There are still an enormous amount of FM stations who have not converted to digital and I was wondering if anyone still tuned their transmitters for minimum AM noise? I remember that Radio Labs had an AM noise monitor on the market quite some time ago but I haven't seen anyone using these lately.
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Anybody Still Tune For Minimum AM Noise?
- Thread starter RF Man
- Start date
I am still responsible for a number of tube transmitters and routinely tune for minimum AM noise to minimize multipath effects. At one location I use the RDL AM Noise Monitor on a Harris 2.5k which is in back-up service. At another, I use the AM noise setting on a Belar FMS-2 which is connected to a Harris HT-5 which has 67 and 92 khz SCAs. Tuning for minimum AM noise is a great way to minimize crosstalk between the main and SCA channels.
With (no tune) solid state rigs like the Harris Z series I simply monitor the AM Noise periodically to make sure everything is legal and working properly.
With (no tune) solid state rigs like the Harris Z series I simply monitor the AM Noise periodically to make sure everything is legal and working properly.
If it's analog only, and you got a Belar monitor, it's easy. Set the monitior to AM NOISE and 'touch up' every stage for a null on the meter. With the Harris boxes, probably the grid tuning is going to be the most critical, but PA tuning and loading will matter as well. If you don't find the AM dip fairly close to the peak power and current dip on the PA, take a look at the stage neutralization and go again. In playing with the late and somewhat unlamented Continental super power FM, we did some checking and found that the Belar monitors, unlike some others, always agreed with the other instruments as to where the minimum AM tuning point was. There wassometimes a difference in the absolute reading, buit the low spot was always correct. (Much of the above picked from the brain of Randy Mullinax some several years ago).
Well, maybe we ought to clarify, what we're actually dong is reducing the synchronous AM. The true AM noise is pretty much power supply noise and noise from the AC filaments. Synchronous AM, OTOH, is produced by the (inherently) limited bandwidth of the entire RF system. Consider that the load presented the transmitter is reactive to some degree at any given frequency. Presumeably, this reactance increases as we move off carrier - assuming the antenna is properly tuned. The reactance will result in some reflected power with frequency change, which change is caused by modulation,. Thus , the signal level will change slightly (acquire an AM component) determind by modulation. If we center the tuning in the system passband, and minimize reactance via the loading for best match, we will have managed to get the system where it performs its best, because we will have it operating in its most linear position. This will give us least distortion of the waveform and produce the fewest intermodulation products. Most of which shows up as garbage in the demodulated waveform, and which can degrade things enough to preclude a subcaqrrier even working.
That being said, empirically, what we want is least synchronous AM. On steady tones, modern stuff can be got down to -65 or so. I've seen -70. Unless there's something drastically wrong with the load - broken elements or the like, you should be able to get a modern system to run better than -45B on Belar monitor under program conditions. Below -45, the performance won't be noticeably degraded by synchronous AM. And, again obviously, this should be done into the antenna at licensed power. The value, and often the tuning point, will vary from the station load to the antenna, the amount of variation being a function of both the antenna system bandwidth and how closely the load and antenna are matched in reactance.
Quick and dirty: Tune it up, put it on the air, get it operating at power, and then touch up the tuning and loading for a dip in synchronous AM as indicated either on your Belar monit6or or your AM noise meter if you got one.
That being said, empirically, what we want is least synchronous AM. On steady tones, modern stuff can be got down to -65 or so. I've seen -70. Unless there's something drastically wrong with the load - broken elements or the like, you should be able to get a modern system to run better than -45B on Belar monitor under program conditions. Below -45, the performance won't be noticeably degraded by synchronous AM. And, again obviously, this should be done into the antenna at licensed power. The value, and often the tuning point, will vary from the station load to the antenna, the amount of variation being a function of both the antenna system bandwidth and how closely the load and antenna are matched in reactance.
Quick and dirty: Tune it up, put it on the air, get it operating at power, and then touch up the tuning and loading for a dip in synchronous AM as indicated either on your Belar monit6or or your AM noise meter if you got one.
littlejohn said:
Would that be the 817A?
That was one busy box.
Noisy too.I recall using a scope to tune for min AM. My memory on this is suspect, but ISTR the trick was to generate a Lissajous pattern, using detected AM from the transmitter's sample jack on the X input, and a sample of the composite on the Y input (or vice versa). Tuning for the flattest line on the scope resulted in best AM noise.
Thje scope will work just fine. When we found out the Belar mionitors showed monimum where the scope showed minimum, we stuck with the monitors. It's easier.
The 817A was a good idea with bad politics behind it. Had there been a tube more suited to VHF service, it would of been an excellent box. Continental swapped them back for paired 816s, and so I'm told sold them less the power supplied and reuseable parts for scrap metal.
The 817A was a good idea with bad politics behind it. Had there been a tube more suited to VHF service, it would of been an excellent box. Continental swapped them back for paired 816s, and so I'm told sold them less the power supplied and reuseable parts for scrap metal.
Any thoughts on AM noise and HD performance? Does the presence of HD carriers mess up the readings of a first generation RDL AM noise monitor? In particular, performance of a Harris HT/HD+ transmitter: My thought is that the RTAC would really be challenged if every stage is not finely optimized, which should result in low noise.
I turn the HD off when I'm measuring the analog on our low level combined plants. The mod monitors want to read the digital carriers as overmod. I haven't tried reading AM with them running, but since you bring it up, I'm going to, just as a curiosity.
wattsup said:
I haven't had the pleasure of working with the HD transmitters but I would think that it would - you are adding signal to the mix.
I worked with HD TV a few years back (Harris) and I know that the setup including RTAC was picky about all the parameters involved. It had to be set up fairly tight to spec to operate properly.
HD radio on FM has its own set of gotchas. The digital carriers have to sit beneath the mask that thye FCC specifies for emission. The problem is that the generate some intermod products which can be fun to reduce below the RF mask. Fairly linear RF amplifiewrs are needed, and active correction via prediustortion helps a lot. Where you're doing high level injection using a 10dB hybrid, matching the waster load helps also. We're all learning the amimal as we go.
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