Minimizing reception at the image frequency is fundamental to useful practical application of the superheterodyne principle—unless the image happens to lie in another amateur radio band, and you want to receive it. This page explores the band-imaging receiver topology popularized in the 1950s by Don Mix, W1TS, and Byron Goodman, W1DX—a design approach that uses a single local-oscillator tuning span to receive either of two bands without oscillator-tank switching, each band the image of the other.

Beginnings

We see its pre-echo in George Grammer, "A Two-Tube Superhet," QST, February 1941, pages 12–15 and 92: In a simple regenerative-detector-behind-a-mixer receiver with a 1.6-MHz intermediate frequency (IF), the same plug-in oscillator coil is used to tune the ranges 5.4–10.0 and 9.5–14.5 MHz. Wrote Grammer:

      It will be noted that the same oscillator coil, D, is used for two frequency ranges. This is possible because the oscillator signal is placed on the low-frequency side of the signal on the higher range. This not only avoids winding a second coil, but also gives somewhat greater stability at the highest-frequency range. . . .
      A word about images. The receiver will, of course, respond to signals either 1600 kc. higher or 1600 kc. lower than the oscillator frequency. The unwanted response, or image, is discriminated against by the tuning of the r.f. circuit. On the three lower-frequency ranges, when it is possible to find two tuning spots on C₁ at which inocming random noise peaks up, the lower-frequency peak (the one requiring the highest tuning capacity at C₁) is the right one. The oscillator frequency is 1600 kc. higher than that of the incoming signal on these three ranges. On the fourth range the reverse is true, since here the oscillator is tuned 1600 kc. lower. Actually, it does not matter a great deal which side is used except for calibration purposes.

The two-bands-for-the-price-of-one story of the Mix-Goodman receiver begins in earnest with the publication in March 1950 QST of "A Beginner's Four-Tube Superhet Receiver," by Don Mix, W1TS. Designed to keep the cost of a first-time amateur radio station down, Mix's receiver provided coverage of two bands at a time (principally, 80 meters [3.5 to 4.0 MHz] and 40 meters [7.0 to 7.3 MHz], although coverage of the 20-meter (14-MHz) band could also be installed, and may have been added as an afterthought) without switching or unplugging any frequency-determining components.

Unlike Grammer's simple receiver, Mix's circuit uses band-imaging as a fundamental feature: The receiver covers its two principal signal-frequency ranges with one local-oscillator (LO) tuning span (5.0–5.8 MHz), which converts both bands to a relatively high (1.5-MHz) IF to allow selection of LO−IF or LO+IF with simple signal-frequency filtering, one range the image of the other. The novelty of Mix's implementation is that coverage of the two bands is arranged end-to-beginning on the tuning dial, with 4.0 MHz on the 80-meter band (LO, 5.5 MHz) corresponding to 7.0 MHz on the 40-meter band. Only retuning the RF-input grid of the 6SB7Y converter is necessary to change from one band to the other, but the bands' end-to-beginning relationship on the dial requires considerable LO retuning to move from, say, the low end of one band to the low end of the other. Shortwave-listening and maritime-traffic code practice possibilities notwithstanding, in effect this arrangement "wastes" revolutions of the set's 6:1-reduction tuning dial on non-amateur frequencies: 4.0 to 4.3 MHz on 80 meters, and 6.5 to 7.0 MHz on 40. This equates to 3/8 of the tuning range on 80 and 5/8 of the range on 40, significantly reducing the tuning-speed reduction achievable with the 6:1 dial.

Three years later, Byron Goodman, W1DX, improved Mix's band-imaging arrangement with the publication of "A Good Four-Tube Superhet" on pages 19–24, 108, 110, and 112 of January 1953 QST. In his improved band-imaging approach, Goodman again converted 80 and 40 meters to a high IF (1.7 MHz), but this time used an LO span (5.2–5.7 MHz) that maximized tuning-speed reduction by spreading 80-meter coverage across the entire dial and 40-meter coverage over 3/5 of the dial. In effect, the two bands were overlaid, with 3.6 MHz on 80 corresponding to 7.0 MHz on 40, one band truly tuning as the image of the other. Using an IF of 1.7 MHz rather than 1.75 MHz avoided reception of spurious signals from IF- and beat-frequency-oscillator (BFO) harmonics at 3.5 and 7.0 MHz.

