John Byrns wrote:

In article , Robert Casey
wrote:

Pat, I just ran a simulation of the use of two cathode followers (I used
12AU7's) and
a 6AL5 diode ddetector. I got very good results, using a 1KHz
audio sine wave, modulated onto a 95% modulated 455KHz carrier, got 55dB
down
of the 2nd and 3rd harmonic. I also threw in a little bit of positive
bias to the detector
diode to partly get it up above the "Knee". About 300mV worth of bias.
Of course
this requires that the AM signal be at least 2Vp-p and that the
modulation index be
about 95% (leaving 5% carrier at the valleys). As I'm pushing a bit
into the "other
side" of the AM signal. IOW, I moved the zero crossing voltage a bit to
make "fake
exaulted carrier".

See a screen capture of my simulation schematic and results in ABPR

Robert,

Your simulated detector seems quite different than Patrick's, mainly
because your bias network tends to turn the diode off, while Patrick's
bias turns the diode on, compensating for his poor AC/DC load ratio, at
least at one fixed carrier level. It took me a while to finally
understand Patrick's diode bias scheme, I finally only realized what he
was doing after John Stewart pointed out a similar circuit in the RDH4,
perhaps I don't yet understand what your bias scheme is doing?

Regards,

John Byrns

The "poor" AC/DC" load ratio does not matter if the output audio voltage asked
from the circuit is
low.
In a triode gain stage, if the DC carrying RL is large, and the cap coupled
load is small,
a similar effect occurs, and positive voltage swing is limited to a lot less
than the B+.
But the first few volts are still quite linear, and simply what one expects
when load liane analysis is used.

My own radio uses a good enough AC/DC load ratio to take the cut off effect
fully into account.

Patrick Turner.





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