[Test-Equipment] FS: Heath IM4180 FM deviation meter

[Richard Knoppow]

----- Original Message ----- 
From: "Gary Schafer" <garyschafer at comcast.net>
To: "'Discussion of Electronic Test Equipment'" 
<test-equipment at mailman.qth.net>
Sent: Tuesday, April 22, 2008 6:35 PM
Subject: RE: [Test-Equipment] FS: Heath IM4180 FM deviation 
meter


> If you really want to get into it, high quality AM can be 
> generated by
> properly phasing two FM transmitters together.
>
> 73
> Gary  K4FMX
>
    There are several patents on transmitters of this type. 
The system is known generically as "outphasing" and a Google 
search will find a good deal of information. One of the 
earliest patents was issued in France to Chireix and the 
system is often called by that name. It was used in the 
1960's for a high power RCA broadcast transmitter called the 
Ampliphase but it was not very successful in practice. 
Outphasing modulation is currently used for some microwave 
transmitters.

    There is a good deal of technical information about FM 
on the web and I wrote off list to Charles with some 
suggestions. Again, a Google search will find both simple 
and quite advanced treatuses on it. FM seems simple but is 
really not very intutive. Really understanding it requires 
some math and aspirins.
    BTW, in theory FM sidebands extend infinitely, however, 
the further they are from the center frequency the smaller 
the amplitude so, beyond a certain limit, they are not 
significant.
    A system of narrow band FM was used in the first Ampex 
video recorders. Its worth looking up because it violated 
the theory of the time indicating that the theory was wrong.

    All modulation can be generated by combining amplitude 
and what is sometimes called quadrature modulation. 
Quadrature modulation is a mathemetical concept and does not 
exist physically however, it is a component of the analysis 
of any type of modulation.

    Sometime around the 1960s a fellow named Kahn wrote a 
number of articles touting a system he had developed for 
using AM transmitters for SSB by splitting the original 
signal into AM and FM components. The AM component went into 
the audio input and the FM component was used for RF drive. 
When everything is balanced right the AM transmitter acts 
like a high efficiency linear amplifer. I have not done a 
patent search but I suspect his patents will show up on a 
Google Patents search. I believe the military used Kahn 
adaptors for some applications.

    I've used the Bessel null method of setting FM deviation 
on satellite uplink transmitters many times. It isn't 
difficult but requires a good narrow band detector of some 
sort (a spectrum analyser is ideal) and an accurate 
knowledge of the audio frequency used for modulation.

---
Richard Knoppow
Los Angeles, CA, USA
dickburk at ix.netcom.com