LTC5507 Power Detector as AM Demodulator

[NA3CW]

Does anyone here have experience using the Linear Tech LTC5507 RF Power Detector as an AM detector?  The spec sheet says it can be used that way but I haven't seen any examples of it being used.  Just wondering how it sounds.

Thoughts?

Tnx es 73,
Chuck
NA3CW

[AB2EZ]

Chuck

Normally, you would want an AM demodulator to produce an output voltage that is proportional to the envelope of the associated RF signal.

Looking at the curves in the LTC specification sheet for this device... when the RF input power increases by a factor of 100 (i.e. from -8dBm to +12dBm), the voltage at the output of this device increases by a factor of 3.15 (i.e. from 630mV to 2000mV). But, if the device were acting as a linear envelope detector, a factor of 100 increase in power... i.e. a factor of 10 increase in envelope amplitude... would be associated with a factor of 10 increase in output voltage. Therefore the output voltage being produced by this device is approximately proportional to the square root of the amplitude of the RF input signal (i.e. the square root of the RF input signal's envelope).

Unless the folks who produced the data sheet made an error, this device would not make a good AM demodulator.

Stu

[flintstone mop]

Great info Stu
Thank you
I am also looking for a detector for AM. I am really guessing that a product detector for  HI-FI AM would be less distortion than the diode detector used in the R390A? The distortion heard during fading. I do not have $700.00 for the Sherwood sync detector.
Some receivers do not produce that distortion as much as others. Maybe a bandwidth problem..
Or should I quit looking/dreaming?
And Chuck...are you building a HB receiver?
Fred

[NA3CW]

Thank you, Stu, for your detailed answer.  It sounded like an odd application for the device but the amplitude modulation referenced in the spec sheet appears to be more on the digital side of things, not analog.

Flintstone, I was just looking to see if there was a reasonable improvement available for the diode detector in my FT-920.  I've got it talking AM pretty well at this point, by all reports, and I wanted to see if the receive side could be better as well.  More of a curiosity thing. 

73,
Chuck
NA3CW

[w4bfs]

this is not my best area but as I recall an infinite impedance detector (cathode or source follower) can provide cleaner audio up to higher modulation indices .... the problem will be keeping up with the avc .... some of the other folk here should know mo

[flintstone mop]

Hi Chuck
I'm in the same boat as you. My approach is for better(?) LESS distortion of AM audio for music. Shortwave listening and the vagaries that take place there.
There must be some secret to unlock here. I do not have $700.00 for that sync detector product from Sherwood.
Fred

[NA3CW]

The Yaesu FT-920 has an IF feed to the AM detector at 8-ish MHz.  If it were, say, 455 kHz, I'd use an "ideal rectifier" circuit using diodes in the feedback networks of op amps. I've used them for years in fields like ultrasonics. They're really linear and work very well. To do it at 8 MHz would take fast diodes and really fast op amps.  I'll have to research it a bit.

73,
Chuck
NA3CW
 

[IN3IEX]

I use the following one by Analog devices.
Great sound and no distorsion with 100% AM modulated signals.
I put it in a Drake R4B. Very satisfied.
I got a free sample from AD.

http://amfone.net/Amforum/index.php?topic=29362.0

It is a squarer followed by a square rooter, i.e. an almost ideal rectifier. It goes from DC to 2.5 GHz. A miracle.

Regards

Giorgio

[NA3CW]

Miracle, indeed!  Definitely going to study this one.

Just before your post I was looking at this device used in full-wave rectifier mode:

http://www.analog.com/static/imported-files/data_sheets/AD8036_8037.pdf

It, too, looks like it has possibilities.

73,
Chuck
NA3CW