Microphone
RFI in microKEYER II
The problem:
microHAM have been receiving reports of distorted microphone audio in
otherwise normally functioning microKEYER II. Although microKEYER
II has been tested for operation in high RF fields, we have found that when the
antenna is very close to the ham shack, the antenna is not well balanced (lack
of effective balun), or open wire feedlines are used, radiated RF can couple to
the shield of the USB cable and from the shield of the USB cable and from the
USB cable into the microphone preamplifier resulting in distortion.
In microHAM interfaces the shield of the USB cable is not
connected to the radio ground. All microHAM interfaces are
designed to provide complete DC and low frequency isolation between the computer
and radio in order to eliminate "ground loops" and the hum/distortion
they cause. Unfortunately, many modern computers fail to provide suitable
RF grounds and bypassing and the ferrite chokes commonly used for noise
reduction reduce the effectiveness of the bypassing that is present. Due
to the difficulties, microHAM have been looking into ways to
improve RF bypassing of the USB cable without losing the benefits of galvanic
isolation.
The
solution: RFI rejection can
be improved significantly while maintaining galvanic isolation and without
causing ground loop problems by installing a capacitor from the shield
to ground. This change has been incorporated in production for microKEYER II
serial number 765 and higher.
Those who are experiencing RFI in the microphone
circuit can add bypassing in the field by installing a 0.001 - 0.01uF high
voltage capacitor (the value in not critical) from the shield of the USB
connector to the metal mounting bracket of the DB37 connector as shown
here.
Installing the bypass capacitor as shown will not
effect warranty. If you need help or are not able to make the
modification yourself, please contact us at
support at microham.com.
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