From my Twitter feed: HF receivers, voltage reference, FDIM

ke9v
A History of HF Receivers | Smoke Curlshttp://t.co/A2uHc38mic #hamr

 

dangerousproto
Erl’s voltage referencehttp://t.co/X0MOESc8BO

I need to build one of these……Dan

 

qrparciFDIM 2013 (Dayton) – Full seminar schedule now online http://t.co/9eWdTI0FEx #hamr#qrp #hamvention

I’ll be attending FDIM. If you see me, say hi….Dan

Video: AA2YV, build a receiver, Wouxoun review

Bill, AA2YV, is not only a fine amateur radio operator, but a professor of German at Nazareth College in New York.

Build a SW receiver with only four transistors!

A video review of the KG-UVD1P, my latest acquisition. Short version: he likes it.

Two Vintage Photos

This morning, I found two links to vintage amateur radio photos in my inbox this morning. The first one if from Wystan Stevens’ Flickr photostream. Stevens is a local historian here in Ann Arbor, MI.

W8ZRF 1953 QSL

W8ZRF is still alive and kicking and an active member of our local amateur radio club, ARROW.

The second one comes by way of the Glowbugs Google Group:

It comes from an article titled, “The Weirdest Photo Research of 2012.” The caption reads, “Sam Harris, of Medfield, MA, trims his beard with electronic scissors controlled by moon bounce signals. Bettmann/Corbis” Glowbug members quickly identified the ham in the photo as Sam Harris, W1FZJ, who is famous for the first 1296 MHz moonbounce contact.

Not only that, they identified the receiver as the Lafayette HE-10 (fully assembled) or KT-200 (kit). Says, Bob, W9RAN, “Really a nice receiver with and RF stage and transformer isolated power supply – definitely a cut or two above the S-38 that the dial was borrowed from.  I like receivers like this for casual listening, as you can just spin the dial and always find something interesting to listen to.  It certainly would have been usable by Novices and on AM, although tuning SSB on receivers like this or my Hallicrafters SX-110 kept the operator busy, tuning to compensate for drift and controlling the audio with the RF gain, but this soon became second nature.”

 

25, 50, and 75 Years Ago in QST

QST publishes a column every month towards the back of the magazine that highlights from issues 25, 50, and 75 years ago. Now that the QST archive is online, it’s really worth taking a look at these articles. Here are a few that were interesting to me this month:

  • October 1937
    • Modernizing the Simple Regenerative Receiver by Vernon Chambers, W1JEQ. This a nicely-designed and built regen using two tubes, a 6K5 pentode and 6C5 triode. I’m going to keep this design in mind if I ever get around to playing with all the tubes I have. As an aside, W1JEQ wrote 87 articles for QST from 1936 through February 1958. This was his third article.
    • Concentrated Directional Antennas for Transmission and Reception by John L. Reinartz, W1QP, and Burton T. Simpson, W8CPC. This article describes two different antennas. The first is a  half-wave loop antenna that the author says works on 2-1/2, 5, 10, and 20m. The second is a square loop antenna called a “signal squirter” for 14 Mc.
  • October 1962
    • In the “New Apparatus” item on page 27, a key made by J. A. Hills, W8FYO, of Dayton, OH is shown under the heading, “New Key Mechanism for Electronic Keyers.” The photo clearly shows a key whose design was adopted by whoever designed the Bencher BY-1 paddle.
    • The Towering Problem by Jay Kay Klein, WA2LII clearly shows that putting up towers have always been a problem for amateur radio operators. This is a humorous take on the problem. What’s notable is that this type of humorous article almost never appears in QST anymore. Amateur radio seems to have lost its sense of humor.
  • October 1987
    • Stalking Those Fugitive Components by Doug DeMaw, W1FB. We often complain about the demise of local parts suppliers, but this article shows that this was a problem 25 years ago as well. W1FB gives some advice that I gave not long ago–stock up on parts, especially when you find a good deal on them, and you won’t have to scrounge around for them when you want them.

Extra Class question of the day: Modulation and demodulation

Modulation is the process of adding some kind of information, including voice and digital information, to a carrier signal. The most common types of modulation that we use in amateur radio are amplitude modulation (AM) and frequency modulation (FM). Single-sideband, or SSB, is a form of amplitude modulation.

To frequency modulate a carrier, a transmitter will sometimes us a modulator that varies the phase of the signal. This is sometimes called phase modulation (PM). One way to generate FM phone emissions is to use a reactance modulator on the oscillator. (E7E01) The function of a reactance modulator is to produce PM signals by using an electrically variable inductance or capacitance. (E7E02) An analog phase modulator functions by varying the tuning of an amplifier tank circuit to produce PM signals. (E7E03)

To boost the higher audio frequencies, a pre-emphasis network is often added to an FM transmitter. (E7E05) For compatibility with transmitters using phase modulation, de-emphasis is commonly used in FM communications receivers. (E7E06)

Amplitude modulation and single-sideband signals are produced using mixer circuits. The carrier frequency and the baseband signals are input to the mixer circuit which produces an amplitude modulated output. The term baseband in radio communications refers to the frequency components present in the modulating signal. (E7E07) The principal frequencies that appear at the output of a mixer circuit are the two input frequencies along with their sum and difference frequencies. (E7E08)

When using a mixer, you must take care not to use too high of a signal at the inputs. Spurious mixer products are generated when an excessive amount of signal energy reaches a mixer circuit. (E7E09)

Single sideband is most often used for phone transmission on the HF bands and for weak-signal operation on the VHF and UHF bands. One way a single-sideband phone signal can be generated is by using a balanced modulator followed by a filter.  (E7E04) A balanced modulator is a type of mixer.  The output of a balanced modulator, however, does not contain the carrier frequency, only the two sidebands.

