Warning: Creating default object from empty value in /homepages/11/d132647312/htdocs/Amfone/mkportal/include/SMF/smf_out.php on line 47
simple interface for receive REA modulation monitor




 
The AM Forum
December 13, 2017, 12:29:51 AM *
Welcome, Guest. Please login or register.

Login with username, password and session length
 
   Home   Help Calendar Links Staff List Gallery Login Register  
Pages: [1]   Go Down
  Print  
Author Topic: simple interface for receive REA modulation monitor  (Read 1031 times)
0 Members and 1 Guest are viewing this topic.
KA7SFL
Member

Offline Offline

Posts: 3


« on: May 19, 2017, 12:02:05 PM »

Hello,
I have been reading over some of the messages dealing with WA1QIX REA modulation monitor. Steve mentions using a simple interface to monitor the receive of a station during a QSO.
I have a REA modulation monitor, and would be interested in how to build the interface. Maybe Steve could post the schematic and hook up diagram here on this site.

Tom W7TJM (formerly KA7SFL)
Logged
steve_qix
Contributing
Member
*
Offline Offline

Posts: 2292


Bap!


WWW
« Reply #1 on: May 19, 2017, 03:17:07 PM »

Anything that will provide about 3 or 4 V RMS (RF voltage) to a direct-connect RF pickup will work.  The circuit I have will certainly work - it contains the detector and some other features as well.

However, there are a number of relatively inexpensive RF amplifiers that can boost a small signal (something under 100mv) and bring it up to at least a watt into 50 ohms.  One of these could also work quite well.

I will post the circuit that I am using to connect the REA monitor to a receiver 455kHz IF.  This circuit has some features such switching the mod monitor from the receiver to the transmitter when the station is transmitting (that is a very good thing to do !), a level indicator and some other stuff as well.
Logged

High Power, Broadcast Audio and Low Cost?  Check out the class E web site at: http://www.classeradio.org
KA7SFL
Member

Offline Offline

Posts: 3


« Reply #2 on: May 19, 2017, 03:33:17 PM »

Thank you Steve, for the quick response and info. I'll be looking forward to seeing your post.
It might even be something you might consider selling along with your modulation monitor.
I am well pleased with the one I purchased from you, along with the two pickups.
Tom
Logged
AB2EZ
Member

Offline Offline

Posts: 1700


"Season's Greetings" looks okay to me...


« Reply #3 on: May 20, 2017, 05:13:52 PM »

Steve: You might want to comment on this:

A few years ago, I experimented with using the amplified/buffered 455kHz i.f. output of a 75A-4 receiver to drive my hardware-based, 1st generation, REA modulation monitor. In those experiments, I included a homebrew, slow acting, carrier level tracking/AGC circuit, in front of the REA modulation monitor, to automatically center the carrier level indicator of the 1st generation REA modulation monitor.  I haven't tried this with my 2nd generation hardware/software based REA modulation monitor... which may have different time constants associated with how it tracks the carrier level of a signal whose carrier level is moving around (due to fading and due to the AGC action of the receiver).

The bottom line was that... at least in those earlier experiments... it was difficult to obtain a useful measurement of the modulation indices (positive and negative) of a received signal because of two factors:

The rate at which received signals fade up and down... in many band conditions... would not allow for the stable centering of the carrier level indicator... even with my add-on homebrew carrier level tracking/AGC circuit in place. [However, I did not experiment with adjustments to this circuit to allow faster tracking of the carrier level]

The AGC circuit of a typical receiver employs a peak detector... and, therefore, responds to the peaks of an AM signal (not the average value... i.e. the carrier level).

I changed the design of the AGC circuit of my 75A-4... to respond to the time-varying carrier level of the received signal. The new AGC circuit consisted of a fast-acting envelope detector (essentially an AM demodulator)... followed by a buffer amplifier and a low-pass filter with a cutoff frequency of around 50Hz. This made the audio output of the receiver sound better on AM signals, compared to the receiver's stock AGC circuit. This also helped, in reducing the effect of moderately slow fading of the received signal on the behavior of the modulation monitor.

Stu
Logged

Stewart ("Stu") Personick. Pictured: (from The New Yorker) "Season's Greetings" looks OK to me. Let's run it by the legal department
steve_qix
Contributing
Member
*
Offline Offline

Posts: 2292


Bap!


WWW
« Reply #4 on: May 21, 2017, 09:31:48 PM »

The new software display modulation monitor has AGC in both the hardware interface and in the software.

