Hello all,
Jim B posted in another thread here:
"Neil, You mention owning a SSTRAN and Ramsey, I assume AM25. I have both yet haven't much success with the AM25. Poor audio mainly, and it doesn't reach a sharp peak when tuning to antenna, therefore not the greatest range. Understand that the AM25 will only do about 80% modulation. Would sure like any help that you'd be willing to share about the unit. JimB".
Hello all,
Jim B posted in another thread here:
"Neil, You mention owning a SSTRAN and Ramsey, I assume AM25. I have both yet haven't much success with the AM25. Poor audio mainly, and it doesn't reach a sharp peak when tuning to antenna, therefore not the greatest range. Understand that the AM25 will only do about 80% modulation. Would sure like any help that you'd be willing to share about the unit. JimB".
I thought it might help to start a new thread on this subject.
I have used the Ramsey AM-25 for a few years now and it is a good unit. It lacks the audio advantage of the SSTRAN unit's processing but is suitable for what I need.
Jim, as you know the AM-25 does not provide for antenna tuning unless you are referring to a tapped loading coil. When I constructed the kit, I used the filter coil and cap. values that Ramsey instructs for a frequency around 1500 kHz. The antenna that I use is mounted horizontally on the ceiling of my basement and works against a water pipe ground (here we go again with ground length). I have a 79 turn #12 coil wound on a 1 1/2" PVC form, with one end going to the ground and the other to the antenna. In parallel with this coil is a 30 to 400 pf variable cap for tuning. The tx. is coupled to the coil with another 7 turn coil wound around the cold end. This was intended to be a resonant transformer rather than a base loading coil ( I was trying to produce a high voltage at the feed to the wire antenna). I tune the cap for max field strength measured with a scope and a test antenna. I get a good peak and decent range considering the antenna is in my basement. I can hear my signal about .1 mile away in my car. I have measured the complex impedance of this system with a noise bridge looking into the primary coil and am working on an analysis of the AM-25 output stage to see if I am close to a reasonable match. The peak I am seeing is probably due to the resonance of the coil rather than a good match.
I have observed the audio problem that you mentioned. If I adjust the AM-25 according to the instructions, the audio on the scope is not fully modulated and is distorted as shown on a trapezoid pattern. If I adjust the unit for 100% mod. with low distortion with R23 and my audio source volume, it goes over the 100 mW. limit. I measured Pin = 237 mW. under these conditions. Very nice looking signal but not legal.
To be fair, I am not using the antenna Ramsey recommends.
I settled on a setting for R23 that gives about 80% mod. and is less than 100 mW. and since I am using a cheap portable receiver and not really broadcasting to my neighbors, I can accept the distortion.
That's where I am with it now. I also use the SSTRAN around my house with the same antenna system, but the range is nowhere near what I get with the Ramsey and the audio does not sound good. The audio problem is probably due the the high Q of my antenna which is lopping off the sidebands. The SSTRAN sounds great when I install R18 with this antenna which lowers the Q but really cuts down the field strength.
I did a simulation on the SSTRAN output stage and found the output resistive component of the unit is around 1.2 kOhms when configured for the base loaded antenna they recommend. My limited range is probably due to a really bad match between my antenna and the tx. Again, to be fair, I am not using either of SSTRAN's recommended antennas.
This is a lot to think about but I hope it helps.
Neil
Thanks Neil. JimB
by MRAM 1500 kHz
Neil,
After you had me re-read 15.215, 15.209 vrs. 15.219 was cleared up for me. By the way, the FCC has not responded yet on that.
Where in the Part 15 rules does it request that power input be measured with modulation applied? You indicated that with modulation you exceeded the allowed power limit. I always picked up on the power ratings being geared to an unmodulated transmitter. I realize the percent of modulation is limited to 100% on negative peaks and 125% on positive peaks but I've not read where the power limits as stated in 15.219 included modulation.
By the way, have you ever heard of Chris Hood Electronics? I'm using a compressor/limiter that can independantly control positive and negative peaks of the modulation. Works fairly well but I have no information on it. It was in a pile of junk pitched by a local radio station. Looks like it was a prototype made in the early '70s. Any info on that one would be great.
Bob
Hi Bob,
The 100 mW. limit applies to unmodulated power input to the final stage of a transmitter. There is no reference in the rules to modulation percentage. The 237 mW. reading I cited was unmodulated. The transmitter was adjusted for best audio at 100% modulation and then I measured the power input with no modulation which is the standard practice for AM.
