Quote:
Originally Posted by G0HZU_JMR
One thing to be wary of when testing your dual gate mosfet mixer will be the amount of noise on the LO carrier 10.7MHz away. This will be at the tuned RF frequency and the image. Also, there would be wideband noise down as far as 10.7MHz.
I've not examined that many synthesisers that get used for the LO for a dual gate mosfet mixer, but I have seen designs that use a VCO at the fundamental VCO frequency and there is often a tuned LO amp. This helps to filter away the unwanted noise. A diode ring mixer is double balanced so this issue is less of a problem with a diode ring mixer. The port to port isolation of the mixer will be much higher with the diode ring mixer so the noise can't migrate from port to port as easily.
Some lab sig gens will have fairly high noise floor at a 10.7MHz offset from the carrier. By contrast, a well designed VCO based LO could be 30dB cleaner at this offset. In other words, if you were to use a typical lab sig gen for your mixer LO and you didn't filter it with a narrow BPF then you could think that your mixer has a much higher noise figure than it really has. I'd expect to see a noise figure of about 7dB to 10dB with a typical mosfet mixer design assuming there are no carrier noise problems with the LO.
The conversion gain could be anything from 10dB to maybe 16dB depending of the mosfet type used. The output TOI of the mixer could be anywhere from +10dBm to about +17dBm but a lot depends on how much you optimise the LO drive level and the bias point of the mixer. I think it's reasonable to expect a +12dBm TOI at the output of the mosfet mixer.
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I would expect significant hurdles to be overcome in the TOI measurement of the dual gate mosfet mixer. I had the intention of using a narrow-band 10.7MHz crystal BPF at the IF port using what I would call "narrow frequency window shifting" technique described by you. It essentially varies the LO frequency to do the IMD measurements in sequential steps to overcome the spectrum analyzer mixer's level limitation. Obviously, the crystal filter would need LC matching networks at both ends. The tricky bit is to design the fairly wideband Pi matching network with a fairly low Q for the input of the dual gate mosfet and the IF transformer with the secondary 50-ohm output impedance for TOI test jig. I haven't had experience with this.
Adding to my predicament, I still lack one VHF signal generator source for such a test. I only have the FY6800 with two channels going up to 100Mhz. I don't have to do any TOI measurements in the VHF frequencies. I will not bother to do it if it gets really time-consuming and expensive. Even I successfully measured the TOI figure and it would give zero impact on my 6-gang FM tuner project. The TOI measurement is for the sake of the measurement itself and nothing else.
The dual gate mosfet mixer has both high input and output impedance, making it easier to implement in a high-performance FM tuner.I would not use a ring-diode mixer for a broadcast wideband FM tuner because the commercially available Mini-Circuits Mixers only work well with low-impedance ports. The impedance matching of the ports is a non-trial problem.
Fair enough if you want to build a QRP receiver or a VHF pre-amp or ring diode mixer with a 50-ohm termination system. There are plenty of circuits to pick and choose from amateur radio communities online. But this is not the direction I want to pursue.
Regarding TOI measurement again, the IF output of a ring diode mixer is very sensitive to the port termination, any significant mismatch that deviates from 50 ohms impedance would result in large degradation of measured TOI. Of course, the addition of an attenuator would "mask" the impedance mismatch, but it would probably be prudent to add a low-Q, 10.7MHz diplexer in front of the 10.7Mhz crystal BPF to ensure 50 ohm termination for the IF port.