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ZL2PD HF RF Signal Generator

A simple RF oscillator which can be used as a basic HF signal generator.

Introduction

This circuit came out of some work I did for the two antenna analysers which are described elsewhere on this site. Those instruments both include an RF signal generator. This is used as the signal source to drive the impedance bridge and detectors used to measure impedance.

Those instruments ended up with somewhat different oscillator designs due to other conflicting design issues. The signal generator outlined here is much simpler, and quite easy to build.

The basic performance can be given in just three lines:

Range:     450 kHz to 60 MHz in three switched ranges
Output :  +10 dBm (2Vpp) into 50 ohms
Supply:   3 to 15VDC (My prototype used a 6V power supply)

The Basic Design

The design is based around the Franklin oscillator configuration. This is perhaps not as well known as other types of oscillator, such as the Colpitts or Hartley. This is also called a "source-coupled FET oscillator" in some books and articles.


This oscillator, shown in outline form above, uses two active devices. Some designs I've seen use two capacitors to couple into and out of the resonant circuit, but in practice, only a single capacitor is actually required with this circuit.    

This simple two-FET circuit forms the heart of this Franklin oscillator. It's really a two-stage amplifier with feedback. A common drain FET first stage is followed by a common gate FET second stage. The first stage gives high gain with a high input impedance to minimise loading on the oscillator's tuned circuit components.  The coupling capacitor is usually a small value, typically less than 10 pF.

The second common gate FET stage has no voltage gain. The low input impedance of this stage permits the stage to be fed from the first FET's source resistor. The output of this stage provides the feedback loop to permit oscillation, and since the output impedance of this stage is high, this minimises loading again on the oscillator's tuned circuit. This is enhanced to some extent by the use of an RF choke which is a high impedance at RF but with low DC resistance for supplying DC to the second FET. This oscillator will operate, with suitable devices, beyond 3 GHz. This is discussed in more detail in Reference 1.

The nice feature of this arrangement is that there is minimal additional capacitance in parallel with the tuned circuit. This maximizes the tuning range possible from the variable capacitor tuning the oscillator and can, if desired, minimise the number of oscillator ranges required. In the prototype, the entire tuning range of 450 kHz to 60 MHz is covered in just three ranges.

Circuit Details

The full circuit diagram of the HF RF oscillator is shown to the left. Right click on the diagram to see it full scale.



The RF choke presents something of a challenge. RF chokes do not present the desired perfect high impedance across the entire RF range covered by the oscillator! Some minor variation of oscillator output levels can be traced to this part. If you experience problems when building this oscillator, try a different brand or value of RF choke, or even a pair of RF chokes in series or in parallel.

The RF choke I used worked quite well. It is a 1mH miniature molded choke. It's shown in the photo above so you know the sort of part to try.

The oscillator's output is taken from the source resistor of the first stage, and goes to a cascade buffer stage using a pair of transistors. Although Q3 and Q4 are shown as BC548B transistors, performance will be somewhat improved above 30 MHz if better transistors such as the 2N2222 are used.

There is a secondary optional output available on Q4 to attach a frequency counter if desired.

The cascade arrangement of Q3 and Q4 mirrors the circuit of the first stage to some extent. There is no feedback in this buffer stage, of course, and the output is delivered via a 4:1 wideband transformer. If a little more output is desired, it is possible to wind this transformer as a 9:1 transformer using three windings rather than two.


In the prototype, I made this transformer by winding 7 bifilar wound turns onto a small ferrite bead. (Right click on the diagram to the left to see the full details)


For those needing a little more information, first get a ferrite bead. These are about 4 to 5 mm long and about 3 mm in diameter. Ferrite beads have a single hole through the centre, so they look like miniature cylinders. Some seem to have smaller holes than others, so if you have a choice, use the bead with the largest hole. This makes winding the transformer a little easier.

Now get some thin enamelled copper wire. You can use anything from 34 to 40 SWG. The wire looks to me like it's about the thickness of two or three human hairs. Get about 2m of wire, and fold it in half so you have two parallel wires. Now, twist these two wires together. I usually put one end of this wire-pair in a bench vise and the other end into the chuck of a small hand drill or into my cordless drill set to rotate at a very low speed .

Twist the wires together until you get about two turns per cm or three or four turns per inch. Wind seven turns of this twisted wire onto the ferrite bead. (See the diagram above) Work out which windings are which on the transformer using an ohmmeter. Then connect these as shown in the circuit diagram above.    
       
As for the other parts, I used a variable capacitor from another old AM/FM receiver for VC1. All of the other capacitors are ceramic types.

Main Coils

Coils L1, L2 and L3 were all wound on coil formers that I had in my junk box.

L1 was wound using wire and formers from an old 455 kHz IF transformer. L2 was similar, but wound on a 10.7 MHz IF transformer from a cordless phone. L3 used a small unshielded coil former measuring 10mm high and about 6mm in diameter with a threaded ferrite slug. It was from an old FM radio.

The best idea, if you want to build this oscillator, is to just try winding some coils with material you have at hand. If you monitor the frequency of the generator with a frequency counter, then you will be quickly able to wind a set of coils which work for you. I took about an hour to wind perhaps six or seven coils to determine an optimum set of three coils. You may need four or even five coils to cover the ranges you want. It just depends on the parts you have available.

Operation

The oscillator can be operated from any supply voltage from 3V to 15V, but the prototype was operated from a nominal 6V supply. The current required is quite low, well under 200 mA.

References:

1.    B. Koster, P. Waldow & I. Wolff, "A unique low voltage source coupled J-FET VCO", RF Design magazine,
       April 2001 pp 58 - 68

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© 2010 Andrew Woodfield ZL2PD
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