I have a problem with the 125% modulation that many talk about. It appears to me that the only way this will happen without clipping the carrier off at zero is to distort the audio on the positive peaks or to supress the carrier which requires sophisticated ciruitry. There is a technique called exalted carrier (it should be called exalted sideband) which hams used to use. On AM, half the total power of the tx. is in the sidebands at 100% modulation. Be careful with this....a .1 watt carrier power tx. at no modulation has .1 watt in the carrier. At 100% modulation this tx. has .1 watt in the carrier and .05 watts in the two sidebands resulting in a total power with modulation of .15 watts. With exalted carrier, the sideband power is boosted above the 50% level for AM so the power in the sidebands exceeds the usual 50% at 100% AM modulation. This appears to me to be equivalent to the 125% modulaton that some speak of. I cannot see how this is done with simple circuits without distortion of the audio. Perhaps this technique is approaching double sideband supressed carrier mode which would not distort the audio. Maybe you or others can help me with this.
No, I have not heard of Chris Hood Electronics. Thanks for the tip. I will pursue this.
Neil
On AM, half the total power of the tx. is in the sidebands at 100% modulation. Be careful with this.
Clarification: sideband power at 100% sinewave AM is 1/2 of the power of the unmodulated carrier, but only 1/3 of the total power output of the tx then. This is recognized in the rest of your post where you say "....a .1 watt carrier power tx. at no modulation has .1 watt in the carrier. At 100% modulation this tx. has .1 watt in the carrier and .05 watts in the two sidebands resulting in a total power with modulation of .15 watts." The ratio of 0.05 W power in the sidebands to 0.15 W total output power is 1:3, or 1/3.
With exalted carrier, the sideband power is boosted above the 50% level for AM so the power in the sidebands exceeds the usual 50% at 100% AM modulation. This appears to me to be equivalent to the 125% modulaton that some speak of.
Most program material has natural asymmetry between the negative- and positive-going alternations of the audio waveform. The audio processors that most AM broadcast stations use utilize this reality, or create it if it didn't exist naturally, and then control peak levels so that positive-going peaks can modulate the tx to +125%, and negative-going peaks are held at -100%. Some audio distortion may be created by doing so, but stations tolerate it for the increase that this provides in signal-to-noise ratio at the receiver. This is the approach used by AM broadcast stations, rather than exalted carrier.
by MRAM 1500 kHz
Neil,
In your post it sounded as though you were measuring input power with modulation,
"If I adjust the unit for 100% mod. with low distortion with R23 and my audio source volume, it goes over the 100 mW. limit. I measured Pin = 237 mW. under these conditions. Very nice looking signal but not legal."
Yes, the Chris Hood Electronics compressor/limiter does produce an asymetrical audio waveform. As such the the audiophile would reject this as distortion but while not overly obvious to the casual listener, it does increase the overall signal/noise ratio. It is tedious to adjust (with no manual) but I can get over 100% positive peaks while not exceeding 100% on the negative peaks. In an article I did see reference to this type of processing. The circuit and explanation of operation were fairly straight forward. I'll see if I still have that article.
I remember visiting a local radio station in 1972 on a field trip of the electronics school I was attending. I distinctly remeber the engineer showing us equipment that was described as doing just what the Chris Hood unit does. He stated that the FCC allows 100% negative peaks and 125% positive peaks. He went on to say that the channel bandwidth for AM stations is 10 kHz and as such they are supposed to limit their audio program to 5 kHz maximum frequency. Those were some of the questions on the Third Class FCC license test that we all went to Cleveland, Ohio to take.
I take it the FCC relaxed the 10 kHz bandwidth rule as that's a hot topic in Radio World News. There's lots of discussion about "Big Radio" lobbying for reduced audio bandwidth (less than the 10 or 15 kHz they run now) to make room for Digital radio. Some were so bold as to suggest reducing bandwidth to 5 kHz (that which we were taught was the limit.)
Bob
By coincidence, I posted something about this on another site yesterday:
The 10 kHz channel spacing in the US dictates that an RF bandwidth of 10 kHz (~5 kHz of audio bandwidth) is the maximum that can be transmitted using conventional AM without creating interference to the sidebands of adjacent channels. Here is the reason why.
