In article ,
flipper wrote:
On Tue, 30 Aug 2011 21:30:00 -0500, John Byrns
wrote:
In article ,
flipper wrote:
snip
when I was measuring
frequency response and getting 'lower amplitude' at 10kHz I was using
full mod. So maybe the small signal response does improve and it's
slew limiting I was seeing limit the amplitude. Maybe just wishful
thinking and I should take it back up to the scope but, for now, I'm
listening to it.
Could be, I will have to take a few minutes to think about what you are
saying
here.
I took 6 days off from thinking about exactly how I will rebuild my AES phono
oscillator with a high level series modulation circuit. In addition to my
wife's project to reduce what I call domestic entropy, which will probably
extend well into 2012, when you are "94", Patrick's number not mine, health
issues crop up which demand attention. The most time consuming though was
purchasing a new car for my wife, which involved scouring the country for one
that was built before, or shortly after, the tsunami in Japan ended the
availability of the optional $395.00 paint job she wanted. The tsunami seems to
have ended the production of a particular type of metallic flakes that were used
in the desired paint.
Well, I did some more testing.
I started with the intent to put my air variable in and see how it
works over the whole AM band and, as suspected, I had to increase C1
to 20pF for it to oscillate on the lower end.
My cap is a typical AA5 dual gang with grounded frame so that meant I
had to put it on the coil big end only, which meant I had to screw the
slug full in.
Still, I was able to get around 540 to 1700 and figured the top end
would drop when I tacked the antenna back on but, to my surprise, not
only did the top end come down but the lower end of the range came up
to, say, 650 minimum.
That's interesting, do you have any tentative explanation for the low end
increasing from 540 kHz to 560 kHz when you connected the antenna, at first
glance it doesn't seem to compute?
The BIG surprise though was the 10kHz amplitude response I talked
about previously. With everything else remaining the same it changes
with carrier frequency from rather lousy on the low end to looking
pretty decent at the very top, and deep mod slew distortion follows
hand in hand.
That shouldn't have been a big surprise, when the tuning is done with a variable
capacitor the circuit Q increases when tuned to lower frequencies, remember that
the grid resistance forms the main load on the oscillator, so the band width
becomes narrower at the low end, plus there is a double whammy because not only
does the Q increase, but the carrier frequency is lower further narrowing the
bandwidth.
I don't understand why but it settles the tuning range question. Pick
C2 so the range is pushed up to the top end.
The "top end" seems to be a better choice anyway because frequency availability
is better up there, and if you are going for a fancy matched antenna to maximize
range, the antenna is likely to be more efficient at the "top end".
--
Regards,
John Byrns
Surf my web pages at,
http://fmamradios.com/