Interfacing your own Analog Meters to MB-1

Below we show a complete example of calibrating an analog meter that has a both a Power scale and SWR scale with the following ranges:

Power: 0 - 130 watts
SWR 1 - 3

Both scales are nonlinear:

The meter was obtained on eBay for $5.  Even though the meter is labeled "Peak Power", once calibrated, it can be used to display any power value (Forward Power, Reflected Power, Delivered Power, Min/Max Power values) as well as SWR measurements.

Since the Power scale and SWR scale of this meter are nonlinear, this meter must be calibrated using the "Non-linear" scale calibration mode in the Panel Meter setup routine.  During calibration, you will be prompted for information about the meter such as the number of power and SWR scales you want to define and the step size of the calibration points for each of the two scales.

The Power and SWR scales are each calibrated independently. Each scale has its own full scale value and step size, and the data for each scale is stored in separate tables in EEProm.

Choose a step size for each scale to provide good resolution and to make the calibration procedure convenient. For example, the FS Power value (top scale) is 130. If we use 26 points for the 130 watt power scale, each calibration point would correspond to a readily identifiable tick mark on the scale and will simplify the adjustment of the needle during calibration (e.g., 5, 10, 15, etc).

Even though the SWR FS value is 3.0, there is at least one more unit worth of real estate on the meter face to the right of the 3.0 tic mark. Therefore, we will add a little more range to this SWR scale by choosing a FS SWR value of 4.0 corresponding to the rightmost tick mark. (We will have to estimate the calibration points between 3 and 4, or temporarily place a piece of masking tape on the meter with spaced tic marks between 3 and 4 to use during the calibration.)  Since this scale starts at 1.0, if we pick 30 calibration points for the SWR scale, this too will give us convenient tick marks (e.g., 1, 1.1,  1.2, ... 4.0).

Give some thought to the number of points for each scale since selecting this number carefully will simplify the calibration process by making use of calibration points that are easy to identify on the scale during calibration.

After defining the Full Scale Values and number of points for each scale, the software prompts you at each calibration point to dial the meter needle to the corresponding point on the meter scale (using either the front panel pot, or the UP/DOWN menu buttons (not to be confused with the Up-Down Scroll buttons), and to save the calibration data for that point using the "SAVE" menu button. This is repeated for all power and SWR calibration points.

The Nonlinear Scale procedure is the most involved, and that is the one we show below. If you follow this example, Linear Scale and Crossneedle Scale calibration should be straightforward. After you become familiar with the procedure, it takes only about five minutes to perform a complete panel meter calibration on scales with nonlinear meters, and even less time for linear scales.

Testing and Editing the Calibration: Before exiting the calibration sequence, the setup routine offers you the option to test the calibration. If you select this option, you can test the calibration by successively pressing one of the menu buttons to advance the needle through each of the calibration points to confirm that the needle lines up with its nominal calibration value.

The Panel Meter Calibration Software also has an EDIT function, so if you see one or more points that need to be tweaked while running the TEST option described above, you can enter the EDIT mode and modify just those points. You do not need to repeat the entire calibration.


Complete Analog Meter Calibration Example


As with all Set-Up functions, first display the applicable menu on line 4 of the LCD (the Panel Meter Menu in this case). Then press the Set-Up button (Top Right Button - Long press).


Select the Panel Meter number:

The software will enter the Panel Meter calibration setup routine bringing up the screen to the right. Press the UP or DOWN buttons (M1 or M2) to select the Panel Meter you want to calibrate. Then press SELECT (M3).

Make sure you use the UP/DOWN "buttons" represented by the first two menus choices on the top line of the LCD using the M1 and M2 menu buttons, and not the buttons with the Up-Scroll and Down-Scroll silkscreen labeling.

Crossneedle Panel meters require two panel meter ports (and therefore occupy two panel meter numbers). Crossneedle meters can only be set up as Panel Meters 1, 3 or 5.

Panel meter ports 1 and 2 are connected to the internal crossneedle meter provided with MB-1. Therefore, use any of the other four panel meter ports (3 - 6). Port 6 is chosen for this example.

Select Operation:

If the Panel Meter port that you selected is already calibrated, that fact will be announced on this screen. If the port is already calibrated and you want to modify some of the settings, press EDIT (Menu Button 1 - M1).

If you want to perform a new calibration as we do here, and if the panel meter port was currently in use, you would press the ERASE button (M2) to erase the old settings.

In this example, the selected panel meter was not previously calibrated. Therefore, press the EDIT button (M1) to start a new calibration.

Select Scale Type:

Select the Scale Type. In this case we select the Nonlinear Scale, which corresponds to menu choice M2, since NONLIN is the second item in on the LCD menu on the top line. 

For this example, we are calibrating Panel Meter port 6, and therefore the Crossneedle selection is not applicable (and therefore not made available for selection).

(Remember that Crossneedle meters can only occupy odd number ports 1, 3 or 5.)



