Electronics World Cover,TOC,and list of posted Popular Electronics articles QST Radio & TV News Radio-Craft Radio-Electronics Short Wave Craft Wireless World About RF Cafe RF Cafe Homepage RF Cafe in Morse Code Google Search of RF Cafe website Sitemap Electronics Equations Mathematics Equations Equations physics Manufacturers & distributors Engineer Jobs Twitter LinkedIn Crosswords Engineering Humor Kirt's Cogitations Engineering Event Calendar RF Engineering Quizzes AN/MPN-14 Radar 5CCG Notable Quotes App Notes Calculators Education Magazines Software,T-Shirts,Coffee Mugs Articles - submitted by RF Cafe visitors Simulators Technical Writings RF Cafe Archives Test Notes Wireless System Designer RF Stencils for Visio Shapes for Word Search RF Cafe Sitemap Advertising Facebook RF Cafe Forums Thank you for visiting RF Cafe!

Distorted Low Freq Sound After IF+Demod - RF Cafe Forums

The original RF Cafe Forums were shut down in late 2012 due to maintenance issues. Original posts:

Amateur Radio | Antennas | Circuits & Components | Systems | Test & Measurement


Charl
Post subject: Distorted low freq sound after IF+demod Posted: Thu Jul 06, 2006 4:20 pm

Colonel


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,
Charl

PS. Schematics!
Demodulator:
http://www.turingbirds.com/electronics/radio/fm_demod/demod_scheml.gif
IF:
http://www.turingbirds.com/electronics/radio/fm_if/if_schem.gif


Top

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

Colonel


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:
http://www.turingbirds.com/electronics/ ... scheml.gif

Kind regards,
Charl


Top

fred47
Post subject: DistortionPosted: Fri Jul 07, 2006 6:25 pm

General


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!
Fred


Top

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

Colonel


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,
Charl







Posted  11/12/2012

RF Cafe Software

   Wireless System Designer - RF Cafe
Wireless System Designer

RF & EE Symbols Word
RF Stencils for Visio
Calculator Workbook
RF Workbench
Smith Chartâ„¢ for Visio
Smith Chartâ„¢ for Excel

About RF Cafe

Kirt Blattenberger - RF Cafe WebmasterCopyright
1996 - 2022
Webmaster:
Kirt Blattenberger,
 BSEE - KB3UON

RF Cafe began life in 1996 as "RF Tools" in an AOL screen name web space totaling 2 MB. Its primary purpose was to provide me with ready access to commonly needed formulas and reference material while performing my work as an RF system and circuit design engineer. The Internet was still largely an unknown entity at the time and not much was available in the form of WYSIWYG ...

All trademarks, copyrights, patents, and other rights of ownership to images and text used on the RF Cafe website are hereby acknowledged.

My Hobby Website:
 AirplanesAndRockets.com

Try Using SEARCH
to Find What You Need. 
There are 1,000s of Pages Indexed on RF Cafe !

height-line