Modern transceivers use digital signal processing to generate SSB signals. The quadrature method describes a common means of generating an SSB signal when using digital signal processing. (E7E13)

At the receiving station, a modulated signal has to be demodulated. Amplitude modulated signals are often demodulated using a diode detector circuit. A diode detector functions by rectification and filtering of RF signals. (E7E10)

For demodulating SSB signals, you want something a little more sophisticated. A product detector is a type of detector that is well suited for demodulating SSB signals. (E7E11) A product detector is actually a frequency mixer. It takes the product of the modulated signal and a local oscillator, hence the name. In an FM receiver, the circuit for detecting FM signals is a frequency discriminator. (E7E12)

Some modern receivers demodulate a signal entirely in software. These receivers are called software-defined receivers. When referring to a software defined receiver, direct conversion means incoming RF is mixed to “baseband” for analog-to-digital conversion and subsequent processing. (E7E14)

Extra Class question of the day: Receiver performance characteristics

One of the most commonly mentioned HF receiver specifications is blocking dynamic range. The blocking dynamic range of a receiver is the difference in dB between the noise floor and the level of an incoming signal which will cause 1 dB of gain compression. (E4D01) Cross-modulation of the desired signal and desensitization from strong adjacent signals are two problems caused by poor dynamic range in a communications receiver. (E4D02)

Another specification commonly bandied about is third-order intercept level. A third-order intercept level of 40 dBm with respect to receiver performance means a pair of 40 dBm signals will theoretically generate a third-order intermodulation product with the same level as the input signals. (E4D10) Compared to other products, third-order intermodulation products created within a receiver are of particular interest because the third-order product of two signals which are in the band of interest is also likely to be within the band. (E4D11)

The term for the reduction in receiver sensitivity caused by a strong signal near the received frequency is desensitization. (E4D12) Strong adjacent-channel signals can cause receiver desensitization. (E4D13) One way to reduce the likelihood of receiver desensitization is to decrease the RF bandwidth of the receiver. (E4D14)

A preselector might help in some cases. The purpose of the preselector in a communications receiver is to increase rejection of unwanted signals. (E4D09)

When operating a repeater, one thing that can occur is intermodulation interference, or simply intermod. Intermodulation interference is the term for unwanted signals generated by the mixing of two or more signals. (E4D06) Nonlinear circuits or devices cause intermodulation in an electronic circuit. (E4D08)

Intermodulation interference between two repeaters occurs when the repeaters are in close proximity and the signals mix in the final amplifier of one or both transmitters. (E4D03) The transmitter frequencies would cause an intermodulation-product signal in a receiver tuned to 146.70 MHz when a nearby station transmits on 146.52 MHz are 146.34 MHz and 146.61 MHz. (E4D05) We get this in the following way:

2 x 146.52 MHz – 146.34 MHz = 146.70 MHz and

2 x 146.61 MHz – 146.52 MHz = 146.70 MHz

A properly terminated circulator at the output of the transmitter may reduce or eliminate intermodulation interference in a repeater caused by another transmitter operating in close proximity. (E4D04) The circulator reduces intermodulation distortion because it helps to reduce the amount of energy from nearby transmitters that might get into a repeater’s final amplifier.

Cross modulation is a form of intermodulation. Cross modulation occurs when a very strong signal combines with a weaker signal and actually modulates the weaker signal. The most significant effect of an off-frequency signal when it is causing cross-modulation interference to a desired signal is that the off-frequency unwanted signal is heard in addition to the desired signal. (E4D07)

Extra Class question of the day: Receiver performance characteristics

In the past, sensitivity was one of the most important receiver performance specifications. Today, instead of sensitivity, we speak of a receiver’s minimum discernible signal, or MDS. The MDS of a receiver is the minimum discernible signal. (E4C07) This is the weakest signal that a receiver will detect.

