Classical vacuum-tube mixers in the amateur radio literature and in vintage amateur radio equipment are commonly grid- or cathode driven with a tube-based LC or crystal oscillator. This is fitting because such circuits commonly require several to tens of volts of local-oscillator (LO) drive at a high impedance. Driving a classical vacuum-tube mixer with a solid-state LC or crystal oscillator that operates at a low power-supply level is difficult.
Figure 1 shows a mixer circuit that solves this problem. Its core is a 6SH7 sharp-cutoff pentode with its control grid resonant at the incoming-signal frequency and its plate circuit resonant at the desired intermediate frequency. LO energy is applied to the gate of a 2N7000 MOSFET connected as a switch between circuit common and the ground returns of the pentode's cathode bias resistor and cathode bypass capacitor.
|Figure 1—Circuit of the hybrid pentode-MOSFET regenerative mixer. Only an LO peak voltage high enough to turn on the 2N7000—per its specifications, 0.8 to 3 V, typically 2.1 V—need be applied to its gate; if necessary, dc bias can be applied to the 2N7000 as shown in Figure 2. Although this drawing shows the pentode screen fed by means of a voltage-dropping resistance, powering the screen via a voltage divider or voltage-regulated supply is recommended to establish a fixed cutoff voltage for the tube.|
|Figure 2—Options for biasing the 2N7000 when the available LO peak voltage is insufficient to turn on the 2N7000. I use Option A in a modified Allied A-2516 receiver and in the modified first mixer of a Heathkit HW-16 transceiver.|
Figure 2 shows options for biasing the 2N7000 in situations where the available LO voltage is too low to turn on the 2N7000. (This is the case in my modified Allied A-2516 receiver, in which the local oscillator—now operating at 9 V instead of 18 V as in a stock receiver—is amplified only by a BJT follower before application to associated circuitry.) Adjusting the bias level by ear while receiving signals in a busy band segment is sufficient for non-critical use.
The 270-Ω resistor in the 6SH7 grid acts to offset the 6SH7's negative resistance to disallow oscillation. It may or may not be necessary—or a value other than 270 Ω may be more optimal—in a given application.
The switched-amplifier mixer can be used in RF-to-IF and IF-to-AF applications. That is, it works as a mixer for RF and also as a so-called product detector for rendering RF signals as audio. I've had success in using the Figure 1 circuit to turn an IF amplifier tube into a detector (6EW6 IF amplifier in a modified Heathkit HW-16 transceiver) and also in using a high-transconductance beam power tube (8106) as the detector in a modified Allied A-2516 receiver. When using the circuit as an RF-to-AF mixer, it's important to minimize beat frequency oscillator (BFO) pickup by the switched-amplifier grid, gate or base; otherwise, that not-caused-by-ac-line-induction species of audio hum known all too well to direct-conversion-receiver experimenters may result.
Whence the Regeneration?
Why the Figure 1 circuit is regenerative may not be immediately apparent. But when we realize that an RF pentode tube has a grid-to-cathode capacitance on the order of 10 pF and that a 2N7000 FET has a drain-to-source capacitance of 10 to 25 pF, we see that we've set up a Colpitts capacitive voltage divider. This isn't speculative; I confirmed on the bench that the circuit can oscillate when the Q of the tuned-circuit inductor is sufficiently high and the parallel capacitance shunting the Colpitts divider is sufficiently low. (A coil of a few microhenries will suffice.)
On confirming that regeneration in the Figure 1 circuit occurs by means of Colpitts-divider-based feedback, I realized that I had encountered this effect (other than in oscillators) before: as the cause of instability in pentode mixers used by Goodman in his band-imaging receiver designs, and as the mechanism underlying output enhancement in an ill-explained "regenerative" frequency-multiplication scheme for transmitters (Figure 3).
|Figure 3—Now that we know that regeneration in the Figure 1 circuit comes from the operation of Colpitts-capacitive-divider-based feedback, we can more fully describe the mechanism underlying output enhancement in this frequency-multiplier circuit from the 1947 Radio Handbook. I can't be the only reader of such treatments to have been frustrated by the ascription of feedback to just "[an] undersized cathode bypass capacitor and large cathode resistor"!|
Replacing the Pentode with Cascoded JFETs
The 2N7000 MOSFET works equally well when switching the solid-state analog of a vacuum tube cathode. Figure 4 shows how junction field-effect transistors (JFETs) can be used to implement an all-solid-state version of the mixer circuit.
|Figure 4—The 2N7000 switch works equally well with the two-cascoded-JFET, synthetic-tetrode mixer described and evaluated by Campbell, Hayward and Larkin in Experimental Methods in RF Design. Here the switching signal is provided by a BJT Pierce crystal oscillator, with the 2N7000's gate-to-source capacitance serving as part of the capacitive voltage divider on which the Pierce depends. For the output transformer core I used a toroid of -43 ferrite material; per EMRFD, -61 ferrite material would provide higher gain. In the implementation of this mixer now in use as the RF-to-IF frequency changer in my modified Allied A-2516 receiver, the output tuned circuit consists of the 1415-kHz BFO transformer from a 3- to 6-MHz "Command" receiver, modified to tune to 1843 kHz, with the 3.3-kilohm loading resistor omitted. (And what's a Regenerodyne? Glad you asked!)|
|Revised December 28, 2015 CE.||Copyright © 2014–2015 by David Newkirk (DavidNewkirk@gmail.com). All rights reserved.|