(Now a bi-weekly program)

March 26, 2013

Join the "CWTD Yahoo Group" for email discussion in between our bi-weekly sessions by clicking here.

1) AM Mode: Principles, Practices and Embedded Implementations
2) RTC Software ... for our Precision Arduino Clock


Our episode this time focuses on two subjects ...

1) AM Mode -- We first consider a venerable operating mode that many of Old Timers grew up using: Amplitude Modulation. While certainly not as popular as SSB and the digital modes, and quite power inefficient, AM is still enjoyed by some today for its audio quality perspective and the nostalgia of operating with the boat anchors of yesteryear. We'll take a brief look at the technology and construct of AM operation, as well as how it is implemented in even one of the most modern SDR rigs today: the SDR Cube Transceiver.

2) Precision Arduino Clock -- Our second focus for the episode is on our continuing group project: the Precision Arduino Clock. In the last couple of episodes we covered the basics of this prescient design, both from a hardware and software perspective, using initial methods for timekeeping and synchronization with WWV and CHU. This time we implement the software for a key element for moving forward: the Real-Time Clock. Surprisingly, the Arduino sketch (software) is vastly simplified and we have the stage set now for making the Arduino and our CWTD shield (hardware) a very useful addition to anyone's shack.

73, George N2APB & Joe N2CX

Listen to the podcast ... here

Online text discussion during the program.

Lots of good discussion in this session! Skip to the bottom of it to see the start of the Whiteboard Notes.

