John at WDCX
How does the inspector get an amp reading at the input to the final without ripping into the circuit?
That's how a FCC Inspector does it at a licensed radio station. In the case of the Rangemaster, you might be required to demonstrate compliance with a meter. Better yet, why not do what a Part 15 operator does in Pennsylvania, take routine measurments in accordance with the user manual for the Rangemaster showing compliance, keep a log, and post on the website if one has one. FCC guys always like to see logs, even though they are not required any more. It shows that you are conscientious.
Hope this helps.
Hello Everyone Here at Part 15 College.
Talking about our ongoing shortwave experiment, here is today's question from the naive.
How is a station's audio bandwidth set? If set by the audio itself, there would need to be a 5kHz wide LP filter in the audio chain.
My little test unit has extremely clean audio, like a medium wave with 10 kHz bandwidth. But that's subjective, judging by ear.
I know what. I'll run sine-tones through there and see what we get. Wait here, I'll be right back.
Hey there:
I was just looking at the old inactive
Medium Wave Alliance web site.
I says that you can put a transistor
in the B plus line and feed modulation
to that. It would go where the
transformer is.
This seems much too easy. I think
I'm going to read up on more approaches
to that, such as how to bias the transistor
and that sort of thing. The schematic
showed just a transistor and nothing else.
If any of you guys have any comments on
that feel free to send them my way.
Bruce, MICRO1690/1700
Yes MICRO1700 I have information about using a transistor to inject modulation and I will gather it together and send it in a while.
MICRO1700 here is another schematic for the collection, this one shows a transistor modulation method.
http://users.belgacom.net/hamradio/homebrew.htm
ON6MU sure has a lot of neat circuits.
I found a few other things myself.
What would we do without the internet?
I do have to say, though, one of my
prized possessions is a 1969 ARRL handbook.
That when it all started for me.
I can also see that biasing is everything.
Good luck with your audio bandwidth analysis.
If you want to keep the sidebands within the
14 KHz window, that is, 7 KHz on each side
of the carrier, you could use an audio equalizer
(one with software or an actual hardware one)
and set the controls to trim off the audio spectrum
you don't want. But you might have an easier
approach but using the right discrete components.
Bruce, MICRO1690/1700
Carl queried "How is a station's audio bandwidth set?"
If a sine wave audio tone (Fa) is applied to a perfectly adjusted AM transmitter operating at a carrier frequency (Fc) three frequencies will appear at the output; (Fc-Fa), Fc, and (Fc+Fa). The bandwidth will extend from (Fc-Fa) to (Fc+Fa) which is BW = (Fc+Fa) - (Fc -Fa) = 2 Fa which is simply twice the audio sine wave frequency.
Program audio is an ensemble of many frequencies so the bandwidth is twice the highest audio frequency in the program. Thus, to limit the RF bandwidth to 10 kHz, it can be done by limiting the audio bandwidth to 5 KHz as you suggested.
Note that on part 15 AM there is no restriction on the bandwidth providing you do not cause interference and provided that the signal stays within the specified operating band. Let's say, for example, the legal upper band edge is 1700 kHz. and the highest audio frequency is 5 kHz. This means the carrier must be set at 1695 kHz so the (Fc+Fa) is 1700 kHz. (This is just an example with made up numbers).
It is useful to keep in mind the audio bandwidth of the receiver since the power used in the transmitter to produce audio frequencies higher than the receiver can process just steals power from the transmitted signal for no useful purpose.
Neil
Carl and Bruce,
Achieving AM by varying the B+ applied to the collector of the output RF amplifier is essentially what is done by use of the transformer. The drawback, though it is not a big one at these power levels, to using an audio amplifier output or a transistor in the collector circuit is that this amplifier must supply both the audio signal and the DC bias needed to power the RF output transistor. A common mistake is to also apply this signal to the base bias circuit which changes the base bias with the audio which can cause problems.
The Ramsey AM-25 modulates by varying the voltage applied to the RF output drain (it is a FET and the drain corresponds to the collector on a bipolar transistor). It has been noted by me and others that this particular circuit does not produce undistorted audio above about 70% modulation.
There is a circuit I used in my class lab which uses a transistor in the RF amp. collector for modulation and it produces the same poor modulation characteristics.
I engaged in a very long and detailed discussion of this either here or on another board and if I can find it I will link to it. EDIT...I found it and it was a thread where Carl and I went over the AM-25 in detail. It is here: http://part15.us/node/1896
This is not meant to discourage either of you from pursuing the B+ approach but rather it is to point out some things which you should be looking for as you test your designs.
