Tag Archives: Elmer

A Block Diagram for the LNR Precision MTR3b

When I was a young child, there was nothing more dangerous in the house than me and a screwdriver. I took everything apart. Nothing was safe. I wanted—no needed—to see the insides of things, and see how they worked. At some point, learning how to put things back together was the only thing that kept me from a seriously sore backside. Still, every bit of it was worth it, and I have retained that kind of curiosity throughout my life and my career. 

Things are a little easier today. You can understand a lot from just looking at the drawings for a device. One of the things I enjoy doing is looking over radio schematics and trying to understand how they work, and the choices that the designers made. In this article, I’d like to do that for the three-band Mountain Toper MTR3b, a radio designed by Steve Weber (KD1JV) and sold by LNR Precision.

The MTR3b is a CW-only, 3-band (20/30/40) radio that can fit in the palm of your hand. The photograph of the radio with an accompanying 9-volt battery gives you some idea of its size, though you don’t truly grasp the compactness of the unit until you hold it in your hand. The 9-volt battery is not just a prop for the photograph. The radio can be powered by a single such battery, though it works better with something a little larger. I use a compact 11.1-volt LiPO battery packs made popular by drone users.

I’m not going to provide a detailed review of the user interface (just 3 slide switches, four buttons, and a single 7-segment LED), or go through its on-air performance. There have been several excellent reviews for this radio in QST and other places. Instead, I’d like to take it apart for you. Well, not exactly disassemble it as much as analyze the schematic and see how Steve Weber made it work.

I was lucky enough to have a fellow named Mr. Davis for my high school physics instructor. He said many wise things (that I only understood long after high school, alas), but one that stuck immediately was his adage, “If you can’t draw the picture, you don’t understand the problem.” How very true! So, I often made models and block diagrams of things to better understand them. These models don’t need to be exact. In the words of George Box, “All models are wrong, but some are useful.” A block diagram of the insides of a transceiver can be useful. Here’s the diagram for the MTR3b I sketched on my iPad Pro tablet a few evenings ago.

Let’s walk through it. The antenna is on the far left. The antenna connects to the radio and is presented to a low-pass filter (LPF). As it turns out, each band requires its own special LPF so there are slide switches on the front of the radio to switch-in the correct filter. Switch position 1 is for 20m, switch position 2 is for 30m, and switch position 3 is for 40m. The way the radio is designed, as we will see, it is important that all three switches be in the same relative position for the radio to operate properly. 

The next thing in line as we walk towards the receiver is a transmit-receive (T/R) switch. When we transmit, the signal path is cut off from the receiver and the transmitted signal only goes to the antenna. When we are receiving, the signal path is routed to the receiver, pictured along the top of the diagram.

After we pass the T/R we have a band-pass filter (BPF), also selected by one of the slide switches, that rejects out-of-band signals. This is the lead-in to the first mixer that generates the intermediate frequency.

We should take a quick detour for a moment and review what a “mixer” does. A mixer is an electronic device that takes two signals in and yields four signals out. It sounds complicated, but it really isn’t. Say we have two signals of frequency A and B. The mixer will output those same frequencies A and B (not very interesting), and two other signals: A-B and A+B. These signals are interesting. 

Superheterodyne receivers like this one mix the received signal with one from the radio (tied to the main tuning system) to produce an intermediate frequency that can then be further processed. By turning the original signal received from the antenna into this intermediate frequency, we can have a system common to all three bands 20m/30m/40m in the rest of the receiver. This common system works because it only has to deal with signals at this selected intermediate frequency. The intermediate frequency selected by Steve Weber for this design is about 4.1 MHz.

After we mix the signal with the first mixer (illustrated as a circle with an X in it), we pass the resulting signals through a crystal filter that allows only the 4.1 MHz-related frequencies through it. This is the A-B output from the mixer. The other products are discarded. Once through the crystal filter, we now have a nice signal from our selected frequency, but it is in the 4.1 MHz range. We need a subsequent step to bring it down to the audio frequency range.

The product detector that produces the audio frequency range signal from the 4.1 MHz intermediate frequency is obtained by passing the signal through a second mixer. When we’re through here, we have something our ears can hear. All that is left is to bring that audio signal through a few amplification stages, and send it to the headphones. This radio is so simple that it doesn’t even have a volume control! 

Just before the headphones is a multiplexor that allows audio to be selected from either the receiver (as we’ve just done above), or from the sidetone generated when we send Morse code. The CPU generates the sidetone signal and controls the multiplexor. This is the whole receiver.

The transmitter is even more simple. The Texas Instruments extremely-low-power CPU watches the paddles, slide switches, and push buttons, and controls the 7-segment LED display. When dot or dash paddles are pressed, it signals to the DDS (Direct Digital Synthesis) frequency generator to output a signal. (The DDS is also used to select the receive frequency.) The output signal is sent through a few amplification stages, the T/R switch is changed to transmit, and the amplified signal is sent to the antenna.

