inexpensive FM tx and rcvr occurred to me back when I first thought I might have to put my tx/antenna system elsewhere. If you only need to go 100′ or less, it’s a good idea. But if you need to go .5 mile or so, which could become a reality for me, using a highly directional wi-fi setup with wireless routers, converters, and mini-dishes from a ‘base station’ seems to be the best solution.
I’ve mentioned this idea (nothing new as far as ideas go, I assume) before, but I haven’t seen any comments. I’d be much-gratified to know if anyone on these forums is using this setup successfully.
Syncing with GPS NIST has also been mentioned, but I haven’t been able to find any articles with pictures of how to set it up … I have no clue how it works.
TIA …