Neil
Thank you for the info, Neil. I intend to
do a lot of reading, and this new source
you presented will be fun to look at.
I have really enjoyed working with this
little transmitter. Carl's is up and
running, and since he is using different
transistors than I am, I still have a lot
of debugging to do. I also have to replace
Q2, which I have blown twice.
I want to try some different circuits
for modulation and also for the power
amp and it's output filter. Then I can
see how they operate under different
conditions.
Thanks again, Neil. I'll be back.
Bruce, MICRO1690/1700
One more step has been taken in our daily tests with the AM Pixie2 Circuit on the air.
Today I unraveled the VHF TV radio from the bathroom shelf where it was tethered so it wouldn't fall off, and turned it on right next to the Pixie2 so as to check for the 3rd harmonic at 54.240 mHz. Whereas before adding filtering to the output this harmonic was receivable in the bathroom, now the filtering has prevented being able to hear it in there.
With the VHF TV radio antenna right alongside the Pixie2 there was a slight trace of signal at 54.240, but swinging the antenna merely 2-feet away it was lost in background.
Previously we checked the 4th harmonic at 3-meters from the transmitter and it was not perceptible.
The only remaining harmonic is the 2nd, located at 27.120 kHz between CB channels 13 & 14, and we intend keeping that since it is an allowed Part 15 frequency, only needing now to verify that it meets the field strength limits.
The transmitter is on the air everyday for a few hours of testing, but will not go on full time until the field strengths have been proven.
Hi Carl:
Thanks for the report on your transmitter.
I'm glad it's working so well.
I still have a lot to do here. I have to
replace the 2nd blown 2N4401, read up
on biasing, and - I'm going to try to
build a modulator circuit with 2 2N4401s
that I saw on the internet. If it doesn't
work, it's no great loss - it's only 6 parts,
I just can't resist trying it.
I bought all the 2N4401s the local Radio
Shack had.
This is going to take a little while because
I am doing a lot of reading on this stuff.
If it seems like I disappeared, I haven't
really. I'm just out here trying things to
make this transmitter work.
Best Regards
Bruce, MICRO1690/1700
MICRO1700
I am glad you are pursuing the experiment with transistor modulation because I am very curious about that method. The SSTran AM transmitter uses a form of transistor modulation and the audio is very good.
Don't rush your work. Take your time and enjoy it. The suspense of waiting for news is another part of the fun here at Part15.us
Happy winter solstice to all. The flying world is making its u-turn back toward summer. It's astronomical!
Meanwhile, on earth, I had a friend with a CB rig tune to channel 13 (27.115 mHz) and there was the audio from my second harmonic, which is actually at 27.120 mHz. We listened to it for a block and a half. Since I've been monitoring these frequencies on my Sangean I haven't heard any CB activity on 13 or 14, but I'd rather not be receivable on people's CBs, someone might report it as pirate activity. That means I need to squash this 2nd harmonic.
A good place for a part 15 transmitter in the 27 mHz region would be some vacancies where the CB allocations skip a notch. Whereas most of the channels have 10 kHz between them, there are several with 20 mHz. For example, 27.145 is a skipped spot between channels 15 & 16.
Hi Carl:
Thank you for your comments. This modulator
I'm about to build sits in the B plus line just
like a transformer. It doesn't seem like the
circuit for the SS-Tran's modulation system,
but because it uses so few parts, and the
part's are available, I figured I would try it.
As Neil said, the circuit has to deliver the
audio and the DC bias to modulate the final
transistor and I hope it will do that. If it
doesn't work, I'll just try something else.
Your 27 MHz emission is interesting. You are
right about the CB channels that have 20 kHz
between them. I think 27.145 is used for
radio control for a lot of the toy cars and such
that are out there. I know there are a few other
frequencies in the 27 MHz range that can be used for
radio control. I have not listened to the 27 MHz
range in a long time but I think it is pretty dead.
I'm sure there are still a lot of truckers and a
few people who run huge linear amps on that
band, but aside from the truckers, I can't see too
many people using CB for routine communications.
I have a special spot in my heart for CB. When I
was twelve years old, and recovering from two
detached retina operations, which were caused
by complications at birth - I used to stand out
in my front yard with a 100 miliwatt
CB walkie talkie for hours on end listening to
CB channels 11 and 14. That was in 1967.
Every once in a while I would contact somebody
on top of a big hill or somebody that was running
mobile. I was not allowed to do much of anything
for fear of going blind again, so having that Radio
Shack TRC-22 really saved me.
Now, more than forty years later, it's Part 15 radio,
AM DXing and 80 and 40 QRP CW.
Best Wishes
Bruce, MICRO1690/1700