Distorted low freq sound after IF+demod - RF Cafe Forums
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Post subject: Distorted low freq sound after IF+demod Posted:
Thu Jul 06, 2006 4:20 pm
Joined: 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 yet.
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.
Unfortunately, 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 cable?
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.
like to get this sorted out before I make my PCBs. Any ideas are much
Post subject: Posted: Fri Jul 07, 2006
Joined: Fri May 19, 2006 5:01 am
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?
help would really be much appreciated.
The schematic is at:
http://www.turingbirds.com/electronics/ ... scheml.gif
Post subject: DistortionPosted:
Fri Jul 07, 2006 6:25 pm
Joined: Wed Feb
22, 2006 3:51 pm
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
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. RATS NEST!!
Nicely glowing EABC80. The diodes for the detector are matched,
by the way.
More godawful ratsnesting:
Tapping the IF from a working (semiconductor
Kind regards and thanks again,