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Author Topic: Frequency Calibration test your radio.  (Read 6126 times)

Posts: 1757

« on: December 25, 2007, 02:36:13 AM »

I've been experimenting with ways to tell if your radio is on the same frequency that it's readout says it is on. I recently got an IC-746Pro and it was clearly off frequency. This lead to fiddling with the dial to make everyone sound ok and wondering how far off the radio actually was.

Using both VFOA and VFOB, I came up with a simple test. At first, I could only get CHU Canada, but I eventually tested it with WWV on both 5 Mhz and 10mhz. Still waiting to hear WWV on 15 and 20 Mhz. Higher frequencies will make for a better final adjustment, but any of these will make decent adjustments.

So here's the method.

Select VFOA, SSB and USB. Tune 1 khz below your target station. You should hear a tone around 1000 hz.

Select VFOB, SSB and LSB. Tune 1 khz above your target station. You should hear a tone around 1000 hz.

Now, you can toggle back and forth between the two tones by toggling VFOA and VFOB. If the two tones are not the same, your rig is off frequency. (we can safely assume that CHU and WWV are not off frequency)

For a graphic display of the frequency difference, you can fire up any digital mode and look at the waterfall for the 1000 hz lines. When you have adjusted your radio correctly, toggling from VFOA to VFOB will leave a straight line and you will not hear a difference in the tone frequency.

At this point, you have two choices. You can just determine how far off your radio is from the standard, or you can make an adjustment so the radio is calibrated. If you don't want to break into the rig, you'll have to settle for the first choice.

To find out how far off your current calibration is, adjust the two frequencies up or down by equal amounts until the tones are the same. For example, if the tones are not the same, move the USB VFOA to 990 hz below the standard and move the LSB VFOB to 1010 above. If it gets worse, go the other direction. With trial and error, you can figure out where to tune your radio to make the two tones the same frequency. The differences will show you how far your radio is off frequency and in which direction.

Now, to make the calibration adjustment. First, return the settings to 1000 hz low and 1000 hz high. Every radio has a different way of setting the master oscillator so you'll have to find that adjustment on your radio before you can change it. Fortunately for me, Icom had the good sense to make it available from the rear of the radio so I didn't have to take any cover screws out. I did have to disassemble my Yaesu FT-857D when I adjusted that radio way back when. However it is done, adjust the master oscillator until the two tones are the same. Many of the master oscillator controls are touchy, so make the adjustments very tiny until you reach the target. Take your time and be picky.

A little research in your manual or online will reveal where to adjust your radio's master oscillator so you can get dead on readings. If you feel uncomfortable with this final step, leave it to a pro and just be aware of what you learned while performing step 1.

Merry Christmas and good calibration to all.

73 de N4CR, Phil

Never believe an atom. They make up everything.

Posts: 594

« Reply #1 on: December 25, 2007, 06:51:40 AM »

Similar test was to switch between U/LSB and compare "tone". Under those conditions, every radio I have owned was off frequency. From a FT-101ee to a 1000MP. I just chalked it up to gremlins or me owning junk.

Merry Christmas

Posts: 4464

« Reply #2 on: December 25, 2007, 07:52:28 AM »

I have an original box-stock IC-756 that needed a minor clock tweak every six months or so but seems to have settled into its groove after the first two years and is now tweakless. If you can apply this idea to the IC-746 Pro it's probably the easiest way to check the VFO accuracy.

Let the rig idle in RX for 30 minutes or better. Go into the options menu and turn on the internal 100 kHz marker generator. Tune WWV in AM mode on whichever frequency is Q-5 with minimal QSB, VFO display showing XX.000.00 kHz. Adjust the trim cap accessible through the hole on the lower right side cover with a diddle stick or medium sized jeweler's screwdriver for zero beat. You do not have to remove any covers nor do you want to.

Can't hear the very lowest tones through the speaker? Not a problem. Watch the S-Meter for wiggle. Set Fast AGC if you like, then tune the trim cap for a steady S-Meter needle. You'll find it takes a bit of patience to get it exactly dead on +/- zilch and you may want to wait for WWV to cycle over to carrier & clock ticks only, but it can be done. Normal up the radio after you have it as close as possible. Switching between LSB and USB on WWV should now sound identical.

Take your time and you'll be very close to spot-on for the next few months. You also have an easy way to check it when in doubt. Internal marker on, WWV in AM, VFO straight up on the zeros. Slow or no wiggle on the S-Meter is O'Tay!

The end of the world will occur on April 23, 2018 ( the day after Earth Day. Go Figure ).  If you're reading this on April 24th look for updates coming soon.  If you're reading this after June first, fuhgedaboudit.....

Posts: 1697

« Reply #3 on: December 25, 2007, 12:02:42 PM »

Here is a quite precise (<1Hz) method that works with modern radios that derive all oscillators (including CW sidetone oscillator) from a single master oscillator.

Set the radio to CW mode and tune in WWV.

Set the radio so you do not transmit with key down, but can hear the sidetone oscillator at the same time as you hear the beat note from the WWV carrier.

Adjust the master oscillator until the sidetone beats EXACTLY with the WWV carrier tone. You can easily resolve less than 1Hz error.

This whole procedure can be done from the front panel of an IC7000 with no instruments, and it can be set to better than 1Hz at 10MHz, accuracy it cannot hold due to master oscillator stability.


Posts: 6642

« Reply #4 on: December 26, 2007, 11:34:51 AM »

Well, please remember that with the "old" radios the dial indicated "carrier frequency" and with the SSB radios it indicates the "center of the passband".
The early Heathkits had separate marks for reading carrier, USB, and LSB on the dial.
Zero-beating the master oscillator is fine if you have a single oscillator (newer rigs) but the older rigs had multiple oscillators... and you are not sure which one(s) are off, so don't go adjusting if you are not sure!
It is always nice to have several "reference standards" in the house.  After all, you are responsible to be within your license's proper frequency limits!


Posts: 1697

« Reply #5 on: December 26, 2007, 01:36:25 PM »

The practice in all modern ham radios that I have come across is that the frequency shown on the radio is the frequency of the carrier or virtual carrier. By modern radios, I mean radios that derive ALL oscillators from a single master oscillator.

Hams seem to follow a convention of denoting an SSB channel by its virtual carrier frequency. That practice is not universal in the comms world at large.

That means for SSB, the nominal frequency of a channel is NOT the centre of the passband, but just lower (~300Hz) than the passband for USB and just higher for LSB.

That is to mean that if you tune a USB radio to WWV with the radio showing 9.999MHz, you should hear a 1kHz beat note from the carrier, EXACTLY 1kHz.

Older radios were not as clever, and some did not change the mix when changing sidebands so the VFO showed the nominal centre of the passband and lines to the left and right of centre showed the virtual carrier frequency. Other older radios try to show the correct frequency with mode changes, but the system gets out of alignment and they are often in error.

Its pretty basic stuff, but not well understood.

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