<20:08:06> "W2AEW - Alan": CW is just AM taken to the extreme!
<20:11:13> "George - N2APB": Taken "down" to the extreme!
<20:11:49> "Mike WA8BXN": W2OY
<20:12:14> "Pete - WB2QLL": The Johnson Invader is a sideband transmitter that sounds lousy on AM unless hugely modded.
<20:13:07> "George - N2APB": I'll next explain that that Invader is MINE!  It's my main station here in the shack, along with the R-390.
<20:13:22> "George - N2APB": Lots of mods, yes, but it actually has an AM poosition on the Mode dial.
<20:13:43> "Ray K2ULR": http://w2dtc.com/w2dtc-sound-bites/w2oy-no-lids-no-kids.wav
<20:14:01> "Howie K3HW": I have permission to operate AM this week during Passover from my Rabbi
<20:14:43> "Joe N2CX": We'll pass over that, Howie.  Make sure your RF is unleavened!
<20:15:11> "Mike WA8BXN": DX-60 net, Sunday mornings, 3880, starts at 8 AM, NCS is in MI, there are some 2 land stations that check in
<20:15:46> "Howie K3HW": Hi Hi Joe - I have matzoh crumbs in my heavily modified DX-60
<20:16:48> "Joe N2CX": George's station will NEVER blow away in a hurricane!
<20:17:40> "Charles WC5MC": LoL
<20:19:26> "W2AEW - Alan": You can view your own tx signal on a scope - here's an easy way to do it: http://www.youtube.com/watch?v=2D83xp3H5Bo
<20:24:53> "Rick K3IND": How about AM stereo?
<20:26:01> "Clint-ka7oei": A low-level CXR can be used in a synchronous detector.  A pilot CXR is used in ACSSB - Amplitude Compandored SSB.
<20:32:10> "Brent KD0GLS": How does DSB sound on the air compared to AM or SSB?
<20:33:06> "George - N2APB": I would think that DSB sounds the same as "normal" AM mode.  Same BW?
<20:33:07> "Clint-ka7oei": DSB is very tricky to tune properly on a DSB receiver.
<20:33:25> "Clint-ka7oei": No problem on a typical SSB RX.
<20:34:14> "George - N2APB": And the audio fidelity?
<20:34:14> "W2AEW - Alan": DSB sound like SSB when received in SSB mode.  It would sound similar to AM mode if product detected, but BFO adjustment for carrier insertion must be exact
<20:36:36> "W2AEW - Alan": s-meter is usually driven by IF-AGC level
<20:36:43> "Clint-ka7oei": Radio Nederland was experimenting with 6dB reduced carrier on their Bonaire SW transmissions many years ago - only a slight increase in distortion on envelope detector receivers.
<20:39:38> "Mike WA8BXN": old rules limited power input, rather than power out as is the case now
<20:40:40> "Joe N2CX": Right Mike - much easier to accurately measure RF out these days.
<20:40:42> "Clint-ka7oei": 1kW DC plate input (on a carrier) yields about 2.4 kW PEP output assuming a 60% efficient RF amp - well above today's 1.5kW PEP!  (On a 100% modulated AM signal)
<20:44:37> "Howie K3HW": I use my modified DX-60 with a Symetrix 528E Audio Processor and an MXL-770 Condenser Studio Microphone - Get close to broadcast quality audio...
<20:49:06> "Joe N2CX": Howie I think the added audio gear costs more than the rig!
<20:49:57> "Howie K3HW": SK estate sale up here in NE PA - $100 for the processor and the mike. Originally paid $0 for the DX-60. $60.00 for the DX-60
<20:58:41> "Mike WA8BXN": program not recompiled each time you run it if no changes made to the source?
<21:04:21> "W2AEW - Alan": are you using an interrupt for the PB, or just doing some sort of query?
<21:06:35> "Lee KM4YY": Where exactly can I can I get the parts to catch up with you guys?
<21:07:14> "Clint-ka7oei": DS3231
<21:10:12> "Pete - WB2QLL": good enough is best :)
<21:11:03> "W2AEW - Alan": Dave - you and me both.  I'm in a hotel room in MA tonight
<21:13:22> "Dave KD2E": Alan....I'm in central Pa...too much lately!  Gonna make me quit my job
<21:15:20> "Dave KD2E": What means "I2C" ?
<21:15:43> "Brent KD0GLS": Inter-IC Comm.
<21:16:21> "W2AEW - Alan": to add to that - it is a fairly simple, low-speed serial data bus between integrated circuits.
<21:16:35> "Brent KD0GLS": Correction: Inter-Integrated Circuit (comm between ICs)
<21:17:03> "George - N2APB": It's a 2- wire, bi-directional serial communications hardware link (with companion software protocol) that allows devices to communicate with each other.  Also called TWI for "Two Wire Interface"
<21:25:18> "Charles WC5MC": Looky O.O ! :) http://www.kickstarter.com/projects/digistump/digispark-the-tiny-arduino-enabled-usb-dev-board?ref=card#WhatMicrContChipIsBeinUsed
<21:25:25> "Brent KD0GLS": Another plus is that that IDE is cross-platform.  
<21:27:34> "AL - N8WQ": http://www.kickstarter.com/projects/18182218/freesoc-and-freesoc-mini
<21:27:51> "Joe N2CX": Kickstarter - I got a Bora, Digital Explorer.
<21:28:45> "Dave KD2E": George...How 'bout driving Nixie's....or blue flourescent??
<21:29:12> "W2AEW - Alan": Love nixies!  I have a nixie clock in my shack
<21:30:31> "George - N2APB": Okay guys, where did you get the nixie kit, or parts, or minimally the design.  I want one too!
<21:31:14> "Brent KD0GLS": My implementation: a 180V CPU-driven boost power supply and 74141 Nixie drivers.
<21:32:12> "Dave KD2E": Got mine about 5 years ago...right after the IEEE did a big article on NIXIE's in one of their publications.  Suddenly, many NIXIE clocks appeared on the market...using surplus Russian tubes
<21:32:41> "Brent KD0GLS": I'm looking at a one-tube Nixie clock right now being driven by a single 14-pin MCU running from 5V USB power.  Homebrew.  
<21:33:55> "Charles WC5MC": very good show Joe and George  :)
<21:34:13> "Charles WC5MC": Horray for linux tinkering.  The more you windows fellas experience a little linux, the better you can discern how the beagle/raspberry boards fit in just above Arduino microcontrollers and other microcontrollers.
<21:34:43> "Pete - WB2QLL": Sadly Nixies have a rather short life in clocks.
<21:35:32> "George - N2APB": Links, links, I need links for the nixie projects
<21:35:59> "Pete - WB2QLL": www.amwindow.org 
<21:42:11> "George - N2APB": Allband SDR for the Masses ... Pete's posting in Softrock group.
<21:42:27> "Dave KD2E": http://tubehobby.com/store.php?cat=1       nixie clock kit with GPS input. They also sell the GPS receiver.
<21:42:31> "George - N2APB": Pete uses a Heath SB1400 power amp
<21:45:27> "George - N2APB": Heath SB-1400 ... Yaesu 747
<21:45:41> "Charles WC5MC": here is the product page for the $8.95 USD Attiny85    Arduino  Dev Board http://digistump.com/product.php?id=1
<21:48:45> "George - N2APB": Johnson Viking Avenger ... http://forums.qrz.com/showthread.php?361632-E-F-Johnson-Viking-Avenger-hybrid-80-10-transceiver
<21:50:36> "George - N2APB": Mods.dk ... Pete's manual for the Avenger, which sold for $247
<21:53:53> "Terry WB4JFI": 16-pin I2C expander is PCF8574
<21:54:51> "Terry WB4JFI": YES, 60 LEDs would look cool.
<21:54:55> "Charles WC5MC": Sounds like fun and experience for I/O Expander
<21:54:59> "Brent KD0GLS": George, look into "Charlieplexing" to minimize I/O pins needed.
<21:55:17> "Clint-ka7oei": Look at the "BulbDial" clock for some inspiration.
<21:55:24> "Clint-ka7oei": That clock uses "Charlieplexing"
<21:55:51> "Clint-ka7oei": Maxim Semiconductor
<21:57:09> "Bob - N2OJJ": there's a cool shadow clock uses 3 rings of leds around a small point
<21:57:14> "Clint-ka7oei": Dual-color LEDs:  Red, Green and also Yellow - all do-able with C'plexing
<21:57:19> "Clint-ka7oei": That's the Bulbdial clock.
<21:57:23> "Bob - N2OJJ": the leds cast colored shadows on the face
<21:57:35> "Terry WB4JFI": PCF8575 is a larger, 16-bit I2C chip.
<21:58:36> "Bob - N2OJJ": check this out  http://www.evilmadscientist.com/2010/on-the-design-of-the-bulbdial-clock/

