RadioBanter - View Single Post - FRG-100 FM pickup problem

March 14th 05, 02:07 AM

In article .com,
wrote:

About a month ago I posted a question asking if others had
experienced local FM station interference at various frequencies on
the FRG-100. I finally found the cause and a solution. The FM gets
into the receiver thru the 12V power cable and any other connections
to the receiver (external spkr, tape recorder or computer connection,
etc.) IT DOES NOT ENTER THRU THE ANTENNA; in fact, you can ground the
antenna terminals and it's still there! The FM signal mixes with the
2nd harmonic of the first local oscillator for some reason, and this
allows either the sum or difference signals of the oscillator's 2nd
harmonic and the FM freq. to fall at various frequencies from 1 to 30
MHz. My worst were on about 5.06 and 13.1 MHz and this computed out
exactly to two of the local 100 kW FM station frequencies.

The FIX: I was tired of that oddball FRG-100 power connector and the
inability to use a standard 5mm coaxial power connector (common for
most receivers)with the radio on camping DXpeditions so I purchased
one of the standard types from Radio Shack (#274-1582), drilled an
appropriate hole and mounted it just to the left of the existing
connector in the blank spot on the back panel. I then soldered a
small powdered iron torodial filter of about 10 uH between the
connector's + pin and the top edge of the rear fuse holder (remove
fuse first!) I then a soldered a 0.1 uf ceramic capacitor to either
side of the toroid and the other side of the caps to chassis ground
forming a pi filter. I then changed the plug on the power supply 12V
cable to the standard 5mm male coaxial mating plug. The FM "intermod"
is now less than S-2 to S-3 with no external antenna as long as I
don't connect anything else to the accessory jacks on the receiver's
rear panel. Once the antenna is connected, atmospheric noise or any
signal that activates the AGC wipes it out.

I hope this helps some of you that may have this problem. I think the
direct ground connection to the chassis helps the most. The oddball
stock connector connects the power ground to the pc board ground
plane very close to the first mixer. This is probably the cause of
the problem due to common mode ground loop problems in the 100 MHz
range.

Something on the order of 100MHz FM has got to be common mode on the
cables going into the radio. I would expect a common mode choke to work
better than the modification you made. Use just a few widely spaced
turns of the power cord on a ferrite EMI type core should do the trick.
The choke will be more effective on the radio end of the cord in this
case.

--
Telamon
Ventura, California