Not everyone can afford expensive RF signal generators, or even several less expensive ones, so here are some thoughts on how to make simple signal generation with a Nano VNA, probably the least expensive piece of test equipment with excellent functionality for the amateur who wants to build some equipment, or even just as an antenna analyzer.
In my case I want to be able to check receivers up to the 10GHz band, maybe higher, so what to do with a simple Nano VNA?
I have been looking a bit around to see what others have done in this regard, so here is my
Up to the 70cm band.
For the 2200m to 70cm bands, even the oldest version of the Nano VNA can be used directly. Even though it does not have a calibrated output (amplitude or frequency) It will provide a (not very clean) signal, but if you know which frequency your receiver is set to, it will work fine. If the signal is too strong it is possible to use attenuators, for the HF bands and lower they can be home made, it is more difficult on higher frequencies.
It is also possible to use the VNA as a "personal beacon" test generator, though a keying circuit (and amplifier) has to be added. What about your own low power CW transmitter? Mind you, some filtering will be necessary, as the VNA signal is not really clean, but for an experiment it will work.
Higher bands.
The simplest way to generate signals on the microwave bands will be a simple diode multiplier (comb generator).
So here we go: Add an amplifier followed by a 3 - 6dB attenuator to the signal generator output. It is likely a good idea to add a simple band pass filter as well. Then connect a diode across the output of the attenuator.
Why the attenuator, you may ask. This is in order to have a good load of the amplifier, giving a more stable output.
Next question: Which kind of diode can we use? Given what others have done, with sufficient input power into the diode, PA3FYM reports + 7dBm is sufficient to generate a 10GHz signal using a standard (dirt cheap) 1N4148 silicon diode, so that is what I would try first. Others suggest using step-recovery diodes for generating stronger harmonics, but why do that if the low cost solution works.
With a 432MHz (filtered) signal from the Nano VNA, signals should be generated on 1296, 5760 and 10368MHz, providing signals for narrow band receivers. About 437MHz will provide a signal in the QO-100 satellite downlink band on 10GHz, and should be within range of the band pass filter.
With 400MHz a 2400MHz signal can be generated.
With 425MHz a 3400MHz signal can be generated.
This covers all amateur band allocations (up to and including the 10GHz band) available to me, and should be sufficient to check receivers on all bands, if no other (better) RF signal generator is available.
Given the relative simple design of this I might try it out. I can test the 10GHz signal with a (synthesized) Satellite LNB, initially I will test without a filter on 432MHz, just to see if it is necessary. I might even try to connect the diode directly to the output of the Nano VNA Simplicity rules ;)
All this is with the cheapest version of the Nano VNA. As you may be aware, there are versions with a larger frequency range. The SAA2 (Nano VNA v.2) works up to 3GHz. More expensive, but much easier to work with, up to and including the 2400MHz amateur band. The diode multiplier should also be useful with this model.
The Tiny SA (spectrum analyzer) should be able to work as an RF signal generator, so this might be useful as well, if you have one.
One other option may be using the generated signal from a scalar network analyzer (35 - 4400MHz. I do not know if the spectral purity is sufficient for those PCBs, or if the frequency can be set to a fixed value, but I guess this has to be tested, too. First more research on the Internet. If this one works, the 3400MHz band will be included, but no HF/MF/LF. Everything is a compromise.
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