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simple interface for receive REA modulation monitor




 
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KA7SFL
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« 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)
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« 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.
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KA7SFL
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« 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
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« 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
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« 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.

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