I am designing the mic input for a ham radio chip I bought, a DRA818V.
Someone gave me an old Kenwood ham radio mic and said I can use that mic for my circuit. The mic element is shown below:
What kind of microphone is this and how can I use/bias it?
My LCR meter says it has 304 ohms of DC resistance. It has 4.653 mH at 1 kHz of inductance. I have an oscilloscope to use to complete my circuit. The mic input of my chip, the DRA818V, is not well described, so I will have to experiment to see what the chip "likes". I expect I will have to make a small signal amp from an NPN transistor to feed the "mic in" of my chip.
My main question is: What is this mic and how do I use it? If I can set it up and see it on my oscilloscope, I can experiment if I need to amplify it or not?
Best Answer
Now that it seems certain that you have a dynamic microphone, let's have a look at what your DRA818V module needs at the microphone input.
On page 3 of the datasheet, there's a table of radio characteristics:
Down towards the bottom is this line:
That's telling you that a 10mV peak to peak signal applied to the microphone input will result in 2.5 kHz of deviation. That's the maximum deviation when using a narrow channel. It isn't clear if you need a stronger microphone signal for full modulation on a wide channel. (It also tells you that this specification is tested using a 1kHz audio test signal.)
What that means is that you need at most 20mV of microphone signal going into the DRA818V to get the maximum possible transmit deviation. That means 20mV to get your spoken voice from the transmit side to maximum volume on the receive side.
What you need to do is to capture your spoken voice on the microphone with your oscilloscope, and measure its peak to peak voltage. From that you figure out how much amplification you need to get to 20mV peak to peak. That shouldn't be much, maybe a gain of 5 to 10.
If your microphone amplifier has too much gain then it will sound distorted on the receiving side, but you won't be breaking any regulations. The transmitter will (should) clip the transmit audio to keep your modulation within the allowed limits.
You might not need any amplifier at all for the microphone. Just connect it straight to MIC_IN and ground and give it a try. I expect the bias you mentioned in comments interfered with the microphone and caused the output to be too weak.