Recently my part 15 AM station has been sounding a bit less than perfect audio wise when monitored on a digitally tuned receiver. Checking things revealed that the transmit frequency was low by 40 Hz. Initially, it was set exactly on frequency by using a trimmer capacitor so why the drift? The transmitter is mounted in a 4 x 4 inch electrical box and when the lid is in place the components heat slowly to about 105 degrees. This rise above the usual ambient of 75 degrees produced a downward drift of 40 Hz which is not acceptable. The drift was so slow that the transmitter would take two days to settle on a frequency. A means to deal with this is the subject of this post.
Starting with a strip of shim brass a crystal heater was developed. The brass was cut and bent so that it would fit tightly around the crystal can. A thermistor and two resistors were then affixed to the brass with epoxy. This is shown in the first two pictures below.
Next, a control circuit was developed to control the crystal temperature. This is shown schematically in the third picture below. More on this later...
The fourth picture is a view of the assembled control circuit and the heater. The circuit was built using the "dead bug" style.
The fifth picture is of the heater and circuit installed in my transmitter. This transmitter was detailed in a previous post here: http://www.part15.us/node/2791 In this view the control circuit is seen at the upper right of the main board and the heater is installed on the crystal.
The heater controls the crystal temperature to within +/- 1 degree F of the setpoint and the frequency of the transmitter is now stable to within +/- 1 Hz. The frequency from a cold start is high by 60 Hz but is stable and on frequency after two minutes of operation.
The reason it is important to keep the frequency of an AM transmitter to within +/- 10 Hz of nominal is that the signals from other stations will heterodyne and cause an audible tone with a frequency equal to the difference between the two signal carrier frequencies. If both (or all) on channel carriers are within 20 Hz the beat will be subaudible though even so some mixing artifacts can usually be heard as a growl.
The operating temperature is set by R1, R2, R3, and T shown in the schematic. T is a thermistor that was salvaged from a PC power supply but any device with a resistance around 1k to 2k ohms at room temperature will work. In this case the T resistance was 1.7k. The target operating temperature was 110 degrees F so a 1K resistor (R1) was used based on the thermistor resistance being about 1k at 110 degrees. Almost any NTC thermistor will work if R1 is chosen to be near the thermistor resistance at the operating temperature. This is not critical but is simply a design guide.
The temperature can be changed by changing any of the resistors (R1 thru R3) and in this design R3 was varied to adjust the setpoint. Lowering the resistance of R3 increases the temperature and conversely. The final value of R3 was determined by experiment to give an operating temperature of 110 degrees. This was chosen to be above the highest inside the box ambient which was measured to be 102 degrees. A potentiometer could be substituted for R2 and R3 to make adjustment easier but experience has taught that pots are not always reliable in DC circuits so fixed resistors are preferred.
The LED gives an indication for heater status, lighting when the heater is on. This is handy but the LED is important to drop the voltage from the output of the op amp which is not zero in the heater off state. The drop across the LED insures Q1 will turn off when it is supposed to.
Nothing in this circuit is critical and leeway for parts values can be used. The values for R5 and R6 were determined experimentally to give a maximum temp. of 120 degrees when constantly powered. This is to prevent a "melt down" in case something fails. The circuit was optimized for a 10 volt supply but higher voltages can be used and R5 and R6 values may have to be changed. The best operation occurs when the heater on time about equals the off time but this is not critical.
It is hoped that enough detail has been presented for those who are interested in building this project. If not post your questions and I will try to help.
Neil
That crystal oven is real upper class and every transmitter should have one, radio8z.
That project is nominated for the Part 15 Museum, which come to think of it, needs to exist so I can refer to it now.
The owners of this website should gather all the great projects and publish a Bible of Part 15. Your crystal oven would be one of the highlights.
Neil, when you make this stuff, like the
crystal oven you just perfected, or that
new AM transmitter - I feel like I have the brain
of a dinosaur.
But I'm really glad for you! Also, it's really
good to have a record of this. Other people
will surely want to try it.
Now I'll just go back into my cave and eat
some of the trees I made for dinner.
Bruce, DINOSAUR RADIO STUDIO 2
Crystal ovens do come in handy with wide swings in temperature.
But..perhaps the device should have had a few vent holes to let some of that heat escape?
Heat is any electronic component's enemy..accelerates component tolerance drift and degradation..even a crystal is not immune to excessive heat.
Nice oven. When will dinner be ready? 😛
A resistor, power source and diode through an op amp makes an excellent regulated oven.
RFB
There are two 1600 kHz stations not too far from
here. They are audible during the day.
They are a few Hz apart. It's funny to tune in the
channel and hear the whoosh whoosh whoosh noise
of the 2 carriers beating with each other. Maybe it's
2 or 3 Hz, or something like that.
