Designing a Percentage Modulation Meter

[ka1tdq]

I just picked up this meter from the Scottsdale hamfest this morning for $5.  It's a percentage modulation meter that goes to 133%.  My transmitter will peg this meter at times, but I still want to build it. 

Firstly, I need to find out what full scale voltage on this meter is.  I'm assuming 50 mV, but I'll find out.

Secondly, what is the relationship between drain voltage and % power?  I have in my head 2 x Vdrain(carrier) = 100% modulation.

So then, I'd just need to build a circuit that would subtract the Vdrain(carrier) and at that point voltage would be in a 1:1 relationship with % modulation.  I would just need to sample that linearly down to 50 mV.

Is this correct?

Jon
KA1TDQ

[KA2DZT]

Unlikely it's a 50mv meter movement.  Look at the very bottom of the face it may say what full scale is.  50mv movements are usually for use with DC current shunts.  The meter you have probably came out of some piece of equipment.  The full scale voltage or current could be anything.  There are ways to find what the full scale is.

Fred

[ka1tdq]

Ah, yes.  It's 400 uA DC. 

[ka1tdq]

Ok, I've come up with this.

I already sample a small voltage off of the antenna output jack for my LED bar graph.  My sample voltage is 1.8 vdc at carrier.  I calculated that 100% modulation would equal 300uA on the meter since 400uA is full scale at 133%.

Doubling the sample voltage to represent 100% modulation would be 3.6vdc.  Using ohms law with the mod meter in series, a 12k resistor would cause 300uA to flow at 3.6vdc.

The opamp circuit has a gain of 1 and simply subtracts V1 from V2.  So at carrier, the output of the opamp is zero volts causing zero meter deflection.  The 1.8 reference voltage would be taken off of a potentiometer to reference the meter to zero with the carrier on.

...I think...

Jon

**Update:  Actually the series resistor with the mod meter would be 6k since the output of the opamp would be 1.8 volts at 100% modulation.

[MikeKE0ZUinkcmo]

If you want to do it the old fashion way, here is a schematic of an old Harris M-6659A meter.

[KA2DZT]

12K would be the meter movement resistance plus the resistor.  You need to find the movement resistance.  Don't use an ohmmeter directly.  Put a series resistor in series with the movement.  Measure resistor and movement then subtract the exact resistor value.  May be very small compared to the 12K, may not need to worry about it.