Mix-Goodman Receivers on Press

I use these published resources as references when thinking about, building, and using Mix-Goodman receivers:

Donald H. Mix, W1TS, "A Beginner's Four-Tube Superhet Receiver," QST, March 1950, pages 11–17; Feedback, QST, June 1950, page 39.
Byron Goodman, W1DX, "A Good Four-Tube Superhet" QST, January 1953, pages 19–24, 108, 110, and 112.
Byron Goodman, W1DX, "The '2B3' Superheterodyne," QST, September 1955, pages 12–15.
Lewis G. McCoy, W1ICP, "A Selective Converter for 80 and 40 Meters," QST, January 1956, pages 38, 39, 130, 132.
Lewis G. McCoy, W1ICP, "The 'Bonus' 21-Mc. Converter," QST, October 1958, pages 33–35, 162.
Byron Goodman, W1DX, "The 'SimpleX Super' Receiver," QST, December 1958, pages 11–14, 178, 180.
"The 'SimpleX Super Mark II' Three-Tube Receiver," The Radio Amateur's Handbook (ARRL), 1962 edition, pages 116–119.
"The 2X4+1 Superheterodyne," The Radio Amateur's Handbook (ARRL), 1962 edition, pages 120–124.
"The HB-65 Five-Band Receiver," The Radio Amateur's Handbook (ARRL), 1965 and 1966 editions.
Wes Hayward, W7ZOI, "Building Spectral Purity into a Band-Imaging Transceiver," QST, August 1990, pages 39–40.

More will be included, from two in Solid-State Design for the Radio Amateur to my 10- and 18-MHz design in the 1988 Radio Amateur's Handbook to multiple 80-and-20-meters-to-9-MHz-with-a-5-MHz-VFO derivatives from the growth-of-SSB-ham-radio era to the Panasonic RF-2200 and related multiband receivers, the HF coverage of which was implemented as three pairs of imaged bands based on a first IF of 1.985 MHz.

Although they cover two bands with one LO tuning range, the 80-and-20-meters-to-9-MHz species and similar band-imaging topologies might more accurately be termed LO imaging, as the bands covered are not images of each other (that is, are not separated in frequency by twice the IF) but rather are separated by twice the LO frequency. Another hallmark of this arrangement is that each band "tunes backwards" relative to the other, as in the following several examples:

LO Imaging 20 and 80/75 Meters with a 9-MHz IF

RF 14.0 – 14.5 MHz
LO  5.0 –  5.5 MHz
RF  4.0 –  3.5 MHz

LO Imaging 30 and 160 Meters with a 6-MHz IF

RF 10.0 – 10.2 MHz
LO  4.0 –  4.2 MHz
RF  2.0 –  1.8 MHz

LO Imaging 40 and 80 Meters with a 5.35-MHz IF

RF 7.0  – 7.2 MHz
LO 1.65 – 1.85 MHz
RF 3.7  – 3.5 MHz

This last topology is impractical because of its significant built-in crossover responses: Harmonics of the LO interfere with IF (at 3LO) and RF on 80 meters (2LO) and 40 meters (4LO) when the LO is tuned to 1.78333 MHz to receive 3.5666 MHz or 7.1333 MHz. Considering these relationships for awhile, we realize—assuming that we want to cover only 80- and 40-meter frequencies commonly used for CW operation (3.5 to 3.6 MHz and 7.0 to 7.125 MHz)—that only an IF higher than 5.4 MHz will do. Better yet, the true band-imaging scheme described by Goodman (using a 5.2- to 5.7-MHz LO to tune 3.5 to 4.0 MHz and 6.9 to 7.4 MHz with an IF of 1.7 MHz) is crossover-free.

Mix-Goodman Receivers at W9VES

None of my Mix-Goodman receivers includes a crystal filter or equivalent close-in band-pass selectivity.

The BG-1 is an 80- and 40-meter, 1.7-MHz-IF Mix-Goodman receiver I've been developing since late 2005. In its initial configuration it used a 12SG7 pentode RF amplifier; BF998 dual-gate-MOSFET mixer; repurposed Kenwood R-599 VFO (4.9–5.5 MHz); 12SK7 pentode (triode-connected) IF cathode follower; 12SJ7 pentode high-C Hartley regenerative detector; 12SC7 dual triode (one section) audio voltage amplifier in the headphone chain; 2N3819 JFET voltage amplifier in the headphone chain; 2N3819 JFET line-output voltage amplifier; and 12A6 beam power tube (triode-connected) as a cathode-follower headphone amplifier.

