[BC] FM Harmonic Measurements

[Richard Fry]

An r-f directional coupler is preferred as the sampling source for harmonic 
measurements, rather than the capacitive probe mentioned elsewhere.  The 
harmonic content "signature" in the transmission line after the 
harmonic/lowpass filter of the transmitter is much less sensitive to the 
location of a directional coupler than to a capacitive probe.

Dielectric Communications makes a series of good r-f directional couplers 
including their 5500-501 (for 3-1/8" OD rigid line) that can be adjusted to 
30 dB directivity or better, and have 6 dB/octave gain over the spectrum 
10-1,000 MHz, at least.  They also terminate the pickup loop in 50 ohms so 
that standing waves are not created on the sample line to the measuring 
gear, which can affect the harmonic measurement.

That sample line should be doubly-shielded to reduce the possibility of 
cabinet radiation entering from the transmitter, and affecting the 
measurement.  A good test for this problem is to move the sample line around 
through various physical orientations while looking at the measured values 
to see if they change significantly.

Even a fairly short length of the small OD coax typically used for the r-f 
sample line has significant and increasing loss at higher frequencies. 
Those losses need to be known, and allowed for in the measurement.

Shameless plug:  Harris "Z" series FM transmitters include an r-f 
directional coupler with the characteristics described above, for customer 
use.

RF