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.

Frank Dresser

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

David wrote:
On 26 Jul 2006 23:00:58 -0700, wrote:



It is an interesting idea, but nobody builds LCR filters. Rather, you
use the LCR filter as a prototype, then build a leapfrog active filter
from signal flow graphs based on the physical LCR filter.


Really? What kind of filters does the Drake R8 series use?

As a demod filter? I would image a low order active filter to clean
things up. Remember, this is the audio band, not RF. I've seen some
write ups on 455khz IFs being done with active filters.

Kiwa sells an active filter for 455Khz
http://www.kiwa.com/kiwa455.html

Note the AR7030 has "tone controls", so certainly it has an active
filter past the demod. The problem with building LCR filters in the
audio band is they are bulky, not to mention often inaccurate. With
active filters, you have more flexibility over component values.

wrote:

As a demod filter? I would image a low order active filter to clean
things up. Remember, this is the audio band, not RF. I've seen some
write ups on 455khz IFs being done with active filters.

Kiwa sells an active filter for 455Khz
http://www.kiwa.com/kiwa455.html

Note the AR7030 has "tone controls", so certainly it has an active
filter past the demod. The problem with building LCR filters in the
audio band is they are bulky, not to mention often inaccurate. With
active filters, you have more flexibility over component values.

Drake uses an LC filter in the IF. They "Get away" with it becuase of
the
lower IF they use. R390s, original not the R390A, and the R392 use
several staged of LC filters and have excellent skirts.

The Kiwa filter you refference is not a "active filter", but a ceramic
filter
with amplification. To me active filter means opamp or norton amp
with feedback to control pass/reject charactoristics. The premium Kiwa
unit is nearly as good as a crystal or mechanical filter and MUCH
easier
to connect. I installed one in a friends R2000 and was impressed by
the quality and how well it worked.

A big advantage of passive LC filters is they are much less "fussy"
then active filters. I like not having to mess with power and proper
bypassing.
And if you are willing to wind your own torroids, it is pretty easy to
get the
L very close to what you want. The C can be built with standard value
caps in parallel.


The Tone-Tilt filter I used in all 3 of our R2000s is active because it
would be
VERY difficult to use LC filters effectively.

Terry

wrote:
As a demod filter? I would image a low order active filter to clean
things up. Remember, this is the audio band, not RF. I've seen some
write ups on 455khz IFs being done with active filters.

Kiwa sells an active filter for 455Khz
http://www.kiwa.com/kiwa455.html

Note the AR7030 has "tone controls", so certainly it has an active
filter past the demod. The problem with building LCR filters in the
audio band is they are bulky, not to mention often inaccurate. With
active filters, you have more flexibility over component values.

For an example of what I consider to be a very usefull filter, please
look at:
http://members.tripod.com/roymal/ReverbTone.htm

With minor component value chages, it is easy to get more or less
cut/booast and/or frequency range.

Please note that I had nothing to do with this page, I found it
usefull.

Terry

wrote:

Dallas Lankford has done some serious research on the cause and cure
of/for
the distortion caussed by ionospheric "hops".

Interesting! In the RTTY world, this is known as selective fading.
Usually an o'scope is used as a tuning indicator, one set of deflection
plates being connected to the "mark" signal and the other set of
deflection plates being connected to the "space" signal, giving the
classic "cross" display (looks like a 'plus' sign on the screen).

The mark and space signal generally used to be 850 Hz apart, commercial
press stuff 425 Hz apart and most of the stuff today using a 170 Hz shift.

During disturbed ionospheric conditions, you would see this selective
fading come into play; i.e., either the mark or space would fade to
nothing while the other signal was solid--even though they were only 170
Hertz apart.

"Carter, k8vt" wrote:

wrote:

Dallas Lankford has done some serious research on the cause and cure
of/for
the distortion caussed by ionospheric "hops".

Interesting! In the RTTY world, this is known as selective fading.

Gee! In the SWL'ing world, this is known as selective fading.

Who woulda thunk it.