Adjust Trim Pot:

When this screen is displayed, the full scale output of the D-to-A chip is applied to the panel meter RCA jack (or two RCA jacks in the case of a crossneedle meter). This screen prompts you to adjust the panel meter's trim adjustment pot so that the needle deflects beyond its full scale reading, but not excessively. Adjust the pot until the needle contacts the needle stop (and then just a bit more). This will give you the extra drive needed if you want to use the Soft Overrange feature below.

After adjusting the trim pot(s), press any of the four Menu buttons M1 - M4. This will remove the full scale drive from the panel meter returning the needle to its resting position.

Select Number of Power Scales:

All Panel Meter calibrations require at least one power range. For the Nonlinear case, you may select from one to three ranges.

For nonlinear meter calibrations, if you select more than one range, the full scale value of the second (and third) range will be x10 (and x100) the value of the first range. This rule is built into the software.

Select FS Value of First (Lowest) Power Scale:

The meter we are using has a full scale power value of 130 watts. We will therefore select the FS value of the first range to be 13 watts. This will give us three power scales: 13w, 130w (13 x 10), and 1300w (13 x 100). With the meter being used in this example, this will require the user to divide by 10 when interpreting measurements on the lowest range, and multiplying by 10 when interpreting measurements on the highest range.

(We could have set the full scale range of the first range to 130, which would have given us scales of 130, 1300, and 13,000. However, the choice above is a better selection for Amateur Radio use.)

Select Number of SWR Scales:

This screen is used to specify the number of SWR scales. Enter 0 if you want to use the meter for power measurements only (no SWR scale). Select 1 if you want to define an SWR scale as well.

Select Starting Val (0 or 1) of SWR Scale:

If you will be setting up an SWR scale as determined in the above step, the screen to the right is displayed. Here, you define the starting value of the SWR scale as 0 or 1.

All "real" SWR scales start at 1, but if you have a meter scale that starts at 0 (a common case on many non-Amateur Radio meters), and if you want to use that meter to display SWR measurements, select the value 0 in this step. The software will then idle the needle at a reading of 1 - a bit unconventional, but without this capability, you would not be able to use 0-based scales at all for displaying SWR measurements.

In this example, we are using an actual SWR scale. Since 1 is the second choice on the menu line, we select it with menu button 2 (M2).

Select FS Value of SWR Scale:

The meter we are using has a full scale SWR value of 3.0. As discussed above, since the scale has some additional room, we set the FS value of the SWR scale to 4.0 and eyeball the calibration points between 3 and 4 during calibration. Alternatively, temporarily place a piece of masking tape on the meter case with spaced tick marks between 3 and 4.

Select Soft Overrange Value:

The screen to the right allows you to specify a soft overrange value - a percentage beyond full scale that defines the maximum travel of the needle when an overrange condition occurs. This allows you to observe an overrange condition on the analog meter without risking damage to the meter movement.

Select Number of Cal Points for Power Scale:

Use this screen to specify the number of calibration points for the power scale. With our 13 watt (lowest scale) full scale value, a choice of 26 points will give us good resolution (e.g., .5w, 1.0w. 1.5w, ...) and convenient tick marks.

Select Number of Cal Points for SWR Scale:

Use this screen to specify the number of calibration points for the SWR scale. If we choose 30 points, this will also give us good resolution (e.g., 1.1, 1.2, 1.3, ... 4.0), again at convenient tick marks.

Select pot or buttons to Adjust needles:

This screen allows you to either use the front panel pot to adjust the meter needle during calibration, or the "UP" "DOWN" buttons corresponding to Menu Buttons 1 and 2 respectively. The pot is easier in most cases although the buttons give you slightly finer control. We use the Pot for the remainder of this example

Announce Start of Power calibration:

After the "POT" choice is made in this example, this screen simply announces that the software is ready to start the Power Calibration sequence. Hit any of the four menu buttons, M1 - M4 to proceed.


Actual Power Scale Calibration:

The actual power calibration now begins. The screen prompts you at each of the 26 power calibration points. The screen to the right shows the third power calibration point (.5, 1.0, 1.5). For each point, advance the needle, using the front panel pot,  to the corresponding location on the meter scale. Then press the SAV button (M3). (Remember that we have defined our low scale at 13, not 130.) 

Even if you selected the pot to adjust the needle during calibration, you can press the TRIM button (Menu button M1) after doing an initial adjustment with the pot (but before pressing the SAVE button). This provides a "nudge" function, giving you very fine control using the UP/DOWN buttons for positioning the needle. This will not be necessary with most meters.

Convention: All menu choices that perform a single function (like TRIM, skip (SKP),  and exit (EXT) in the menu to the right) are requested with a short push of the corresponding menu buttons (M1, M2, and M4).
Since there are only four menu buttons (M1 - M4), those menus that have five choices like the one shown to the right on line 1 of the LCD are handled as follows. For this menu, the third menu item SAV/PRV controls both the SAVe function and the PRV (previous) function (backs up to the previous calibration point). For these dual menu cases, you select the first function (SAV) with a short press of the corresponding menu button (M3 in this case). You select the second function (PRV) with a long press of the corresponding menu button (M3). 

