Distorted Low Freq Sound After IF+Demod - RF Cafe Forums
Post subject: Distorted low freq sound after IF+demod
Posted: Thu Jul 06, 2006 4:20 pm
Fri May 19, 2006 5:01 am
I'm still hard at work at building a
digital tube radio. I have wound my own inductors for the IF coupling
and they appear to be doing okay - though I haven't had a chance
to look at their bandwidth/freq response in a spectrum analyzer
My setup is as follows: I have a working radio with
a known-good 10.7MHz IF output. I then use about 20cm of shielded
audio cable to pass this on to my IF amplifier. There it is capacatively
coupled into the grid. The tube then amplifies it and sends it through
my homemade IF transformer to the demodulator.
the low frequencies on the audio sound distorted. I have switched
the IF transformer for a known-good one, but it's still the same.
Power lines are clean. Could this distortion be caused by the audio
My circuit isn't built too cleanly (soldered around
tube sockets, supported by their leads), but I don't think it would
amount to this distortion only at low frequencies.
to get this sorted out before I make my PCBs. Any ideas are much
Post subject: Posted: Fri Jul 07,
2006 4:39 pm
Joined: Fri May 19, 2006
Okay, did some
debugging work. The IF stage is fine, and the cables don't pose
too much of an impedance for the signal either. I conclude that
the issue must be in my demodulator.
Could this be an issue
with my homebrew transformer (which works okay as a plain IF transformer)?
Or is it in the circuit?
Any help would really be much appreciated.
The schematic is at:
Post subject: DistortionPosted: Fri Jul 07, 2006
Joined: Wed Feb 22, 2006 3:51
There are two problems:
1. The circuit looks like a cross between a Foster-Seeley discriminator
and a ratio detector. In the end, however, it doesn't appear to
have the benefits of either. It appears to have the poor noise performance
of the Foster-Seeley discriminator, combined with the variable output
level of the ratio detector.
2. In both circuits, the performance
of the IF transformer is critical, as the IF transformer provides
the frequency-dependent phase shift that makes either of them work.
I would recommend the ratio detector if you don't have much
experience with tubes. A dual-diode tube is preferable in either
case, due to the benefits of matching. A common (USA, sorry - I
don't have a European cross-reference here at work!) tube used was
the 6AL5. As usually implemented, the ratio detector uses a capacitor
from the primary to the center tap, rather than inductive coupling.
This can be made adjustable, so that you have two adjustments: tuning
and phase shift.
Post subject: Posted: Sat Jul 08, 2006 4:31 pm
Joined: Fri May 19, 2006 5:01 am
Thanks a bunch
for your reply. You're right about the mix of ratio detector and
Foster-Seeley. I did some reading in my copy of the Radiotron handbook,
and based on some further reading on the 'net decided to go for
the Foster-Seeley. It was quickly prototyped, and works a whole
lot better than the previous setup. Not perfect, but I'm sure that's
due to some other issues.. *cough* - see below.
I won't bore
anyone with further details until the PCBs are made. But here are
some pictures for your entertainment:
The prototyping setup.
Nicely glowing EABC80. The diodes for the
detector are matched, by the way.
More godawful ratsnesting:
My homebrew IF transformer:
Tapping the IF from
a working (semiconductor based) receiver:
and thanks again,