I was able to get all this just by perusing the schematic included at the rear of the MTR3b manual. You can download the manual for free from the LNR precision website and follow along from the block diagram I’ve provided.

Every time I do this for a radio I feel like I’ve learned something new. It is also gratifying to use a radio that you understand, and know how it works under the hood. 

http://www.lnrprecision.com

Scott, NE1RD

N1FD Multi-Op Team #1 in Area 1 in 2018 Rookie Roundup SSB

ARRL Rookie Roundup SSB Scores
ARRL Rookie Roundup SSB Scores

2018 ARRL Rookie Roundup SSB Results

The scores are in.  The N1FD Multi-Op entry placed first in Area 1 in the 2018 ARRL Rookie Roundup SSB.  We also had the highest score of all the operators in the contest!

Congratulations to the Operators:

It was great to see how much the Rookies’ operating skills improved the longer they operated.  They are now seasoned operators who are ready for Field Day and the 13 Colonies events!

Dan, AC1EN logs with Keith, KC1IMK at the Mic
Dan, AC1EN logs with Keith, KC1IMK at the Mic

Thanks also to the members who elmered the Rookies, Jamey, AC1DC, Aron, W1AKI and Fred, AB1OC!

The ARRL Rookie Roundup SSB is a 6-hour contest for amateurs who were first licensed in 2016, 2017 or 2018.  The N1FD multi-op entry operated from AB1QB and AB1OC’s QTH on Sunday, April 15th.

Rookies and Elmers who joined the weekly repeater net.
Rookies and Elmers who joined the weekly repeater net.

Members who operated or elmered in the contest joined us after the contest in the weekly repeater net.  The live stream from our shack can be found on the N1FD Facebook Page.

Next Rookie Roundup

The next Rookie Roundup will be the Rookie Roundup RTTY to be held on Sunday, August 19th.    All Rookies or anyone who has never operated digital before are welcome to join us – hold the date!

Extra License Class – April 27-29

The Nashua Area Radio Society will hold our final license class of the Spring 2018 season, the Amateur Extra License class, on Friday – Sunday, April 27-29.

The Extra Class License is an upgrade from General Class.  Extra class will give you access to more frequencies on the HF bands, especially where those rare DX like to operate from.

Spring 2018 Extra Class Flyer

Day 1 of the class will be held at AB1OC/AB1QB’s QTH.  We will have lots of demos to help reinforce the material including test equipment, antennas and operating.  Days 2 and 3 will be held at Dartmouth-Hitchcock.

Gordon West Extra Class Book
Gordon West Extra Class License Book

Class preparation

To prepare for the class, you need to read the Gordon West Extra Class book, which can be found on Amazon.com.  When you register, we will send you a pre-work assignment based on the materials in the book.

We also recommend that you take practice exams like those from AA9PW or HamTestOnline.

To register for the class, visit https://www.n1fd.org/amateur-radio-license-class-registration/

Any questions?  Contact Anita, AB1QB at [email protected].

Calling All Rookies! ARRL Rookie Roundup is April 15th

Were you first licensed in 2016, 2017 or 2018?   Are you hoping to operate at Field Day and want to get some QSOs under your belt?  Do you want to try out contesting but need some practice before operating in the CQ WW contest?  Join us on April 15th for the ARRL Rookie Roundup!

ARRL Rookie Roundup

The ARRL holds 3 contests for “Rookies” each year.  The ARRL Rookie Roundup SSB is coming up on Sunday, April 15th from 1800z (2:00pm Eastern time) to 2359z  (7:59pm Eastern time).  The Rookie Roundup RTTY is on August 19th  and the Rookie Roundup CW is on December 16th.

2016 ARRL Rookie Roundup SSB Certificate
2016 ARRL Rookie Roundup SSB Certificate

We will be fielding a multi-op entry using the N1FD callsign from AB1OC/AB1QB’s Station.  This will be a great opportunity for newly licensed Hams to get on the air.  We will be here to Elmer you along with some experienced “Rookies”.

Elmering Session

We will hold an elmering session starting at 11:00 am on Sunday to help familiarize you with operating, using our station, and the contest rules.

During the contest, operators will work in pairs, with one operator logging and the other behind the mic.  This will give you 2 sets of ears for identifying callsigns.  The operators can switch off halfway through their operating time.  Depending on the number of operators, we’ll switch operator teams after 2 hours or so.

If you are interested in joining us, or have any questions, please email Anita, AB1QB at [email protected]

For more information on this event, see the Calendar entries at https://www.n1fd.org/event/elmering-session-for-arrl-rookie-roundup/ and https://www.n1fd.org/event/arrl-rookie-roundup/2018-04-15/