These are needed for several things; the standard way of testing linearity in amplifiers is to apply two tone. A series of mixed tones such as 2F1-F2 or 2F1-F2 are created at so many dB below the tones.
Observing these on a Spectrum analyser allows a quantitative measure of goodness - the IMD performance.
An ordinary oscilloscope can be interpreted qualitatively with skill.
I hope to use a SDR to observe the spectrum
The site http://www.w0qe.com/Technical_Topics/imd_testing_of_amplifiers.html
Explains how to measure IMD in transmitters (or linears).
You can use two audio tones injected into the mic socket of the complete transmitter or two RF tones from two oscillators. It is important to keep them well isolated - usually by having a 6dB attenuator on each and then combine them with a combiner such as a simple return loss bridge. The output of this can drive a low pass filter (since harmonics from the oscillator would cause inaccuracy).
If using audio tones, e.g 700 and 1900 hertz - not harmonically related. If RF then they can be very close one application note says 1kHz, another says 1.2 KHz typical for amateur radio use or says use 2kHz or 10kHz - whatever is easy to detect.
Two tone testing is also used to test receiver input circuitry - and there a spacing of 20KHz is often used. Such IMD is related to how a reciever performs when receiving a weak signal when a strong signal is nearby - it would better to use 5kHz since that is a better real life test, however the advantage of the 20kHz separation is that you can compare your design with figures published for many commercial transceivers.
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I will create two XTAL oscillators probably using identical XTALs, one with a series inductance and one with a series trimmer capacitance - to pull lower and pull higher the output frequencies.
P53 of Eamon's book give a suitable circuit (figure 38)
Each oscillator has a MPSH10 followed by a 2n5109 (with heatsink) into a 7 pole filter (3 x T50-6 inductors and fixed caps into a 4dB pad and a return loss bridge acting as a combiner - it has a FT37-43) final output is +10dBm per tone. Might be worth adding better voltage regulators, one per oscillator. Also feedthru caps, or at least carefully soldered coax between VERY well shielded sections.
Alternatively the site http://www.robkalmeijer.nl/techniek/electronica/radiotechniek/hambladen/fa/2003/page1141/index.html gives circuits and useful text - in Dutch it translates ok in Chrome
more later when I actually build something!
Observing these on a Spectrum analyser allows a quantitative measure of goodness - the IMD performance.
An ordinary oscilloscope can be interpreted qualitatively with skill.
I hope to use a SDR to observe the spectrum
The site http://www.w0qe.com/Technical_Topics/imd_testing_of_amplifiers.html
Explains how to measure IMD in transmitters (or linears).
You can use two audio tones injected into the mic socket of the complete transmitter or two RF tones from two oscillators. It is important to keep them well isolated - usually by having a 6dB attenuator on each and then combine them with a combiner such as a simple return loss bridge. The output of this can drive a low pass filter (since harmonics from the oscillator would cause inaccuracy).
If using audio tones, e.g 700 and 1900 hertz - not harmonically related. If RF then they can be very close one application note says 1kHz, another says 1.2 KHz typical for amateur radio use or says use 2kHz or 10kHz - whatever is easy to detect.
Two tone testing is also used to test receiver input circuitry - and there a spacing of 20KHz is often used. Such IMD is related to how a reciever performs when receiving a weak signal when a strong signal is nearby - it would better to use 5kHz since that is a better real life test, however the advantage of the 20kHz separation is that you can compare your design with figures published for many commercial transceivers.
---
I will create two XTAL oscillators probably using identical XTALs, one with a series inductance and one with a series trimmer capacitance - to pull lower and pull higher the output frequencies.
P53 of Eamon's book give a suitable circuit (figure 38)
Each oscillator has a MPSH10 followed by a 2n5109 (with heatsink) into a 7 pole filter (3 x T50-6 inductors and fixed caps into a 4dB pad and a return loss bridge acting as a combiner - it has a FT37-43) final output is +10dBm per tone. Might be worth adding better voltage regulators, one per oscillator. Also feedthru caps, or at least carefully soldered coax between VERY well shielded sections.
Alternatively the site http://www.robkalmeijer.nl/techniek/electronica/radiotechniek/hambladen/fa/2003/page1141/index.html gives circuits and useful text - in Dutch it translates ok in Chrome
more later when I actually build something!
