• Poor receiver selectivity will allow out of band signals and interference to raise the noise floor, mask weak signals, and cause drop-outs, Herringbone, or Flashes in video from PCS Packets.  All SIGNALS appearing in the mixer add together to reduce sensitivity. Our SuperFilter/LNA was designed especially to prevent this.  The MA2GU and MA2GUX receivers are even more sensitive, and signal levels higher than -45 db can cause problems.

• Poor Local Oscillator Stability will cause a low but audible "swishing" sound in audio subcarriers, "flickering" and/or Color Flashes in video, and will make a digital video adaptor useless.  A Spectrum Analyzer can easily detect this.

• Although a receiver may be rated for a maximum on-channel RF Signal Level of -20 dbm, it becomes useless when you are trying to receive a -70 dbm signal on YOUR channel, and the ADJACENT channel has a -20 dbm signal on it.  Keep signals BELOW -36 dbm if you use a Frequency Agile Receiver with an LNA stage.  This includes Brand New receivers.  Unfortunately, your new Antenna System can also easily overload, and cause signals to appear on YOUR channel. Please keep in mind that the digital PCS Signals on a Spectrum Analyzer may be many times higher in (peak/instantaneous) power than what you observe on the screen. Replacing your Antenna System's LNA with a lower gain, High Dynamic Range LNA will actually improve range, and reduce fades in  STL's.

• Receiver Overloading is one of the chief problems when replacing an old receiver in 7 & 13 GHz Duplexed STL Systems.  We can easily retune your Duplexer for increased rejection, and you can increase fade margin up to 20 db or more!

• Many ENG receivers have not worked correctly for years, some since day one! One of the most common problems is the IF amplifier or other gain stage that appears to be working correctly, but has a stage (or two) that oscillates.  This has occurred in MRC and other receivers.  This can result in lines through the video, color "flashes," and noise or "squeals" in audio subcarriers.

The problem is with the MMIC (Miniature Microwave Integrated Circuit) amplifiers used. These amplifiers look like regular metal can transistors, but usually have four or more leads. The makers of the receivers designed the high gain IF amplifiers to work at 70 MHz, but some of these devices have high "gain-bandwidths", and are capable of working (and oscillating) well beyond their rated frequencies. As an example, the ICs used in the AGC stages, usually oscillate between 800 to 1200 MHz, while one of the output transistors in the 70 MHz IF Filter Module of WLOX-TV's MRC receiver, was clocked at 1.8 Ghz.

How do you know if you have a receiver problem?  Unplug the input to your 70 MHz IF. Plug in a spectrum analyzer at the 70 MHz output on the rear panel of your MRC or other radio. Look at all frequencies up to about 3 GHz, and especially between 1 and 2 GHz.

You can check the output of each stage by using the 70 MHz output
line to the rear panel, and plugging it into the "output" of each stage.

If you find a signal present hundreds of MHz above the IF frequncy (excluding harmonics), you have a problem! You can usually find the bad stage by puting your finger on or near trasnsistor/MMIC cases in various stages. When you find one that makes the oscillation change frequency or go away, you've found the offending stage. Here's what you need to do to solve a typical problem:

MRC's 70 MHz IF Filter Module.
Output stages are in the upper right corner.

Q13, on the right may be oscillating. Q12 on the left, is usually OK.
Notice that the "tabs" on the transistors (7 o'clock position) are not soldered

Solder the tab of Q12 & Q13 to ground.  Use a 100 watt soldering gun,
for as short a time as possible. Using a low wattage soldering pencil
will cause excessive heating of the transistor, and possible failure.

Add a tin or brass "ground strap" to the chasis. You need to provide a solid
ground path.   #22 wire does not work well at microwave frequencies.

If you have further questions, call us at 1-.  If you want us to check a receiver for proper operation, or do troubleshooting,  the charge is $250.  In addition, many other problems can be easily detected with our extensive equipment.

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