Part 1: AM Mode ... An Old Friend Coming Home

A discussion of one of the early voice modes - principles, practices and embedded implementations

Amplitude modulation, also known as Ancient Mode or Admirable Mode

The oldest voice modulation mode ...

Watch the YouTube video


Spark was an AM mode as well – carrier modulated by raspy bark of spark gap

AM Rigs of “Today”

EF Johnson Viking Invader 2000                                        Collins R-390 Receiver

Yaesu FT-9000D


The waveforms

o Carrier … A stable sine wave at the RF frequency, say at 3900 kHz
o Modulation … An audio waveform like voice, or perhaps a 1 kHz tone shown
o Modulated Carrier … the time-based view of the transmission.

Transmitting AM … the AM Modulator

- Can be designed using tubes from “yesterday”, or semiconductors (today’s technology)

Traditional AM: feed plate of power amplifier device with DC and modulating audio

Or mix carrier and modulating audio on base of transistor …

SSBSC - filter or phasing method

Full carrier SSB – generate SSBSC and reinsert carrier

Transmitted waveforms (in Time Domain)

o The modulation process creates sidebands at Carrier +/- the frequency(s) of the modulating signal

o Upper Sideband (USB) and Lower Sideband (LSB) contain identical information of the modulating signal

Transmitted waveforms (in Frequency Domain)

o Carrier plus two sidebands containing the modulation

Presence of carrier allows easy demodulation of AM with simple diode detector

◦ But the total sideband power is only half the unmodulated carrier

Single Sideband Suppressed Carrier (SSBSC) gives much better efficiency by eliminating one sideband and the carrier

All power concentrated in the one sideband

Spectrally efficient – whole signal in half bandwidth
Less selective fading than AM
But more difficult to demodulate and often has “donald duck” sound

Full carrier SSB (Compatible)

Uses only one sideband (upper or lower) plus carrier

Spectrum efficiency twice as good as AM

Simple diode detection works with it

Less selective fading than AM


Let's say we are running a 1 kilowatt (output) AM transmitter, legally. When there is no modulation the output power is 1,000 watts. When it is modulated to 100 percent the power in the carrier does not change but additional power is put into the sidebands. As mentioned earlier the voltage of each sideband when a test tone is being transmitted it 100% modulation is 50% of the carrier voltage. 50% voltage is 25% power, test it your self with ohms law and P=IV. That means the power in each sideband is 250 watts. So the total for the carrier plus both sidebands is 1500 watts. That extra power comes from the modulator tubes.


Peak envelope power (PEP) is the average power supplied to the antenna transmission line by a transmitter during one radio frequency cycle at the crest of the modulation envelope, under normal operating conditions.

The PEP output of an AM transmitter at full modulation is four times its carrier PEP; in other words, a sold-state, 100-watt amateur transceiver is usually rated for no more than 25 watts carrier output when operating in AM

Similarly if you have a two tone signal modulating an SSB(SC) transmitter, the PEP of the composite signal is four times the power of a single tone.

This is because the peak voltage of the two signal combination is twice the voltage of each one individually.   Since power is the square of voltage, the power is four times for two signals.

This is typical for AM and SSB since the transmitter becomes non-linear as power is increased and this occurs at the peak of the modulated envelope. Thus the PEP rating is at or (more typically) less than the CW rating to keep the power amplifier from becoming non-linear.