The BG-1's high-C regenerative detector is the result of experiments inspired by the stability of the 40-meter regenerative receiver I published in September 1992 QST. The detector in that design used a total of about 200 pF as its tuned-circuit capacitance—at 7.04 MHz, a reactance of 113 ohms, which, scaled to the 1.7-MHz IF of the BG-1, equates to about 820 pF. With the notable exception of the intentional use of relatively high C by George Grammer, W1DF ("Rationalizing the Autodyne," QST, January 1933, pages 11–16 and 23), ham-radio practice up through the 1930s generally held that regenerative detectors should be relatively low-C—that is, that a regenerative detector's tuned circuit should exhibit a relatively high impedance for the purpose of achieving higher tank voltage. Yet my experience with high-C detectors reveals that a storied principal weakness of the regenerative detector, "pulling in" when the frequency difference between the detector and strong signals is no more than a few hundred hertz, can be greatly reduced by using a high-C detector tank.

Ongoing experiments with the BG-1's audio lineup resulted in its present configuration: 12SC7 dual triode (one section as audio-filter "send" amplifier, the other as audio-filter "receive" amplifier); 12SC7 dual triode (one section as audio voltage amplifier in the headphone chain, the other section as audio voltage amplifier for line output); and triode-connected 12A6 beam power tube as headphone amplifier. Switchable surplus audio filters between the sections of the first 12SC7 allow a choice of no filtering; 400-Hz low-pass filtering; and 1-kHz low-pass filtering.

The BG-2 began as 3- to 6-MHz "Command" receiver (IF, 1.415 MHz) that I obtained with the intention of expanding its tuning range through band-imaging by building out its RF amplifier stage to include switchable coils to cover spectrum below its local oscillator (its original design) and above its local oscillator (giving 40-meter amateur radio band coverage in the resulting 5.83- to 8.83-MHz range). In evaluating the receiver, however, I discovered that a previous owner had modified it to cover only 3.33 to 3.83 MHz by removing all but three rotor plates from each of the three sections of its tuning capacitor. (Hint: Removing plates from a variable capacitor to reduce its capacitance range is almost never necessary; paralleling a variable with additional capacitance and seriesing the pair with another capacitance is one non-destructive, reversible alternative.) A toggle switch, two air-dielectric trimmer capacitors and a 40-pF padding capacitance later, I had modified the local oscillator to provide reception from 3.475 to 3.640 MHz and from 6.995 to 7.180 MHz.

The BG-3 receiver (originally 6AZ8 pentode mixer, triode Hartley local oscillator; 6AZ8 pentode 1.7-MHz, high-C (≅755 pF) Hartley regenerative detector; 6GV8 triode AF voltage amplifier, 6GV8 triode-connected-pentode cathode-follower headphone amplifier) began as a vacation-station-in-box project and (as of mid-August 2009) continues as an experimental platform for improvements to and variations on Goodman's 2B3 and SimpleX Super designs of the 1950s and 1960s. Changes from its initial lineup include:

• High-C (420 pF at 3.5 MHz), single-tuned mixer grid for good-enough input selectivity. Using the set in conjunction with the additional signal-frequency selectivity afforded by the station's coax-to-ladder-line antenna tuner, I have yet to hear an other-band image.


• Solid-state audio. As good as the receiver sounded through the transformerless pentode-triode-connected-as-a-power-cathode-follower headphone amplifier, with heater power considered the 6GV8 dissipated over 10 W no-signal and unnacceptably heated the receiver box. Replacing it with one stage of AF filtering (half of a TL082B dual operational-amplifier integrated circuit configured as a Sallen-Key 1-kHz Butterworth low-pass filter) followed by an AF power amplifier (an LM386 configured for 26 dB of gain) gave performance equivalent to the 6GV8 with practically zero heating and the practical plus of steep high-frequency AF rolloff. The triode section of the set's second 6AZ8, unused in the initial lineup, now serves as an AF cathode follower between the detector and TL082 input for the purpose of avoiding damage to the TL082 by transients that could occur if it was capacitively coupled directly to a circuit point operating at a dc level on the order of the set's plate supply (≅165 volts)


• Oscillator buffer amplifier for better oscillator and mixer stability. In his writeup (December 1953) for his first Goodman-Mix design that used a pentode mixer with cathode LO injection (Figure 1), Goodman mentioned encountering oscillator-pulling and mixer-regeneration (that is, mixer oscillation) effects as the mixer grid is tuned through its range. (In my experience, the pulling is worse when the mixer grid tuning is varied around 3.5 MHz [at the LO frequency, a changing inductive X at the grid] than at 7 MHz [at the LO frequency, a changing capacitive X].) Direct capacitive coupling between the sets' mixer cathode and LO tuned circuitry is the cause; addition of a triode cathode follower between the oscillator and mixer is the cure, entirely eliminating mixer instability and reducing ocillator pulling to zero (as the mixer grid tuning is varied around 3.5 or 7 MHz) or to a hundred hertz or so (as the mixer grid is tuned through the LO frequency). I added a LO buffer in the BG-3 by using a 5963 (premium 12AU7A) dual triode as combination LO and buffer tube. For economy, the buffer and 6AZ8 pentode mixer can share a common cathode resistor (910 or 1000 Ω); using separate mixer and buffer cathode resistors (1500 and 2200 Ω, respectively) and ac-coupling the buffer and mixer cathodes with 0.001 µF also works. The addition of the LO buffer slightly strengthens the set's 2LO–IF response (circa 8.7 MHz), perhaps as a result of LO-signal distortion in the buffer.



