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I have found the pre-production prototype of the new K7DYY modulation monitor kit very useful for my AM operation. You
could use it with any AM gear, from Johnson Ranger class transmitters, right up to full legal limit. It is all analog, no
noisy embedded microprocessors.
(Click on images below to enlarge in new tab)
K7DYY Modulation Monitor front view
(You may customize yours as you wish.)
My legal limit AM station
(See below for components.)
K7DYY Modulation Monitor top view
(Note size of PC board furnished!)
My legal limit AM station components, top to bottom:
- Homebrew Transmatch
- K7DYY 80 - 40 375 Watt Transmitter
- K7DYY Modulation Monitor
- K7DYY 160 - 80 375 Watt Transmitter
- Inovonics 223 multiband audio processor
- BBE EQA231 Graphic Equalizer
- Behringer VX2496 audio preamp/compressor
To obtain your K7DYY Modulation Monitor kit, contact Bruce Franklin at
indexlabs@gmail.com. Pricing should be on the order of $200.
FEATURES
- CARRIER OUTPUT LEVEL, about 25 watts up to 500 watts (legal limit) on LINEAR SCALE
- Carrier meter will reveal carrier level shift during modulation. Carrier shift may happen in legacy transmitters which
share a common high voltage supply between the RF and Modulator tubes. This can be impossible to read on competing displays.
- Carrier level final calibration can be adjusted to agree with your own shack reference wattmeter
- NEGATIVE MODULATION peaks in percent
- POSITIVE MODULATION peaks in percent (can exceed 100%, if your transmitter is capable)
- Both Positive and Negative modulation accurate calibration is easily accomplished using an internal DC reference
level (no exotic test equipment needed)
- ON AIR LED indicator when carrier is present; modulation % accurate when lit
- OVER MOD LED indicator automatically flashes when negative peaks hit 100%
- OVER MOD LED is FACTORY CALIBRATED, no user calibration needed
- AUDIO output from a low distortion detector of actual RF signal (HI impedance, about 1 volt) which could be used to drive
a headphone amplifier or a distortion analyzer
- The enclosure shown (not included) is a rack panel from
frontpanelexpress.com. Besides the engraving and painting, it already has the large holes
for the meters. This saves a lot of machining, and gives a finished professional look. The software for duplicating the panel is
included, so you can simply order the panel without having to design it yourself.
- Custom meter scales for recommended meters are also included, or make your own
- REMOTE MONITORING of a converted AM broadcast transmitter located in a garage, with the meters at your desk operating position
CUSTOMIZATION OF ENCLOSURE AND METERS
The K7DYY Modulation Monitor Kit can be configured in a number of ways. Mine is a 2U rack panel, to match my K7DYY Super
Senior transmitters. You could also use a smaller stand alone box, depending on the size of your analog meters, even edgewise
meters. If you are building a home brew transmitter, you could incorporate the K7DYY Modulation Monitor into the cabinet of
your rig. This would be equally at home in a classic tube transmitter as well as a class E solid state AM rig. It would also
look great in the rack of a repurposed former broadcast transmitter.
The meter scales shown were created using software at http://www.tonnesoftware.com
Use the tool called METERBASIC. The scales supplied by K7DYY are 120% full scale. You could make your positive peak meter
full scale reading something like 140%, if you wanted to. You could even use a VU meter scale similar to the REA modulation
monitor, for a more classic look. Tonne software
also has programs for filter design and other useful products for RF designers. Check it out.
WHAT IS INCLUDED IN THE KIT?
- An RF sampling head suitable for up to 2000 watts PEP, calibrated for 50 ohm RF loads in the amateur HF spectrum.
- The electronics board, about the size of a Triscuit cracker
- Plugs and cables suitable to interface with the meters and power supply
- Sets of custom meter scales sized to fit the meters recommended below
- Software files used by Front Panel Express for custom front panels to fit the recommended meters
OTHER PARTS YOU NEED TO FURNISH TO COMPLETE THE KIT
CHOICE OF METERS FOR THE MODULATION MONITOR
The meters I used were from allelectronics.com.
They are only $14.50 each!
The exact part number for the one I recommended, 1 mA meter is PMD-1MA. Here is the link:
https://www.allelectronics.com/item/pmd-1ma/1-ma-dc-panel-meter/1.html
Bruce also recommends a somewhat larger 1mA meter Uxcell Part Number 44C2/1ma which is available from
harfington.com as part id1082663 and also from Amazon as Uxcell
Analog Current Panel Meter DC 0-1mA 44C2/1mA.
If your junkbox contains a matched set of 1 mA meters in a classic round style, you could even use those for your vintage
station. Meters in the uA range may be more damped in mechanical response. I preferred the action of the 1 mA meters I tried.
IMPORTANT: SET R22 AND R24 TO MAXIMUM COUNTERCLOCKWISE BEFORE POWERING UP FOR
THE FIRST TIME TO AVOID METER DAMAGE. The adjustment pots are wide range to accommodate your choices, but
the meters can be damaged if the control is misadjusted. These are high quality 25 turn pots for easy adjustment and best accuracy.
COMPARISON TO OTHER METHODS OF MODULATION MONITORING
There are some meters (DOSY and Micronta) you will see at hamfests targeted at the CB market, but they do not display positive
and negative modulation separately. Their accuracy outside of CB frequencies is unknown, and they do not include an OVER MOD flasher.
The K7DYY Modulation Monitor kit, at less than $200 might be just right for you too. The 3 meters will cost an additional $14.50,
but you might have some in your junkbox that will work. The enclosure is as fancy as you want to make it, and could be built
right into a homebrew AM rig. It does give you a lot of freedom to create a monitoring system exactly to your liking.
HOW DOES THE K7DYY MODULATION MONITOR WORK?
It has similar circuitry to some legacy commercial AM broadcast monitors. The sampling head is well made. I tested mine using a
Siglent SSA 3021X spectrum analyzer and it is flat to better than 0.5 dB over the MF/HF spectrum. The sampled RF goes via a short
SMA cable to a frequency compensation network and then to a precision detector comprised of a balanced pair of transistors.
The RF detector drives an AGC network which generates a stable carrier reference DC voltage for the modulation comparators.
A photoresistive device is used in the feedback loop for linear response. It results in a linear RF power display on the Carrier
Level Meter. The AGC voltage also drives the ON AIR LED circuitry.
The precision detector also provides low distortion audio (for an external output) as well as the detected modulation,
which is routed to precision rectifiers (one for positive modulation, one for negative modulation). Those signals are applied to peak hold
circuits for positive and negative varying DC proportionate to the modulation peaks. The final output to the meters compares the
detected Carrier Level with the processed audio signal, and displays it on a linear scale.
REFERENCE READING MATERIALS
Commercial AM Broadcast Modulation Monitor Manuals
DISCLAIMER
I, Janis Carson AB2RA, am not an employee of, nor do I receive any compensation from, index labs.
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