EMRFD Message Archive 12989

Message Date From Subject
12989 2016-06-30 18:52:40 davidpnewkirk Fun with the 2N7000: Frequency locking and a hybrid pentode/MOSFET m

The relatively low input capacitance of the 2N7000 allows it to be easily driven by ac-coupling to its gate crystal oscillator operating at 12 V. (Further, if that crystal oscillator is a Pierce, the 2N7000's input capacitance can serve as part or all of the the oscillator's anode-to-common feedback capacitance.) What we then have is an RF-clocked switch that lets us do some fun and useful things when it's installed in the cathode/source/emitter return of an RF oscillator. (We can expect the BS170 and similar devices to do the same.)

Installed between the cathode/source/emitter of an oscillating device and its tuned-circuit tap (in a Hartley) or its cathode/source/emitter impedance (resistor or choke) and feedback network in a Colpitts oscillator, the 2N7000 switch allows us to inject a signal at multiple of f to use the oscillator as a locked divider. The 200-mW-output crystal exciter at W9BRD uses this technique to get 80-m output from crystals at 7 MHz (crystal/2) and 10.7-ish-MHz (crystal/3), which signal can then be filtered for output at the locked-oscillator fundamental or a harmonic (commonly 2f at W9BRD). So when W9BRD operates with crystal control at 3560 kHz, that's with a 3.5-MHz oscillator locked to 3560 with a 7120-kHz crystal controlling the switch. And when W9BRD operates with crystal control at 7118 kHz, that's the second harmonic of the 3.5-MHz oscillator locked with a 10677-kHz rock. Listened to with a receiver, the effect of tuning the LC oscillator through lock sounds and feels so much like merely switching the oscillator on and off at the lock frequency that it's almost creepy. Lock is easy to achieve and maintain, but I do monitor my transmitted signal just in case lock fails.

Another 2N7000 application in daily use at W9BRD (when the BGCD superhet-regen [ http://dpnwritings.nfshost.com/ej/pictures/pictures1.htm ] that uses it in the operating lineup du jour) is a hybrid 2N7000/beam-power-tube mixer (shown with a pentode in the page at http://dpnwritings.nfshost.com/ej/hybrid_mixer/ ). Here, a BJT Pierce oscillator drives the 2N7000 to switch the tube's cathode return--resistor _and_ , very importantly, its cathode bypass capacitor--of a tube otherwise configured as an amplifier with its grid tuned to RF and its plate turned to IF. The Colpitts divider formed by the grid-cathode C of the tube and the net drain-to-common C of the 2N7000 makes the tube somewhat regenerative, so I usually offset the grid negative resistance a few hundred ohms between the grid its associated tuned circuit. (Coming to an understanding of why this mixer is regenerative was extra fun because as a result of doing so I came to understand the mechanism of a regenerative frequency multiplier topology that I have never found adequately explained in its heyday, and why the LO-driving-the-cathode 6AC7 and 6U8 pentode band-imaging mixers used in several receivers by one of my mentors [By Goodman, W1DX] were also incidentally regenerative "out of the box.")

My page about the hybrid mixer shows a 6SH7 pentode, which I used until recently in a modified Allied A-2516 receiver. In the BGCD Regenerodyne the mixer is a 12A6 beam power tube.The mixer circuit works well both in terms of gain and resistance to overloading in the sense that I have yet to experience blocking or IMD with it. In contrast to this, a J310 cascode standing at 12 V/10 mA and operating as a mixer source-switched by a 2N7000 IMD-overloaded readily on 40 meters.

It's import to peg the mixer screen voltage with a voltage divider or voltage regulator, or else the "key up" voltage at the mixer cathode (actually the cutoff voltage of the tube) will soar, with cutoff "chasing" the screen voltage's rise with falling screen current demand. I paid extra attention to this after feeding the 12A6 screen through just a dropping resistor and measuring the 2N7000's drain voltage (rated at 60 V Vds) as over 80 V with the oscillator crystal removed!

Best regards,


amateur radio W9BRD

12991 2016-06-30 20:55:07 Ashhar Farhan Re: Fun with the 2N7000: Frequency locking and a hybrid pentode/MOSF
what an amazing body of work! this never showed up in the endless searches i have made on 2n7000s. it will take some time for us to assimilate all this stuff.

- f

12992 2016-06-30 22:14:26 iq_rx Re: Fun with the 2N7000: Frequency locking and a hybrid pentode/MOSF
Hi Dave,
Delightful, entertaining, and though-provoking.  It's always nice to see someone dive in and embrace a different aesthetic.  Vacuum electron devices get us thinking in different ways, and that's always worthwhile.  In addition to your historical insights and new contributions, it looks like you are having a lot of fun.
Your description of the regenerative mixer was particularly interesting, and yet another instance of a circuit from the By Goodman era that "works better than it ought to, given the simplicity of the circuit."
Best Regards,
Rick KK7B
12994 2016-07-01 08:41:05 kb1gmx Re: Fun with the 2N7000: Frequency locking and a hybrid pentode/MOSF
FYI, the 2N7000 and its cousins are good to VHF.

I built a 108-136mhz supper regen using 2n7000s a decade ago.  One for RF amp (common gate),
a common gate supperregen and a third for first audio for headphones.

Generally fets/mosfets are good to VHF maybe higher and the input capacitance is not a 
limit either (look ad the input capacitance for UHF LDmosfets!).

Also the regenerodyne using fets circuit you can use 2n7000s in place of the j310s.  The only change
apply positive bias to the lower mosfet (2n7000 is enhancement mode and the J310 is depletion mode).

Good devices and good fun.