AM Mode on the SDR Cube Transceiver

A major feature “AM Mode” has been added to the SDR Cube Transceiver software, enabling the operator to select and use the AM mode for voice communications. Mode selections of either AM▲ _(upper) or AM▼ _(lower) provide a unique and powerful approach for AM reception, and an efficient method is implemented in software to generate AM transmissions. This latest software release gives the SDR Cube owner a distinctly capable way to communicate in AM mode, and when coupled with some RF power amplification he will be able to participate in AM discussion nets on the air with other hams using this classic mode.

AM Reception

AM reception is accomplished in SSB mode, by selecting and listening to either the upper sideband of the transmitted AM signal, or to its lower sideband. The spectral information is the same and the operator can decide which sideband to listen to. This is a unique and useful feature in that one can avoid interference and noise signals by selecting the quieter sideband. No ordinary AM receiver can do this.
Hence we use AM▲ and AM▼ symbols for the Mode indicators on the Cube display. In receive, the Cube’s bandscope shows the AM signal at its original place around zero (carrier) frequency.

AM Transmission

AM transmission always occurs at the dial frequency. With I-Q modulator, the AM carrier generation is somewhat unconventional. First we generate an audio signal (a CW tone) that is amplitude modulated with the baseband audio signal, and then the AM carrier offset is added . This signal is then fed to the SSB modulator where the carrier frequency is set “CW offset” amount off so that the AM carrier appears at the dial frequency. The easiest way to verify this is to transmit AM without modulation so just the carrier frequency peak would be visible in a spectrum analyzer.


Where is AM Used on the Bands?


AMfone came into existence as we all know it in the year 2002. It is the combination of three websites that were started in the 90's.

- Brian WA5AM's - AM North America
- Steve WB3HUZ's - The AM Window BBS, and
- Gary W2INR's - The AM Classifieds

Here is Brian's original concept for AM North America (AMfone)

“I started this site back in the late part of 1997 with the intent of promoting the use of Amplitude Modulation on the Amateur bands. It was, and never will be intended as my personal home page, but rather a page for each and every enthusiast of AM.”

Much, much more is at … http://amfone.net/Amforum/index.php


1. http://en.wikipedia.org/wiki/Amplitude_modulation
2. ARRL Handbook – any publication date
3. Peak Amplifier for an RF Wattmeter- <http://vk1od.net/module/PepAmp/PeakAmp.htm>
4. Agilent Fundamentals of RF Power Measurement - <cp.literature.agilent.com/litweb/pdf/5965-6630E.pdf>
5. Modulation Basics .. http://www.angelfire.com/electronic/funwithtubes/Basics_06_Mod.html
6. AM Fone Website … http://amfone.net/Amforum/index.php
7. Wadsworth Sales … http://www.wadsworthsales.com/Pages/amphone.aspx
8. Retro-75 AM Transceiver, by Small Wonder Labs

Part 2: Real-Time Clock Software for our CWTD Precision Arduino Clock

This time we present the software "sketch" used for storing and recalling time from the Real-Time Clock chip

Project Resources

Shield Schematic -- see below

Phase 0/1 Sketch -- Software for basic clock with manual register count-up (for software training purposes) and sync with tone decoder via CHU signal. Also handles external sync input.

Phase 2b Sketch -- Software now uses the RTC chip for date/time retention and timing. Current time set from computer when you first load the sketch onto the Arduino ... no more need for cumbersome serial console and "user time setting" menus!! Also no longer depend on "wait 1000ms" for timing ... we now let the RTC chip do it and we look for the transition of its output pin, clocking at 1 Hz interval. LED added to show RTC heartbeat. Software greatly simplified. Needs the RTClib (see next item).

RTClib library... unzip, rename folder to RTClib and place into your Arduino libraries folder.

DS1307 Data Sheet (RTC chip)

LMC567 Data Sheet (Tone Decoder)

Arduino Uno R3 Data Sheet

Rookey Tone Decoder Board   

Precision Arduino Clock Functional Flow Chart


Precision Arduino Clock Breadboard Prototype


Phase 0: Tone detection at top of minute from WWV or CHU

Phase 1: Addition of 1pps input signal from GPS for seconds sync

Phase 2: Addition of RTC chip to maintain date/time

Phase 3: Addition of NMEA input from GPS for date/time setting & sync

Phase 4: Addition of USB serial input from PC for sync via Internet

Phase 5: Addition of MC145442 modem chip to decode WWV digital data


"Official" info on I2C communications with the Arduino using the Wires : <http://www.arduino.cc/en/Reference/Wire>

Sample project using I2C expansion with Arduino: <http://www.neufeld.newton.ks.us/electronics/?p=241>

Sample I2C Arduino sketches: <http://digitalcave.ca/resources/avr/arduino-i2c.jsp>



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