Figure 1—After having used a 6SB7Y pentagrid first converter in his initial band-imaging receiver (January 1953 QST), Goodman used variations on this circuit, a pentode with cathode oscillator injection, for all but his final band-imaging design (the 7360-beam-deflection-tube-mixer-based HB-65 receiver of the 1965 and 1966 editions of the ARRL Radio Amateur's Handbook). The direct coupling between the oscillator and mixer result in two shortcomings that make this arrangement problematic: Couplng the oscillator's feedback coil to the mixer cathode makes the mixer regenerative—it can oscillate, especially when its grid is lightly loaded and/or with lower values of cathode resistance—and tuning the mixer grid pulls the oscillator somewhat, even at 3.5 and 7 MHz. (Versions that use a triode-pentode tube with the pentode suppressor grid hardwired to the pentode cathode [6U8, 6EA8, and so on] for the mixer and oscillator functions also have the shortcoming of capacitively coupling the LO signal to the pentode plate; using a 6AZ8, 6CH8, or 6KT8 and grounding the correct heater pin [5]; or using a 6CL8 triode-tetrode, solves this problem.) This concept-only depiction does not show the double-tuned mixer grid circuitry Goodman used to improve image rejection in several variations of this circuit.

Figure 2—Adding a buffer amplifier between the mixer cathode and oscillator removes the cause of mixer regeneration and reduces pulling to essentially zero at 3.5 and 7 MHz and to tens of hertz at the LO frequency. This depiction shows a Hartley oscillator; the Armstrong oscillator (tuned grid with feedback via a plate ticker coil) used by Goodman would work as well with the buffer grid coupled to the oscillator plate. The mixer and buffer cathode resistances can be combined into a single resistor if the cathodes are dc-coupled; capacitive coupling between them allows separate cathode-bias adjustment of both stages. Both approaches have been tried and work well. With a 47-Ω resistor between its grid and input tuned circuit, Goodman's original high-transconductance-pentode band-imaging mixer (based on the 6AC7 as described in December 1953 QST) is also stable across its full input tuning range with this circuit.



• Subharmonic locking for reduced detector pulling. I experimented with lightly coupling (through 1 pF) the tank circuit of the set's 1.7-MHz 6AZ8-pentode regenerative detector to the plate of a 3996.5- or 5100-kHz Pierce crystal oscillator (12J5 tube) and adjusting the detector tuning for subharmonic lock with the crystal frequency. Having used this technique to obtain transmitter drive at crystal subharmonics, my thought was that pulling an oscillating detector to a very strong local signal could increase its practical dynamic range by making it less pullable by strong on-air signals. This experiment failed because the high-C detector was already so pulling-resistant that I could not make it lock stably across the expected regeneration-control range (from below to well above critical), and because even with the detector ostensibly locked, unpleasantly loud unlock-relock thuds sometimes occurred as regeneration was adjusted through critical in the presence of strong on-air signals.


• Resistive RF input-level control. A 1-kΩ potentiometer, stator toward the mixer and rotor toward the antenna, provides the RF level adjustment capability necessary for bringing strong signals within the frequency-pulling dynamic range of the set's regenerative detector. (I have long considered the detuning-for-level-control technique unacceptable.) Eliminating mixer regeneration through the addition of the oscillator buffer was key in maximizing the smoothness of this control; with regeneration present the mixer was more prone to oscillation with the control adjusted to either end of its range.

The IF-Agile Receiver exists only in idea form at present. I envision it as using a single LO range (5.1 to 5.7 MHz or so) to cover multiple MF/HF ham bands through the use of switchable front-end filtering and selectable IFs at appropriate CPU-clock-crystal frequencies, with NE602 Gilbert Cell ICs used as its converter and product detector. David White, WN5Y, has pioneered the IF-agile idea with his high-performance Electroluminescent Receiver, a four-band band-imaging design with switchable IF filters at 3.547 and 4.000 MHz.

[to be continued...]