Bruce, DOGGRADIO STUDIO 2
The 2 or 3 hz difference will produce the "woosh" noticeably.
Wonder how that managed to get set up like that..two stations in close proximity on the same frequency. I bet it's a nightmare during the daytime for someone in between these stations locations trying to tune in to one or the other!
The crystal oven thing...I would imagine if it's starting out at room temp 60hz high, then that crystal needs replacing with a better more precise crystal with a better temperature tolerance.
I bet within a few days since the problem is heat related, that oven might bring things back into prospective..but what's to keep that crystal from drifting if the cabinet has no venting to let that heat escape in the first place which would prevent the frequency drift by heat problem to begin with?
An odd way of resolving the problem...but interesting anyway.
RFB
Yes. They are too close together. One is a daytimer
in Mass. about 30 or 40 miles north of me.
The other one is in New York City - It actually bought
ANOTHER 1600 on Long Island - so they could shut the
Long Island 1600 off and boost their power. They built
a four tower array. .
I guess I'm sort of in the middle. The Mass. 1600 is
north of me, and the New York city 1600 fires it's
pattern to the south east of me. The NYC 1600
is 25 kW during the day. I have always been confused
about that. I know some regional channel stations
run more power now - but I keep on forgetting this.
So 25 kW on 1600. I still think that's weird, because
I've been in the hobby so long.
Bruce, DRS2
Isn't that ironic. Two licensed stations within interference range of each other and it's allowed.
And the NAB is worried about 100mW causing nighttime skip interference????
SAY WHAT?!!
DOH!
Back to the subject...have you considered putting vent holes in that enclosure? I ask because even with that oven correcting the crystal frequency at power on, that heat building up over a few days due to lack of venting is not being solved with a crystal oven.
I bet that crystal would settle pretty darn close to spot on frequency if your enclosure had some venting and would not need that oven. You should try running it without the cover over the same amount of days it was running with the cover on and track that crystal's drift with the heat being allowed to escape.
RFB
And there is verifiable evidence that it takes 20-Watts to generate a sky wave reflection. Do you know how much more 20-Watts is than 100mWatts? The answer is that it's 19.900-Watts more.
Therefore 100mWatts is very safely well below the skywave mark.
Is that an insult, or what?
It's all politics and lobbying playing the favorites to the big boys and their noisy toys.
RFB
Captain, could you tell Starfleet
that the subspace radio logs got
lost in the ship's galley?
oops
RFB raised a couple of questions.
Why not vent the cabinet? My intent is to have the option of eventually mounting this outside and I wanted to preserve the waterproof cabinet.
Lousy crystal? Probably. This was salvaged from an old digital modem and was probably not spec'd for high accuracy or stability. My other part 15 txs. hold frequency pretty well but they are in a vented cabinet in a fairly constant environment.
The other problem with this particular design is I foolishly located the modulation/power supply transistor next to the crystal. In the picture you can see this at the top left of the tx. board. This was one of those "gotchas" that only became apparent after the thing was built. Arrrrggggh.
Even with venting, a better crystal, and better layout I anticipate that with the expected 90 degree ambient swing outdoors some sort of controlled crystal heating will be needed.
Neil
"The other problem with this particular design is I foolishly located the modulation/power supply transistor next to the crystal. In the picture you can see this at the top left of the tx. board. This was one of those "gotchas" that only became apparent after the thing was built. Arrrrggggh."
I was going to mention about that T0-39 with the wing heat sink sitting right next to that crystal and trimmer cap. A "gotchas" indeed!
Maybe create another cabinet using the same type you have now with the exception of a vent in the style of a weather head like a utility drop, or a vent with a small cover head like that of a dryer vent's covering. Either method might be a way to solve the heating problem to some degree. Heh, made a funny there! 😉
RFB
I like the idea of having a sealed box for the transmitter since here in addition to moisture it seems that insects find their way into everything outdoors.
One thought I had was to split the transmitter so the crystal oscillator is inside at the control head and the rest is outside. This design uses a 74HC gate to drive the final and maybe sending the carrier signal differentially on the feed line would work.
OTOH, what I have works but it is yet to be tested at the expected extremes of temperature.
If the transmission line restriction would go away things would be easier.
I recall postings from others who have their transmitters outside and would ask if any have monitored their frequency as the weather changes.
Neil
This is the year of the radials here, as the outdoor installation has been drawn on paper and will be put in place within months.
Until you just mentioned it, I had not thought of frequency stability over the range of temperature and humidity.
A.) What means is required to properly monitor frequency...... frequency monitor rack mounts are expensive.....
B.) Would it be helpful to have temperature/humidity sensors in the transmitter cabinet?