It's multipath distortion.

dxAce
Michigan
USA

Well, it works.

I have been playing with the ELPAF since last autumn; first on my
R-390A, which, despite having done the AF Deck mod, does have its
quirks with regard to audio quality. The ELPAF cleaned up audio
admirably. Mostly doing MW DX then. Then, this summer together with a
modified IC-703 mostly on SW. It practically eliminates the distortion
caused by fades, as well as high-frequency hiss and noise giving an
audibly better signal to noise ratio. The trade-off is of course a
more limited audio response. Personally I can live with that - I never
use bandwidths wider than 6-7 kHz anyway. My ELPAF has a bypass switch
so it is easy to compare audio quality.

I used to have an SE-3 as well, and enjoyed the excellent audio it
produced. The ELPAF does little less with regard to audio recovery. I
am thinking about doing an A-B comparison between the two later on.

BM

"Telamon" wrote in message
...

[snip]


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.


There's a linear region in the usual model of a semiconductor diode (a fixed
voltage drop with a series resistance), but that model is only an
approximation. The other model, the square law model, is also just an
approximation, although it's supposed to be close enough over small parts of
the curve.

However, the diode doesn't have to be linear in order to have a fairly
linear diode detector circuit. Imagine we have a diode whose forward
resistance drops in a square law with the voltage. At 0.1V the forward
resistance is 1 meg. At 0.2V the forward resistance is 1K. At .0.3V the
forward resistance is 32 ohms. At 0.4V the resistance is 5.6V, and so on.

Now, let's put this nonlinear diode in series with a linear load resistance
and decide that the circuit is pretty much linear once the diode resistance
drops to 10% of the load resistance. Well, it's obvious that diode detector
circuits which work into higher resistance loads will linearize themselves
at lower voltages than diode detectors which work into lower resistance
loads.

Below a certain voltage, the diode's non linear characteristics will
dominate the detector. Low voltage signals will have much more of their
waveform in this funky reigion than high voltage signals, even at the same
modulation index.

So, as I see it, there's alot more to know about a diode detector's audio
distortion than only the modulation index. There's the actual
characteristics of the diode, the resistance of the load and the signal
voltage the detector is operating at.

There's also the RF filtering, which will tend to "sawtooth" the audio a
bit, much as the rectifier and capacitor do in a power supply. There's also
some resistances/capacitances in the AVC line.

But I could be wrong. If so, let me know!

Frank Dresser

wrote:
wrote:

As a demod filter? I would image a low order active filter to clean
things up. Remember, this is the audio band, not RF. I've seen some
write ups on 455khz IFs being done with active filters.

Kiwa sells an active filter for 455Khz
http://www.kiwa.com/kiwa455.html

Note the AR7030 has "tone controls", so certainly it has an active
filter past the demod. The problem with building LCR filters in the
audio band is they are bulky, not to mention often inaccurate. With
active filters, you have more flexibility over component values.

Drake uses an LC filter in the IF. They "Get away" with it becuase of
the
lower IF they use. R390s, original not the R390A, and the R392 use
several staged of LC filters and have excellent skirts.

Except we are not talking about IF filters .The "fading" filter is at
the end of the chain, i.e. past the demod.


The Kiwa filter you refference is not a "active filter", but a ceramic
filter
with amplification. To me active filter means opamp or norton amp
with feedback to control pass/reject charactoristics. The premium Kiwa
unit is nearly as good as a crystal or mechanical filter and MUCH
easier
to connect. I installed one in a friends R2000 and was impressed by
the quality and how well it worked.

A big advantage of passive LC filters is they are much less "fussy"
then active filters. I like not having to mess with power and proper
bypassing.
And if you are willing to wind your own torroids, it is pretty easy to
get the
L very close to what you want. The C can be built with standard value
caps in parallel.

Except this is at audio frequencies, where the component sizes are much
larger. Again, this is not at IF frequencies.




The Tone-Tilt filter I used in all 3 of our R2000s is active because it
would be
VERY difficult to use LC filters effectively.

Terry