Receivers can limit their RF bandwidth to 10 kHz, yielding an audio response to 5 kHz. But that will not remove modulation from adjacent channels that exceeded 10 kHz of RF bandwidth on the adjacent channel. Those modulation components will lie in the RF passband of the receiver, and cause interference to the desired signal. The higher the audio frequency response on the adjacent channel, the more it will interfere with a receiver tuned one channel away.
For most locations, the only time that 10 or 15 kHz audio response would be usable for AM broadcast would be in the daytime (not that many of today's receivers could deal with it). At night, all stations with that audio response would have to cut it back to ~5 kHz if they wanted to avoid creating skywave interference to stations on adjacent channels. This is just a reality of physics.
Probably most people would choose listening to clear, intelligible program audio with less audio bandwidth than listening to "wideband" audio with a lot of interference in it.
Rich,
I really blew it!! 1/2 the carrier power in the sidebands makes them 1/3 the total as you stated. This was a point my students would miss and that's why I posted "be careful with this". Then I messed it up....maybe I learned too much from my students.
Rich and Bob,
If I understand your answers correctly, the 125% mod. is due to audio processing. That makes sense to me and confirms my suspicion that audio distortion is produced. I just couldn't see using supressed carrier techniques on part15 tx. because of the complexity.
Thanks for helping me with this.
Bob, when I measured the 237 mW. it was after I had adjusted the tx. for 100% and then I turned the audio off. It was measured with a dead carrier.
Neil
In the early 1950's AM broadcast audio processors were simple compressor-limiters. Eventually engineers played around with clipping the negative-going side of the audio, understanding that phase amplitude shifts in the human voice, after a compressor-limiter, produces positive sine wave excursions less than 100% or levels equal to negative side. A couple of very smart broadcast engineers developed audio processing that sensed the most advantageous phase shift and flipped the audio polarity automatically. Eventually, the circuits shifted the audio without any audible processing artifacts.
Today, processors like the Innovonics 222 do all the sensing and shifting and clipping digitally, maintaining very natural sounding audio with 95% negative peaks and 125% positive peaks. Price is about $700.00, much less than the $5-10,000 original units in the 60's.
The FCC does not regulate percentage of modulation any more, except to establish maximum limits. The Part-15 reg's determine power by the voltage/amperage input to the final stage(PA) of the transmitter. All done minus any modulation. Otherwise power would be determined by peak-to-peak or peak envelopepower which is beyond the capability of most low power broadcasters.
Someday, take a look at an asymetrically processed audio sine wave on an O-scope. That is the picture that has been making very smart audio engineers scrotch there heads for decades.
Around the middle '80's, the FCC adopted the NRSC audio-mask standard for AM broadcast narrowing the audio bandwidth to 10Khz plus or minus the carrier frequency. This standard also prescribes audio pre-emphasis (75 u/sec) just like FM broadcast to make up for the poor receivers used to pick up broadcasts. Commercial and non-commercial licensed full-power AM radio stations are required to comply with this standard now. Part-15 stations are not required to use the NRSC standard. However, on most receivers the non-NRSC audio will sound tubby and life-less.
Theoretically, all of this introduces considerable sine-wave distortion. However, without the processing many AM signals simple are not "competitive" in the listener's ear. So, for all intents and purposes, listeners have demonstrated over and over they would rather have less noise and acceptable levels of distortion instead of pristine audio. Even FM broadcast stations carry up to 5%-10% processing distortion on their final audio product these days.
Being a licensed broadcast engineer for the past 38+ years has allowed me to work with all kinds of audio processing and transmitters. Not everything I have had to do to please the program director, general manager and listeners had anything to do with good audio practices. Its just my opinion, but getting a tenth of a watt to the listener will require some exceptional measures in light of the poor quality of most modern day AM receivers and the susceptibility of AM to atmospheric noise.
And finally, I too have an AM-25 transmitter. I have had to modify it with bigger heat sinks and take extra care with supplying low-ripple DC power, a good ground system and adequate audio processing. Matching the antenna to this transmitter is well beyond most Part-15 users though. Without a network analyzer for tuning the antenna, providing this transmitter with a proper 50 ohm/resistive load is close to impossible. Much of the challenge is because of the shortness of the radiator itself in relationship to wavelength and the tenth of a watt power level. My AM-25 works satifactory. However, because of a recent FCC action against Ramsey, I am moving to the more expensive FCC certified AM-1000 transmitter.
KK7cw,
Thanks for your post. It is interesting reading.
73
Neil
The new thread starts here:
www.part15.us/node/637