IMD or Intermodulation distortion testing is in RF systems usually done with 2 signal sources, quite close in frequency, and one determines the level of the unwanted responses. The actual measurement procedure is shown quite well other places in the literature [1].
Here I will show my design for an high performance crystal controlled RF source delivering 100mW or so into 50 ohm at HF. The obtained phase noise from this generator are better than the resolving bandwidth of my phase noise measurement -140dBc so there should not be any problems from the phase noise of the signals.
The oscillator circuit used is an common base Butler circuit, as described in Matthys [2]. There are a total of 5 frequency dependent components, the crystal, the inductor, and 3 capacitors. The oscillator have limiting to reduce the oscillation amplitude and to improve the frequency drift.
C5+C8 is selected such that the middle value falls in the center of the trimmer C5 range. The chosen crystal should be an HC49 size, not one of the smaller models, as those tend to drift more than their larger counterparts. Measured drift for this oscillator on 3.7MHz is less than 10Hz from start (red trace). Crystals can be ordered from [3] or [4]. Crystal ovens are not recommended, if temperature drift is a problem, fit the crystal with an Styrofoam bead to insulate.
The oscillator is followed by an 3. order low-pass filter to remove harmonic products from the limiting of the oscillator. There exists several types of filter one can use if one wants, I have calculated the values for an half wave filter. This have the simplicity of calculation that the reactances of the components are equal to the system impedance. In this case, for an given frequency :
where f is the crystal operating frequency in Hz. Different filter topologies like Chebychew or Butterworth can be used to improve the harmonic attenuation of the filters. Information on the calculations of such filters can be found in [1]
Following the filter is an attenuator that can be inserted if the max output level is higher than the desired operating amplitude. Suitable attenuator values can be found several places on the internet. If not used, insert an 0 ohm resistor into R8.
The attenuator is followed by an variable gain hybrid cascade amplifier. This consists of an cascade of an J-fet (MMBT310) and an bipolar transistor (BC847). The J-fet is impedance matched to the 50 ohm system with an balun made by winding 4 turns bifilar on an BN-43-2402 ferrite binocular core. The output is in an similar way matched to the power amplifier with an transformer wound on an BN-43-2402 core. The primary side against the hybrid cascade consists of 12 turns, the side against the power amplifier consists of 2 turns. Wind these such that the leads protrude on each side of the core. The amplifier gain can be adjusted with the trimmer R11 such that the output amplitude is at an fixed level, or this can be taken to an potentiometer mounted in the box such that the output power can be varied if needed.
The power amplifier consists of an NE46134 medium power RF transistor, operating in class AB and delivering a nominal 100mW output power. The inductor is wound on an FT37-43 ferrite core, and 10 turns should suffice for all HF frequencies. Following this is an 5.th order low-pass filter. This filter uses the same capacitor and inductor values as calculated for the 3. order filter before. Observe that C23 is double the value of the other capacitors. The inductors should be wound on iron core toroids. T37 size is recommended. -2, -7 and -10 material should fit the frequency range from 2MHz to 50MHz.
The PCB for this design is available from OSHpark:
https://oshpark.com/shared_projects/trtilfzL 3 boards for $36 with delivery worldwide.
The PCB have 2 small isolated holes on each side of the crystal and transformer to secure those to the PCB. The PCB is made to fit inside an Hammond 1590B die cast enclosure. All the used components are 0805 size. 1206 should fit, although a bit large.
In order to combine the generators, some kind of combiner is needed. There exists several kinds of couplers that are common to combine generators. 6dB hybrids are easy to construct and work over an large frequency range. For an small bandwidth range, like these crystal controlled oscilators, an Wilkinson divider gives 3dB split, and are easy to construct.
The PCB for the Wilkinson splitter can be ordered from OSHpark:
https://oshpark.com/shared_projects/9O4qDXVJ 3 boards for $9.90 with delivery worldwide.
A limited amount of kits may be available from me. Documentation and PCB files available on request.
[1] Hayward et. all, Experimental methods in RF design (EMRFD).
[2] Matthys, Crystal oscillator circuits.
[3] Expanded spectrum systems http://www.expandedspectrumsystems.com/prod4.html
[4] ICM https://www.icmfg.com/
RF and electronics blog LA3PNA 