However, as Stu pointed out, selective fading will adversely affect the readings from the modulation monitor.  Under good, stable conditions, the readings will be reasonably accurate.

The quality of the receiver (particularly the AGC system) is very important if you want accurate off-the-air readings.   As Stu very eloquently stated, the AGC system in many communications receivers reacts to peaks and not the average carrier level.  This means more AGC is generated on peaks than if the signal is unmodulated.  This is very bad for AM in general, and any receiver exhibiting this behavior should be modified so that the AGC works off the average and not the peaks.

I use the AMM-SD1 mod monitor on my receiver all the time.  The receiver is sufficiently broad to allow for good, accurate modulation readings from received stations.

But, the readings are only as good as the receiver and band conditions.

Logged

High Power, Broadcast Audio and Low Cost?  Check out the class E web site at: http://www.classeradio.org
KH6KK
Member

Offline Offline

Posts: 4


« Reply #5 on: June 01, 2017, 06:51:23 AM »

Thanks for that schematic. I've been wanting to interface the monitor to an SDR which has no 455kHz IF. I wonder if I can remodulate the demodulated RX audio to 455kHz with a DDS transmitter and than feed it into the Mod monitor interface.?

 I have one of these TX that might work :

http://www.ebay.com/itm/DDS-AM-TRANSMITTER-MODULATOR-100KHZ-2MHZ-GENERATOR-NO-PLL-/282489191858?hash=item41c5aafdb2:m:m1hOgP7bHDdOITCavcFbX0A

 Do you think this is doable? Not sure on the accuracy as it might defeat the purpose of the Mod Monitor except for watching the meters swing on RX? Wink Thanks, Joel
Logged
WA2SQQ
Contributing
Member
*
Offline Offline

Posts: 452


« Reply #6 on: June 01, 2017, 08:06:16 AM »

Same situation here - wish it were possible to get the mod monitor hooked up to a Flex radio
Logged
KD6VXI
Contributing
Member
*
Offline Offline

Posts: 1632

Making Amplitude Modulation GREAT Again!


« Reply #7 on: June 01, 2017, 12:12:55 PM »

That's something that needs to be programmed into the flex software.

A couple years ago, a European ham pasted a plug in for SDR# that gave pos and neg peaks both with an oscope display and in numerical format.

Once you've demo dilated the audio, you can't remodel ate it into something (like someone said about using it at a 455 kHz oscillator and impressing demod audio on to it.  You'll then be measuring the modulation pct on your dds, not the original carrier.

For you sdr guys w no if, maybe the sdr# plug in can be made to work with your software?  It was posted to this site a while back.

--Shane
KD6VXI
Logged
steve_qix
Contributing
Member
*
Offline Offline

Posts: 2292


Bap!


WWW
« Reply #8 on: June 01, 2017, 08:34:06 PM »

Same situation here - wish it were possible to get the mod monitor hooked up to a Flex radio

You can definitely use the mod monitor with a flex, (or anan or whatever), just not for receive.  The primary purpose *is* to monitor transmitters, and not received signals, which are subject to all sorts of problems that interfere with the accuracy of the readings.

Logged

High Power, Broadcast Audio and Low Cost?  Check out the class E web site at: http://www.classeradio.org
DMOD
AC0OB - A Place where Thermionic Emitters Rule!
Contributing
Member
*
Offline Offline

Posts: 1052


« Reply #9 on: June 05, 2017, 01:17:00 AM »

I purchased an REA monitor recently and it works very well.

I noticed that even with my low power SGM rigs, it will capture low power signals after it "learns" your power level. I assume that's the AGC feature?

Phil - AC0OB
Logged

"What kind of Koolaid do they make you drink in the Physics Department?" Charlie Epps to Dr. Larry Fleinhardt, NUMB3RS   Smiley
KA7SFL
Member

Offline Offline

Posts: 3


« Reply #10 on: June 05, 2017, 10:47:17 AM »

Hi Steve.
By any chance do you have circuit boards, or a complete LPAM/receiver interface available.
I have essential tremors, so my soldering skills now are terrible.

Tom
Logged
Pages: [1]   Go Up
  Print  
 
Jump to:  

AMfone - Dedicated to Amplitude Modulation on the Amateur Radio Bands
 AMfone 2001-2015
Powered by SMF 1.1.21 | SMF © 2015, Simple Machines
Page created in 0.083 seconds with 19 queries.