HUM REDUCTION IN LP AM & FM TRANSMITTERS

     It is convienient and easy to use a wall transformer with FM stereo or AM transmitters, but often unwanted hum appears on the transmitted signal. This problem can be frustrating and difficult to correct. Depending on the design, layout, and packaging of the transmitter, several sources of hum can be present at the same time. These include power supply hum due to poor DC supply filtering, AC hum pickup in the transmitter audio section and the audio input lines due to poor shielding, poor circuit layout, ground loops, and RF pickup. Also, induced hum modulation on the transmitted signal can be encountered even after the previously mentioned problems are cleaned up. THIS IS NOT DUE TO INSUFFICIENT FILTERING OF THE DC SUPPLY VOLTAGE. Rather, it has to do with induced modulation of the RF ground return impedance due to the nonlinearity of the rectifier diodes in the wall transformer. What happens is that the case and PC board ground foil and any other conductive material acts as a ground plane for the antenna. Since the power supply cord is connected to the transmitter, it is part of this ground plane. The windings of the transformer in the wall supply are closely coupled for RF energy, therefore, even the AC house wiring acts as part of the ground plane. However, the rectifier diodes are in series with the power cord and the AC house wiring, as far as RF is concerned. The diode impedance for RF is forced to vary at the 50 - 60 Hz line frequency, which effectively modulates the ground plane impedance. This introduces incidental hum modulation into the transmitted signal. While this is largely AM component, no FM system is perfectly free from AM susceptibility, and it is easy to wind up with a hum component that is 40 or 50 DB below peak audio level. Since the hum component is largely AM, especially with AM Transmitters, this hum is annoying and very noticeable. It has nothing to do with the transmitter circuit design, being a function of the wall transformer and the environment. It is reduced by bypassing the rectifier diodes, using RF chokes in the supply lines, using a good earth ground, and/or a large ground plane immediately under the antenna, or in the case of FM, by using a balanced antenna such as a dipole with a good balun transformer. However, these measures are not very practical as wall transformers are generally sealed and most users will want to simply use the LP AM or FM stereo transmitter with a small whip antenna mounted on the rear of the unit.

This phenomenon was demonstrated on our original model MPXX multiplex transmitter prototype by substituting a battery for the wall transformer as a power source, while still leaving the wall transformer connected to the MPXX case, and using a small bulb (#1487 lamp) drawing about 150 ma to simulate a load on the transformer. Even with the battery supply running the MPX transmitter, the hum persisted. Disconnecting the wall transformer stopped the hum. This transmitter appeared in the March 1988 issue of Radio Electronics Magazine and used a VCO and a multiplex generator made up of conventional discrete components. (This magazine has had its name changed to Electronics Now) The transmitter then was equipped with a built in 120 V : 12V transformer and bridge rectifier. Proper RF bypassing and grounding techniques were used and no hum problems were evident in the transmitted signal.

Our current PLL synthesized transmitters, the MPX96 and the AM88, were designed to help reduce this problem by using high level audio circuitry (AM88) and a MPX generation scheme (MPX96) that runs at high audio signal level and very simple, compact layout with plenty of grounding to help reduce possible hum pickup in the audio circuitry. The low level MPX signal generation approach used in the BA1404 (now obsolete) has been avoided. While the MPX96 uses much larger MPX audio signal levels, we still recommend using a regulated 12V supply or batteries if possible to avoid any possibility of hum. Since most wall transformers are poorly regulated and generally not RF bypassed, we really do not recommend using one if possible. However, for those who want to try, a wall transformer delivering at least 15VDC at 150 ma (MPX96) or 250 ma (AM88) is needed. Instructions are given in the kits for this mode of operation, but as we have no control over the wall transformer and its RF environment, there may still be the possibility of some hum. We cannot guarantee total freedom from hum with these modifications but they seem to work reasonably well.

BACK TO MPX2000         BACK TO MPX96         BACK TO AM88
 
NORTH COUNTRY RADIO
PO Box 200, Hartford NY 12838-0200
EMAIL: support@northcountryradio.com                 Tel 518-854-9280          Internet http://www.northcountryradio.com