One parameter that affects receiver sensitivity is the noise figure. The noise figure of a receiver is the ratio in dB of the noise generated by the receiver compared to the theoretical minimum noise. (E4C04) Lowering the noise figure of a receiver would improve weak signal sensitivity. (E4C08)

A related specification is the noise floor. When we say that the noise floor of a receiver has a value of -174 dBm/Hz, it is referring to the theoretical noise at the input of a perfect receiver at room temperature. (E4C05) If a CW receiver with the AGC off has an equivalent input noise power density of -174 dBm/Hz, the level of an unmodulated carrier input to this receiver would have to be -148 dBm to yield an audio output SNR of 0 dB in a 400 Hz noise bandwidth. (E4C06)

A receiver’s selectivity is the result of a lot of things, including the filters a receiver has. 300 Hz is a desirable amount of selectivity for an amateur RTTY HF receiver. (E4C10)2.4 kHz is a desirable amount of selectivity for an amateur SSB phone receiver.(E4C11)

In addition to a 300 Hz filter and a 2.4 kHz filter, high-end receivers also have filters called roofing filters. A narrow-band roofing filter affects receiver performance because it improves dynamic range by attenuating strong signals near the receive frequency. (E4C13)

Back in the day, when superheterodyne receivers had intermediate frequencies, or IFs, in the 400 – 500 kHz range, image rejection was a problem. If there was a strong signal present on a frequency about two times the IF away from the frequency your receiver was tuned to, you might hear that signal. Accordingly, 15.210 MHz is a frequency on which a station might be transmitting if is generating a spurious image signal in a receiver tuned to 14.300 MHz and which uses a 455 kHz IF frequency. (E4C14)

One solution to this problem is to select an IF higher in frequency. One good reason for selecting a high frequency for the design of the IF in a conventional HF or VHF communications receiver is that it is easier for front-end circuitry to eliminate image responses. (E4C09) A front-end filter or pre-selector of a receiver can also be effective in eliminating image signal interference. (E4C02)

Another way to get rid of image signals is to use a narrow IF filter. An undesirable effect of using too wide a filter bandwidth in the IF section of a receiver is that undesired signals may be heard. (E4C12)

Because most modern transceivers use digital techniques to generate a local oscillator signal to tune a receiver, synthesizer phase noise might be a problem. An effect of excessive phase noise in the local oscillator section of a receiver is that it can cause strong signals on nearby frequencies to interfere with reception of weak signals. (E4C01)

Finally, here are two miscellaneous questions on receiver performance characteristics. Atmospheric noise is the primary source of noise that can be heard from an HF receiver with an antenna connected. (E4C15) Capture effect is the term for the blocking of one FM phone signal by another, stronger FM phone signal. (E4C03)

A ham radio Halloween story

On About.Com, Mary L. writes about her experience with “haunted ham radio.” She writes,

The next day, sometime in the early evening, I plugged the radio into the wall and began tinkering with it. Then, the most terrifying thing happened. The static faded completely. The silence came back on as did the sound of breathing. Suddenly, the creepy voice of a man came on and said, “Hello, little girl.”

Yipes! :)

You Learn Something New Every Day: S-Meters

On the IC-746PRO mailing list, a fellow asked, “On the 2m band, why doesn’t the S-meter reading on my IC-746PRO match the S-meter reading on my 2m mobile radio?” My answer was that while there is a standard (it’s actually a recommendation), it’s rarely followed, so it’s not a surprise that the two readings don’t agree.

Mark, K5LXP, corrected me, noting that the recommendation specifies one value for an S9 reading below 144 MHz, and a different value above 144 MHz.  The recommendation reads:

STANDARDISATION OF S-METER READINGS

  1. One S-unit corresponds to a signal level difference of 6 dB.
  2. On the bands below 30 MHz a meter deviation of S-9 corresponds to an available power of -73 dBm from a continuous wave signal generator connected to the receiver input terminals.
  3. On the bands above 144 MHz this available power shall be -93 dBm.
  4. The metering system shall be based on quasi-peak detection with an attack time of 10 msec ± 2 msec and a decay time constant of at least 500 msec.

This corresponds to a voltage of about 50 microvolts below 144 MHz and a voltage of 5 microvolts about 144 MHz.

It would be interesting to know how the software of the IC-746PRO handles this. I say software because the S-meter on the IC-746PRO is a digital meter whose readings are controlled by the rig’s microcontroller. Theoretically, the software could calculate the S-meter readings differently based on the band that the rig is set to, but I don’t know that the programmers would go so far as to do that.

Two Radios for You to Build

On the AMRAD mailing list, Andre, N4ICK, posted a link to the YouTube video, “12AU7 regenerative radio on a tin bake plate” (see photo at right) It’s a very cool regenerative radio made with a single 12AU7 vacuum tube. The only problem is that there are no links to the schematic for the radio.

So, I Googled a bit, and came up with this circuit. The cool thing about this circuit is that it uses a 12V power supply, not the high voltage power supply normally required for a tube circuit.

Over the weekend, I went to a big rummage sale sponsored by the local Kiwanis club. They had quite a few aluminum baking pans there for very little money. I should have picked up a couple of them. :)

Simliar Radios
VK3YE has also posted a YouTube video
of his experiments with a similar circuit. Unfortunately, he also doesn’t include a link to a schematic.

Another related YouTube video is for a one-transistor radio. This is actually a very well-done video. It shows you step-by-step how to build the radio. This is something that we may actually be able to do down at the museum with some kids.

At the very least, watching the videos is amusing. At best, maybe they’ll inspire you to do a little experimenting.