Announce Start of SWR calibration:

After all 26 power points have been calibrated, this screen is displayed. This screen simply announces that the software is ready to enter the SWR Calibration sequence. Hit any of the four menu buttons to proceed.

Actual SWR Scale Calibration:

The actual SWR calibration now begins. The screen prompts you at each of the 30 SWR calibration points (1.1, 1.2, 1.3, 1.5, 1.6..., 4.0) The screen to the right shows the sixth SWR calibration point (1.6).

For each point, advance the needle to the corresponding location on the SWR scale using the front panel pot. Then press the  SAV button (Menu Button 3).

As with calibration of the Power scale, the TRIM function is available for each point by pressing M1 (TRIM) if you want to use the "nudge" function.


Save Settings to EEProm:

After you have saved the setting for the last calibration point, you are given the option to save the Panel Meter calibration settings to EEProm by pressing SAVE (M1).


The screen on the right confirms that the calibration values have been save to EEProm.

TEST or EXIT option:

The software now gives you the option to EXIT (Menu button M2) resuming normal meter operation, or to TEST the calibration (Menu button M1). We choose TEST in this example. The results are shown in the table below.



Testing the Meter Calibration
 (using  the Set-up software)

The table below shows the procedure for testing the calibration before exiting the calibration procedure.

The TEST procedure causes the software to sequence through each calibration point, displaying the nominal value of the meter reading on the LCD, and driving the meter needle to the corresponding point on the scale. The two should match for each calibration point.


TEST or EXIT option:

This is a continuation from the table above. This is what you will see if you chose the TEST option to verify the calibration.

Test of a Power Scale calibration Point:

In the TEST Mode, repeated pressing of NEXT-VAL (Menu Button M1) will advance the needle to the next calibration point.

The meter needle should read the exact value as shown on the LCD display. The tenth power point (5.0)  is being displayed in the sample screen to the right.

If you were careful during the calibration, the points should line up exactly. If not, note the calibration points that need to be tweaked. You can then use the Panel Meter EDIT function to fine tune those points without redoing the entire calibration.

Test of an SWR Scale calibration Point:

After all of the power calibration points are exercised, the SWR calibration points are tested in a similar way.

The meter needle should read the exact value as shown on the LCD display. The fourth SWR point (1.4) is being displayed in the sample screen to the right.

As with the power calibration points, the SWR calibration points can be tweaked with the EDIT function if required.


Displaying the Panel Meter Calibration Data on the LCD

You can display a summary of the Panel Meter calibration data at any time using the DISPLAY function.
Panel Meter Menu:

As with all DISPLAY functions, first display the applicable menu (Panel Meter Menu) on line 4 of the LCD. Select the Panel Meter you want to display the calibration data for (Panel Meter 6 in this example).

Then press the DISP button (Top Right Button - Short press).

View Display Data:

The display function starts. The first page of the Panel Meter calibration data is displayed on the LCD as shown to the right.

The first page contains the following information:

- Panel Meter number
- Scale Type
- Number of Power Ranges
- Number of SWR Ranges
- SWR start value (0 or 1) if an SWR scale is defined

Since this display has multiple pages, press the DISP button again (short press). Each press advances the display to the next page.

Power Ranges:

The second screen displays the Power Ranges. (Remember that for Nonlinear scales and Crossneedle meters, the power scales are confined to be x1, x10, and x100 scales of the single range specified in the setup routine.)

(For Linear Scale calibrations, which can have up to 12 Power Ranges, there could be multiple pages of Power Range information.)

SWR Ranges:

If an SWR range was defined, the screen on the right displays its full scale value.




Test or Exit Prompt:

After the last page of information is displayed, you are given the option to test the calibration, which works identically to the TEST function described above.

Therefore, at any time, if there is a question about a Panel Meter's calibration, enter the DISPLAY mode for that Panel Meter, as shown here. Cycle through the informational data. Then when given the TEST/EXIT choice, simply press TEST (M1) to verify the calibration point by point as was done above.

Display Function Exits:

In this example, we press EXIT (M2 in the above screen). The meter now resumes normal operation.






Verifying the Meter Calibration Operationally  

After calibration, you can operationally verify the accuracy of the calibration at a couple of points if desired. Since we are only verifying the Panel Meter calibration in this example, the Simulator can be used since we are only verifying agreement between MB-1's view of the measurements and the Panel Meter Display. The Simulator is extremely useful for this purpose.

You can also exercise the Panel Meter by running a real test, which is shown in the figure below.

In the following figure, the MB-1 internal 7-segment displays have been configured to display the following:

Top Left - Instantaneous Power
Top Right - Panel Meter Range
 (Panel meter FS range is 130 watts)
Bottom Left - SWR Minimum
Bottom Right - SWR Maximum