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Tech548
Moderator
Username: Tech548
Post Number: 13
Registered: 5-2004
| | Posted on Thursday, May 06, 2004 - 1:52 am: | 
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Tuning for Frequency Accuracy
This text is for those who love playing "in" their radios or find it a challange to fix and/or correctly align a radio that someone has already boogered up. My hat is certainly off to you guys. I also realize that most of you do not have the big-buck equipment it takes to do a great majority of the required alignments. So, what I'm attempting to do here is show you a few ways to do some of these alignments with the minimum amount of equipment and tools at hand. I will try to keep these as simple as possible. This article is aimed at the individuals who are interested in learning a bit of radio alignment tricks and tuning.
The main purpose of this Article is to help you correctly align or realign a radio that "screwdriver jocky" has already knocked out of alignment. I realize that the average CBer does not have all of the high-tech electronic equipment needed to do these AM and SSB alignments
correctly.
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However for those of you that have a good RF Meter, an "Accurate" Frequency Counter, a good Dummy Load, a few alignment tools and most important, a good "ear", will be able to do many of the required alignments yourself.
In order to accomplish this alignment procedure, the most important thing you need to have is a very accurate frequency source. And where do you get that? For those of you that happen to own a Short Wave radio with USB and LSB capabilities or better yet, an HF rig, then you alreadyhave a Frequency source.
These methods are simply taking advantage of the features of the WWV signals that can be found on the Ham bands at 2.5--5.0--10.0--15.0 and 20.0 Mhz. These tone signals can be used to put anf frequency display of a HF SSB radio "Dead-On" frequency.
After we have established an accurate frequency source, I'll show you how this can be applied to your SSB CB Radioto get the Clarifier to be Dead-On frequency at center-slot.
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The following procedure is for calibrating the main oscillator to operate at the very same frequency as what is being seen on the VFOs' frequency counter. This calibration should be checked from time to time for frequency accuracy. Even though your HF radio may be brand new, that's not to say that it is "Dead-On" frequency. Some are and most aren't.
Once we have established a correct frequency source, I'll show you a few ways (using a WWV tone) how it can be applied to a SSB CB radio to get the clarifier to be "Dead-On" frequency at center-slot.
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I'm sure that most of you dedicated CB sidebanders have already discovered that 9 out of 10 brand new out-of-the-box SSB CB radios are NOT on frequency. But first, for those of you that have a HF radio, lets get the oscillator tuned to (right on) frequency.
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Most of today's modern, synthesized HF Ham radios use a single master oscillator to control the frequency of operation. It is very common for this master oscillator to be 20MHz. This frequency will be used as an example throughout this text. However, the ideas presented here apply equally well even if the master oscillator is a different base frequency.
The frequency standard of the WWV time broadcasts that we will use is readily available to all amateurs or anyone with a HF radio. As stated above, these stations broadcast on 2.5, 5.0, 10.0, 15.0, and 20.0MHz. While these broadcast frequencies are very accurate, the only other instrument used will be your ears.
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USB & LSB
The WWV broadcasts are in the Amplitude Modulation (A/M) mode. There is a 500Hz audio tone transmitted in the background during the first 45 seconds then followed by the time announcement. The A/M signal is double-sideband, meaning that the modulated signal will be heard in both upper and lower sideband.
The method involves first tuning in WWV on the highest frequency where it can be heard. This means tuning in at 10.0, 15.0 or 20MHz. Use the highest frequency that you can hear depending upon propagation conditions. The reason that you want to use the highest receivable frequency is because the frequency error will be the greatest and easiest to detect.
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Now for the fun part: Once tuned to the strongest WWV A/M frequency, switch to USB. Listen for the 500Hz tone that is transmitted in the background for the first 45 seconds or so after the time announcement. While listening, switch to A/M again, and back and forth. If your master oscillator is accurate, there should be no difference in tone/pitch between listening to the signal on USB or A/M. However, if you do hear a pitch change, this means that your oscillator is a little bit off frequency.
If this is indeed the case, it is nothing to worry about because you can easily recalibrate the oscillator. Let's assume that you are tuned to 20MHz WWV, and you hear a difference in pitch of the 500Hz tone that is about 20Hz (510Hz on USB and 490Hz on LSB). This amount of frequency difference is fairly noticeable to hear. Assuming your master oscillator is 20MHz, this means that it is off by only 10Hz. This kind of accuracy is very good. However, if you are hearing a pitch change of 200Hz, which is very easy to detect, then the oscillator would be off by 100Hz. You can easily adjust this out.
Your ear can detect very small changes in pitch, particularly at low audio frequencies. The adjustment method is to simply adjust the master oscillator and repetitively switch between USB and A/M until you CAN NOT detect a pitch change between the different modes.
NOTE: A very important thing to remember is, before doing this frequency calibration, make sure your radio has had plenty of time to warm up so that any thermal drift has already stabilized.
The method for adjusting the oscillator, via a trimmer capacitor will vary from radio to radio. Check your operators manual to find the proper procedure for your particular HF radio. Most oscillator calibrating procedures are pretty much the same from radio to radio. On most models, just removing the bottom cover will reveal this tiny trimmer capacitor that is located on the side of the chassis.
