Re: AM detectors, was re: OPT equalising circuit

"Patrick Turner" <info@[EMAIL PROTECTED]
> wrote in message
news:481F27AD.428815B9@[EMAIL PROTECTED]
>
> It works, try it. The CF idle dc current of 1mA is OK.
> But the gm is much higher than 0.5mA/V.
> As the tube turns on harder gm much increases.
> It wouldn't work well to get a negative going voltage.
> But OK for a positive going voltage.
> THD is well under 1%, better than 5% as in most detectors.

Patrick, do you have a quality signal generator capable of  nearly 100% AM
to test?
Otherwise, how could you measure the detector THD?

I guess such AM generator can be built on the basis of a DSP and a good
DAC... Possibly it might be my next weekend project.

> > For the detector to begin detecting, obviously the AC current through
the
> > diode shall exceed the DC component (90uA). It will require about
150mV
on
> > the input to the detector or conversely 400mV on the grid of the CF.
> > Sensitivity of such a detector is rather poor (as you know,
sensitivity
of a
> > pure p-n diode is about 25mV.)
>
> Doesn't matter because the signal to be detected from the IFT is so
> large
> that one can get about -12Vdc of AFC voltage, more than needed.

Yes, agree. With such a strong signal any detector will work, even the so
called cathode detector -- just remove the silicon diode, remove the 33K
pull-down resistor and use the sharp cut-off tube solely.
>
> >
> > It is easy to improve the result 10 times -- just reduce the bias.
Connect
> > R17 not to the ground, but to point "R" (+31V). After that the diode
will be
> > biased only by the self bias of the tube, which is about 4V.
Sensitivity
of
> > the detector will increase 10-fold.
>
> The sensitivity you mention isn't needed or wanted.

Understand.
>
> The R16 and R17 network are 330k and 100k = 430k,
> as as the cathode of CF is at about +33Vdc, there is a dc current
> through the diode = 33/430 mA
> or about 0.09mA, and this stays fairly constant with changes to AF
> voltage in the 220pF.
> I found having a virtual constant current discharge of this C was much
> better than having
> a large R taken to a voltage not much different to Ek.
>
> I suggest you build this detector and give it a decent test using a
> 455kHz modulated test
> signal. The main advanatge is that at high percentages of modulation
> used these days the distortion
> remains low until very close to 100%.

My favourite detector is the emitter detector preceeded by a source
follower. Its sensitivity is 25mV. A great disadvantage though that it can
not stand a carrier more than 1.5...2Vm because of the reverse breakdown
voltage of the transistor. Linearity is perfect.
>
> In many radios the detector suffers cut off distortion as % mod rises.

Agree. Some can not stand more than 60% of modulation, even at low audio
frequencies.
>
> With very small signals, the detector suffers no diode threshold effects
> because the
> diode is 'on' at all times.

Yes, the diode is always ON, but it does not do good, contrary to a common
belief. Because of high diode bias, the detection (rectification) will
begin
after the 455KHz AC component will exceed the DC component. Below that the
diode is virtually a resistor of about 250ohm. So your CF is loaded by a
250ohm in series with 220pF, representing about 1.6K impedance. From that
you need at least 150mV of the signal on the diode (more on the grid) for
AC
exceed DC.

Regards,
Alex
>
> Patrick Turner.
>
> >
> > Regards,
> > Alex