RF.AMX10

DIY RF Condenser Microphone
Schematic

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(Just click on the image to download a PDF of the schematic)

The schematic shows the current version of the circuitry employed in the creation of this simple amplitude modulated RF oscillator based microphone. This latest version (v.2.2) includes a number of revisions which have been made to further improve the overall perfomance of the microphone.
The PCB layout remains the same.

You can find a copy of the original version of the schematic and associated parts list HERE

Some additional project notes - together with schematics optimised for use with an alternative edge terminated 'C12 style' capsule - can be found HERE

The circuit concept is based on some of the techniques described by audio engineering legend Peter Baxandall in THIS PAPER from 1963.
This circuit is designed to be connected to a low impedance, balanced XLR microphone pre-amplifier, which must have the option of providing 48V phantom power.
It consists of 4 distinct functional sections: RF oscillator - Modulator/ Detector - DC power and Audio Output... (The last two functions utilise the elegant and much copied 'Schoeps' style circuitry)

RF (Radio Frequency) Oscillator:
The circuit components connected to Q1 form a 10MHz crystal controlled Colpitts oscillator. Around one third of the c.20V(p-p) 10MHz sine wave present at Q1 emitter is connected to the primary winding of T1 via R3. With the values shown, the oscillator draws around 4mA from the 48V phantom power supplying the circuitry.

Modulator/Detector:
TI and T2 are 'off the shelf' 5.3uH tunable IF cans, available from Spectrum Communications here in the UK.
One end of the grounded centre tapped secondary winding of T1 is connected to a 68pF capacitor, the other end of which is connected to one terminal of the microphone capsule. The other end of the capsule is connected to the remaining end of T1 secondary.
This arrangement provides a slightly unbalanced tuned 'bridge' circuit, where the 'balance' is further modified by the changing capacitive value of the capsule, as it responds to audio stimulation.
This change of the bridge balance state will enable amplitude modulation of the RF oscillator in proportion to the applied audio stimulus.
These changes in capacitive value tend to be extremely small - in the order of 0.001pF for an alternating pressure of around 1 dyne/cm² (i.e. normal speech level signals at around 30cm.) - according to Baxandall.
T2 is an identical tunable IF can to T1, and one end of the primary winding is connected to the centre of the capacitor 'bridge' across T1 secondary. The other end is grounded.
T2 secondary winding is loaded with a 47pF capacitor, which allows the inductor core to be adjusted to resonate at 10MHz. The high 'Q' of T2 when tuned will allow the tiny changing bridge imbalance signal to be stepped up by the T2 turns ratio to a higher AC voltage - without introducing any further noise - for presentation to the gate of Q4...
Q4 acts as both an infinite impedance detector and an audio phase splitter. The infinite impedance detector function takes the RF signal present on the gate, and in conjunction with the time constants R10/C13 and R4/C9 will output audio to C5, with opposite polarity audio to C10.

DC Power:
Power for the unit is derived from the 48V phantom power presented to the circuit via the 3 pin XLR male connector XLR1. Q2 and Q3 are configured as emitter followers, with commoned collectors connected to the positive terminal of C7. This arrangement is generally known as a 'Schoeps' style circuit, and is widely used for this type of device.
R11 and R12 - together with C15 - form additional low pass filtering to remove any noise that may still be present from less than perfect 48V phantom power supplies.

This smoothed DC voltage is connected to the junction of R2 and R10, to provide DC power to both the RF oscillator based around Q1, and the infinite impedance detector/ phase splitter based around Q4.

Audio output:
The audio outputs present at the drain and source of Q4 are connected - via C5 and C10 - to the bases of Q2 and Q3. These are configured as emitter followers, and present a low impedance audio path via R7 and R8 to pins 3 and 2 of XLR1 respectively.
L1 and L2, C11 and C12 are included as lowpass filters to remove any residual RF from the final audio output.

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