PWM Modulator
PWM Modulator
The Pulse Width Modulator uses the WA1QIX PWM Generator, PWM Output, and Efficiency Meter printed circuit boards. The audio input is configured to accept a line level signal from an external audio chain.
PWM Modulator Component Map
Click image to view component labels
Dial-ups beware - Large!
The high voltage power supply is metered for voltage and current, and has a switched output for tune (low), operate (high) and off (no) power. There is also a booster transformer to raise the high power output, activated by an internal switch. The HV power supply primary and secondary is switched by an open frame TR relay, which is activated by an external 120vac keying voltage that includes several interlocks (see the TR sequencer description).
The PWM modulator circuits described below were designed by Steve Cloutier WA1QIX. A great deal of detailed information on these circuits, including the PWM Generator, Efficiency Meter, and PWM output boards may be found at www.classeradio.com
PWM Generator Board
The PWM signal is provided by a UCC25701 IC on the PWM generator board. This board also features circuits that provide a microphone preamp, audio processing, anti-aliasing filter, audio phase select, negative peak limiter, overload shutdown, and power supply filter/regulator.
Efficiency Meter Board
The efficiency meter board implements a circuit (patent pending) that provides a relative efficiency meter reading. Efficiency is derived by comparing RF amplifier input power to RF amplifier output voltage. A front panel potentiometer is used to adjust the rf amplifier output relative signal level, which is rectified and filtered by an external sampling circuit. This can be used to tune the class E transmitter without the use of an oscilloscope, and it works very well! Also on this board are circuits used to provide overload trips to quickly shutdown the transmitter in case of a fault. This feature was put to the test during my initial shake-down, and also worked very well! A trip of the overload circuit will open a relay contact on the board, which will de-energize the modulator TR relay. Also, a trip will signal the PWM generator board to turn off its PWM output pulses in a very short period of time, protecting the rf amplifier from damage.
PWM Output Board and Filter
The PWM output board utilizes two IRFP460N MOSFETs to amplify the PWM signal using the HV power supply output. The output board has its own power supply to prevent interactions with the low level circuits on the PWM generator board. The PWM output is then filtered by the PWM filter, which is implemented using two CWS/Bytemark CH777060 toroids. The modulated DC filter output is then routed through the efficiency meter board for voltage and current sensing, current meter shunts for the front panel meters, then output to the RF amplifier.
PWM Output
Keying the modulator output is accomplished by an external 120VAC signal. This voltage energizes a small relay that provides a ‘reset hi’ signal to the efficiency board, starting the PWM generator board output pulse train. This 120VAC signal is also routed through a normally open contact on the efficiency board relay to energize the open frame TR relay.
The modulator was designed by Steve Cloutier WA1QIX. The design revisions used to build this modulator are detailed in these PWM Modulator and Efficiency Meter board schematics. The current revision of these schematics may be found at www.classeradio.com