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One final thought before we relate all of this to the CB band. Using nothing but your ears, it is easy to adjust the master reference oscillator in your rig to an accuracy of 1ppm or better by simply taking advantage of the WWV broadcasts. Use the highest WWV frequency you can hear to make the inaccuracy more pronounced. By using USB and LSB reception, you are down-converting the master oscillator frequency into the audio band, where you can hear the inaccuracy and simply adjust it out.
OK, so much for calibrating a HF Ham radio to dead-on frequency. With that done, now we have a "True Frequency Reference" whenever one is needed. With this, you can calibrate ANY external frequency counter and/or ANY SSB CB radio to have the correct 1.5Khz frequency shift when switching from A/M to upper or A/M to lower sideband.
Now, for the "how to do it" part. There are a number of very simple ways to do this but in this segment, I will be showing you the easiest way to do it without the use of expensive electronic equipment other than your "ear".
We will be using the Grant-XL and Cobra-148 as examples because they seem to be the most common and they also use the same PC board. Before we get started, I would like to point out that for those of you that have put a 10 to 12Khz "slide" on the clarifier of your 148 or XL, the accuracy of the clarifier will only be from center/slot to 5Khz up and from center/slot to 5 Khz down. Something strange begins to happen beyond those two points.
On either USB or LSB, the transmit and receive frequency begin to split as you go farther into the "slide". In other words, the TX freq and the RX freq no longer track together by the time you get 10 or 12Khz down. This is a very poor way to pick up the "A" channels since they can easily be had at the flip of a switch right off the PLL chip. For those of you that ONLY use your 148 or XL in the A/M mode, this frequency split caused by the 10 to 12 Khz clarifier "slide" will never be noticed.
Method #1 is for those that own both a HF rig and a Cobra-148 or Grant-XL. What we are going to do here is to get the correct 1.5 Khz frequency spread between A/M and USB and also between A/M and LSB.
NOTE! Your SSB CB should also have had at least 45 minutes to an hour of idle warm-up time before doing this alignment.
Also have your radio laying up-side down with the cover screws already removed. To make coil identification easy, you may want to mark them in some way so you will know at a glance which one is AM, LSB, and USB. This can be done with a Magic Marker by making the letter "U" on coil L-59, and the letter "L" on coil L-22, and the letter "A" on coil L-23.
1- This is the easiest method of all. With the HF rig on USB, tune the VFO to center frequency in the CB band. 27.205.0 (CH 20) will work fine for this. Be sure to disconnect the coax so as not to overload the HFs' receiver.
2- Put a dummyload on the CB that will be frequency calibrated.
3- Now put the clarifier at EXACTLY center-slot and switch to USB.
4- Key the radio and say a long "ahhhhh" or use a steady whistle into the microphone. NOTE: (DO NOT use a power mic or any more Mic-Gain than is neccesary). Too much modulation can set up a second harmonic and you may hear more than 2 tones.
5- At this point, you will probably hear two different pitches. If so, with a non-metalic tuning tool, adjust coil L-59 to where you hear the same pitch coming from the speaker of your HF radio as the pitch of your whistle. To put this another way, adjust L-59 to bring the two pitches together until they sound like one single pitch.
6- Tune back and forth on the slug slowly until you have a smooth clear tone with no fluttering sound.
7- Once you have it, switch both the HF rig and the CB to LSB and repeat the process while tuning coil L-22.
When this is completed, you will now have the correct 1.5Khz frequency spread when switching between A/M to Upper and A/M to Lower sideband. This also gives you a leg to stand on when someone tells you that you are "off" frequency.
At this point you may be wondering why I made no mention of tuning the A/M coil. This can only be done with an accuratly calibrated frequency counter. A 6 digit counter will get you close but a 7 digit counter will get you right on the money. Another reason why I didn't mention tuning the A\M coil is because most dedicated sidebanders could care less about the A/M mode anyways. But for those of you that do have a accurately calibrated 7 digit counter, do the following. With your clarifier at center-slot, adjust L-23 for 27.205.00 or as close as you can possibly get it.
NOTE! If the AM frequency calibration is to be included in this overall frequency alignment, do the AM alignment BEFORE you align the Upper and Lower coils.
As mentioned above, for precise A/M tuning, an accurate 7 digit counter is needed when doing precise synthesizer alignment's, but for average everyday use, any accurately calibrated after-market 6 digit counter is all that's really needed when reading a particular frequency.
Jeff
Tech548
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Radi0head
New member
Username: Radi0head
Post Number: 4
Registered: 5-2008
| | Posted on Sunday, May 18, 2008 - 1:32 pm: |
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Great article. The WWV method works very well for aligning HF rigs.
73 |
Mslisaj
New member
Username: Mslisaj
Post Number: 1
Registered: 12-2015
| | Posted on Saturday, December 12, 2015 - 10:57 am: |
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Thank you for this article. I have a known good Icom HF radio and it tested well with your method and then I moved on to the CB and got it pretty close. At the very least I am getting good reports on my own transmissions now.
73's
Lisa |
Dale
Senior Member
Username: Dale
Post Number: 2254
Registered: 12-2002
| | Posted on Sunday, December 13, 2015 - 9:25 am: |
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i dont own a hf rig. i wonder how would a shortwave work in place of the hf rig?
dale/a.k.a.hotrod
cef426
cvc#64
454 [dx numbers]
38lsb
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