Discussion in 'Amplitude Modulation' started by N2DTS, Oct 23, 2017.
More then likely their using a linear amplifier after the transmitter.
I'm pretty sure the elliptical low pass filter in that book is what Inovonics used for the low pass filter design in the 222. It is very effective. Nothing gets by the cutoff.
I gotta find out what Radio MiAmigo is running for processing. Very effective...
Inovonics uses a 9 pole elliptical LPF preceded by an overshoot compensation circuit.
Elliptical filters suffer from overshoot, and without a compensation circuit on its input, there will be distortion added to the signal.
The LPF filter in the Inovonics uses "girators" as tuned circuits in each pole of the filter and each one is tuned to a specific frequency.
My 222 is modified for 5Khz and after tweaking the nulls in each section, using the frequencies in the conversion sheet, putting 0dB in I got 0dB out at 5Khz and at 5.16Khz in, I got -62dB out. (a very sharp slope).
If you wanted better than that you would have to go with a DSP filter.
I fabricate and test elliptical, Butterworth, Bessel and constant delay HP, LP BR and BP filters as my day job.
Just my $0.02 that nobody asked for, audio bandwidth limiting is a must for AM operation, especially if you are running QRO power and want to stay in good graces with your ham neighbors.
No need to.
Just parallel more FETS and up the power supply current.
There is no limit to that?
Right now I am using a graphic EQ with everything above 5 KHz cut 12 db plus the tone controls on the mic preamp setup turned down, and it seems to be ok.
I also have 5k cut 6 db.
I have an Audio Tecnnica 3035 condensor mic that used to sound very good, but over time, the high end went away (a lot).
After it sat for a year it sounded good again, for a few days, then the high's went away again....
Is that a common problem with condensors?
The D104 did the job of limiting the extreme highs very well all by itself...and after 30 years or so still sounds the same...
Nothing tops the sdr filters (on RX or TX) but I suppose some filter that would reduce anything over about (or around) 5000 Hz and up 20 Db or so would be a big help.
Some guys run the hifi stuff into a modified transmitter and pass stuff well past 10,000 Hz, not just class E rigs, most of my transmitters will pass 10 KHz or more audio.
I guess its a combo of high power and no passband limiting that makes some stations so bad.
It would be nice to think about your signal quality, but many guys just fire up the Heathkit or Johnson gear with no monitor of the signal at all.
Oh well, it is what it is, and it seems like there is nothing in particular about the class E rigs that makes them worse then any other transmitter type.
I'm curious, so how many FETS and how much power supply current is needed for a 7500 watt carrier?
I haven't opened up an AT but I'm a longtime Neumann mic repair guy. One thing that can happen is that the diaphragm can develop a perforation with moisture getting into the element. This can cause a sputtering sound and/or loss of top end. Letting it sit for a while can dry it out... The only cure I know for that is to replace the capsule.
edit: Shoot, Brett, one thing to try is to clean the capsule. This is done by taking the mic apart and holding the capsule over a sink at a 45 degree angle so water can flow across the face. Have a friend dribble distilled water while you gently move a soft model paint brush over the face to remove the film of gook that collects on the gold surface. This gook may be taking on water from the air (breath) and weighing down the diaphragm. It's worth a try before replacing the capsule (nothing to lose).
Blot it dry with little pieces of paper towel. Water will just roll off the face but collect in an edge and you just use a small piece of PT to wick it up.
The biggest ham radio transmitter I know of consists of 4 big tubes in parallel. As far as I know, it's still around. There was a Collins 21E on 160 meters a number of years ago, but that one is gone as far as I know.
Getting high power from class E rigs is actually hard to do. You can't just parallel up FETs. It doesn't work that way. You get into complex power combiners, balancing sub-assemblies and some fairly involved circuitry. Not particularly practical for ham radio, but I've seen it in commercial transmitters on short wave and the AM broadcast band. The big challenge is keeping the impedance of the thing in a range to be workable with mere mortal components.
It is *much* easier to parallel up tubes because the impedances are still reasonable, even with a lot of tubes in parallel. The most tubes I've ever put in parallel in an RF amplifier was 6. It was a 500 watt transmitter.