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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
Posts: 25
Location: Netherlands
Hello everyone,

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.

I'd like to get this sorted out before I make my PCBs. Any ideas are much appreciated!

Kind regards,

PS. Schematics!


Post subject: Posted: Fri Jul 07, 2006 4:39 pm


Joined: Fri May 19, 2006 5:01 am
Posts: 25
Location: Netherlands
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: ... scheml.gif

Kind regards,


Post subject: DistortionPosted: Fri Jul 07, 2006 6:25 pm


Joined: Wed Feb 22, 2006 3:51 pm
Posts: 104
Hi Charl!

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.

Good Luck!


Post subject: Posted: Sat Jul 08, 2006 4:31 pm


Joined: Fri May 19, 2006 5:01 am
Posts: 25
Location: Netherlands
Dear Fred,

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:

My homebrew IF transformer:

Tapping the IF from a working (semiconductor based) receiver:

Kind regards and thanks again,

Posted  11/12/2012
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