In article
,
"Frank Dresser" wrote:

wrote in message
oups.com...

Frank Dresser wrote:

Am I reading the nifty formulae wrong? It looks to me like he's
deriving the distortion of a diode detector from the modulation
index only. My sense of these things says that a 50% modulated
signal at a tenth of a volt is going to have much more distortion
than a 50% modulated signal at 10 volts.

Frank Dresser

Very few radios drive the detector with anything near 10V. The R390
and R392 have the highest diode drive voltages I have seen and I
think they are less then about 3V.

The range is extreme, but not outlandish.


Most modern, IE "solid state", receivers I have measured have less
1V. All that I have seen that use discrete diode detectors as
oppossed to ICs, have farily high AF gain stages.

But I'd expect considerably less distortion at 3V rather than 1V.

And I'd also expect that no radio really uses a square law detector
to detect the audio. Real detectors try to linerize a diode's
operation by lightly loading the detector with a reletively high
resistance and trying to minimize operation in the diode's "square
law" area. Both voltage and AC/DC impedance are important
considerations in determing diode audio detector distortion.

I suspect the term "square law detector" is the same sort of term as
"first detector" -- what's now known as a mixer.

I know I've been tripped up by these archaic terms before.

I'm not a radio circuit designer but detectors circuits are designed for
a certain situation and will not produce the expected output if the
expected input conditions do not exist. All RF carrier and sidebands
(tones) are an alternating wave forms. To recover the AM modulated
information the sideband tones are rectified and averaged, which is the
low frequency audio modulation. The sideband tones are usually much
lower than the carrier but the detector rectifies all of these signals.
For the detector design a minimum signal level is required for it to
rectify the side band tones and the designs have depended on the carrier
to be there so that the detector is switching on and off into the liner
region of the diode. If the carrier is not there then the sideband tone
signal is switching the diode on and off resulting in a lot of
distortion.

The sync detection uses a PLL circuit to lock a local oscillator to the
received carrier and that is summed with the received carrier and side
band tones so that when the received carrier disappears due to selective
fading the locked local oscillator signal is enough to keep the detector
operating in its liner region with just the side band tones present.

The same thing happens using a BFO or when you switch to SSB mode on a
radio but here the local oscillator is not locked to the received
carrier and you have to tune the radio very carefully to get it spot on
the received carrier frequency so the side tones are reproduced at the
original modulation audio frequencies.

Before sync detection circuit designers would use diodes with smaller
non-liner switching regions using germanium for example with lower
forward voltages. These diodes would need less signal power to turn on
and off into the liner region of it operating curves so less energy from
the carrier would be needed to keep the detector in its liner region.
This is a help when the received carrier only fades a little but does
not help if fades a lot or disappears.

Some detector designs would use a DC bias on the diode to put it on the
edge of its liner region to improve its small signal sensitivity. The
optimum bias voltage will depend on the diode characteristics.

--
Telamon
Ventura, California