A JFET-based Franklin oscillator reprises 50’s valve design in this rough & ready 400KHz- 12MHz offering.
Yep, I believe mine was built using a couple of EF80 TV pentodes – ‘cos they cost – nothing!
My memory of it is shaky, but quirky and temperamental are adjectives that spring to mind.
So it was then, that I came across a paper on the ‘net: A unique, low-voltage, source-coupled J-FET VCO, which I downloaded and read the article with interest.
Further searches referred heavily to this article, including: ‘ZL2PD HF RF Signal Generator‘, and after downloading and re-arranging/breadboarding, I came up with the following schematic:
No claims to originality are made for the above then, just sufficient alterations to arrive at a repeatable and stable design using available FETs. A 6-pole switch, collection of coils and a 12pF-176pF capacitor comprise the main frequency-determining components as follows:
The arrangement I used gives 6 overlapping ranges from 386KHz to nearly 11MHz.
Most of the inductors I used were modern small-size axial units, however a 5uH component refused to coax the VCO to life, and suspecting a low Q, I substituted a home-made bifilar-wound 5uH HF choke from several I had made nearly 40 years ago – The VCO sprang into life!
I noted earlier that my memories of the pentode-based circuit included the circuit being temperamental, and as such, this is the main drawback with this oscillator also.
Instability will result if the Drain load of Q2 is not decoupled, and any such decoupling capacitor will affect tuning range. Similarly, the low value feedback capacitor (I used 15pF) has to be chosen carefully to avoid VCO failure at lower frequencies. There is also no efficient AGC in the VCO, and the waveform shape suffers as a result. This can be mitigated somewhat by ‘fiddling’ with the source-bias resistor and drain load values. (if you like fiddling) Despite the foregoing, the frequency stability is excellent using a stabilised supply of only 5 volts. In fact, the slightly distorted output from the unit gives rise to a rich set of harmonics, far above the original VCO frequency. In some minds probably a disadvantage, but for my use a positive plus.
I built a simple phase-shift oscillator to modulate the unit, and had a lot of fun building it into a cheap tin sandwich box, together with a simple MOD 1 plastic-gear reduction drive, connected to the drive-shaft of an antique air-spaced capacitor bought on eBay at 5 for £3.00! (GBP).
The simple modulator schematic is given below:
The above makes provision for altering the modulator frequency. (within limits) If this facility is not required, then a 27K resistor should be substituted for R9 and a link placed across X4. I adjusted circuit values for best compromise between sine-wave shape and amplitude, and the unit outputs an amplitude-modulated VDD of around 4.3 volts with roughly 10% modulation of 440mV. Provision is made in the schematic for connecting a modulation ON/OFF switch.
Both the above circuits could be combined and implemented as a single PCB. I decided to keep them separate. The component layouts and foils for the two boards are as follows:
Note that the inductors L2 and L3 are actually one component. I twisted a length of 26 SWG (25 AWG) gauge enamelled wire together and fed 7 bifilar turns through a small (10mm) ferrite toroid. The start of winding 2 is then connected to the finish of winding 1, giving the tap shown in the schematic.
As mentioned above, the enclosure for the unit is a sandwich tin purchased for £0.99 (GBP) at BM. (here in the UK) (External dimensions: (Imperial) 7.8 inches X 5.1 inches X 2.6 inches; (Metric) 200mm X 130mm X 66mm) The tin-plate used for this is so thin, it has the resilience of soft toffee, so a piece of aluminium was cut to size, which fits the bottom of the tin exactly, providing the necessary strength and rigidity to support the tuning capacitor and it’s associated gearing assembly, as well as the 50 ohm BNC sockets, frequency range switch, modulation frequency pot and switch.
I made a front panel/drill template using Front Panel Designer and this is shown below:
I left the panel paper-white, but laminated the printing, so that I could write the various frequency markers onto it with a fine-tipped CD marker pen. Also, I haven’t shown a drill-hole for the satellite gear – Your choice of gearing may differ from mine.
The six coils were mounted directly between the switch tags and the box. I suggest a soldering iron with a power rating of at least 25 watts, for soldering directly onto the tinplate. I also soldered the heads of the PCB mounting screws directly to the box bottom. (which is now the front panel) I confess to using 6BA screws for this, as I had no metric screws made of brass. An audio out socket and DC power connector were fitted to the side of the box. Detail of box wiring is shown in accompanying photographs.
The 1st photo shows the capacitor and coils I used. Also shown is the satellite gear support bearing, made from another MOD 1 plastic spur gear. The crazy offset angles the fixing screws are set at are deliberate, allowing fine adjustment sideways and down, for correct meshing of the satellite gear on the front of the panel. A 4mm bush with grub-screw holds the 4mm axle-rod firmly in alignment.
The arrangement on the right shows the collection of coils covering the various frequency ranges. The only home-made unit being my monster 5uH bifilar-wound choke – wound nearly 40 years ago! Several strengthening solder joints can be seen on the box, holding the bottom and side panels together. (The box as bought has only crimped joints)
A completed internal view of the unit is below. On the right hand side the DC input socket, audio output jack socket and BNC sockets for output to the unit under test and a frequency meter. Note the ferrite beads on both the DC supply lead and audio feed. The red LED DC power indicator, and series resistor are just visible under the audio jack wiring. In the centre the VCO and modulator boards. Note the printed circuit track can be seen through the high quality half-thickness FR4 laminate board I used for the modulator. On my prototype VCO I fitted link connectors to try out 4 alternative modulation points. Unwanted links have been removed on the current PCB. To the left are the modulation frequency pot and on/off switch, with the coils and frequency range switch above.
Below detail of the large gear wheel and knob. Note the hole in the plastic gear clears the capacitor shaft completely, the assembly being held onto the shaft with the grubscrew in the large knob. To ensure concentricity of the knob and gear, a smaller hole is drilled first in the gear, a friction fit to a 1/4inch (6.35mm) shaft, the knob is fitted on top and the gear and knob super-glued together and left for 24 hours, before the clearance hole is drilled.
Not for me a wimpy piece of flimsy perspex with engraved black line as a cursor. My piece of small diameter brass rod makes an efficient, easily-cleaned and most importantly, cheap, pointer.
Below, a close-up photo showing the satellite gear and knob assembly. I drilled a 4mm hole down the centre of a short off-cut from a plastic pot shaft (6.35mm (1/4inch) diameter). The grubscrew in the small knob depresses the thin wall of the pot shaft onto the 4mm rod, keeping the knob firmly in position. The small MOD 1 spur gear is held on the 4mm shaft using a grubscrew, with a small number of 4mm washers adjusting the height of the gear above the front panel to match that of the main gear.
Last but certainly not least, a view of the DC jack socket. Note the RF decoupling capacitor and ferrite bead, together with a 1 amp diode which will short-circuit an incorrect polarity connection to the unit, thereby protecting it without suffering a serial voltage drop.
The FrontDesigner front panel project is here: http://joebrown.org.uk/images/SigGen/SignalGenerator.FPL
A full-size image of the above is here: http://joebrown.org.uk/images/SigGen/SignalGeneratorFPL.JPG
Zipped Eagle file for the schematics and PCBs are here: http://joebrown.org.uk/images/SigGen/SigGen.zip
Zipped full-size images of the schematics and boards are here: http://joebrown.org.uk/images/SigGen/SigGenImages.zip
ESR can supply all electronic components and knobs, PCB materials etc., including small inductors, but not ferrite toroids, or the ‘antique’ tuning capacitor.
Technobots supply the MOD 1 plastic gears I used.
I purchased a selection of ferrite materials, including the toroids I used from: http://www.bardwells.co.uk/