Quote:
Originally Posted by Radio Wrangler
If you're doing a VCO and are looking for best RX performance, a trick is to do the VCO for double the frequency you first thought of, and use a logic divide by two. This can give a more symmetrical waveform to the mixer and give you an extra dB or so on TOI.
Having a symmetrical mixer driver with long-tailed pairs that apply a current squarewave to a diode ring mixer transformer is another step along this route.
I used to work with Hugh Walker ("Sources of intermodulation distortion in diode ring mixers" Proc. IRE in the early seventies) and I picked up a few tricks.
David
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I see. With sinusoidal input of LO and RF, the diodes have a turn-on switching voltage. There will be dead spot near the zero crossing on the waveforms of the mixing products when the diodes are momentarily off. As a result, the waveforms are not as symmetrical as the theoretical mixing output created by square waves. The ring diodes work on the non-linearity behaviors of the diodes (seems paradoxical!). The waveforms of the mixer output look like the spikes on the back of Puff the Magic Dragon with superposition of high and low frequency harmonics contents.
I have attached the wide and narrow spectrum IF output of a +7dbm Mini-circuit ring diode mixer (with FY6900 sig gen).
RF input f1 = 88MHz at -30dbm,
LO f2= 98.7MHz at +7dbm,
and IF = 10.7MHz.
I can see the intermodulation products from some of the markers( I am running out of markers to label every one of them! So I have attached two plots with wide and narrow span):
M3: 2f2-f1= 109.4MHz, 3rd order
M5:2f2 = 197.4MHz, 2nd order
M7: 2f2+f1 = 285MHz, 3rd order
M8: 3f2=296MHz, 3rd order
I would expect 3rd order product, 2f1-f2 = 77.3MHz but I get 62.87MHz instead (see Marker 2 in the 2nd attachment). I dont know why....
I have also attached the harmonics measurements at the IF output of ring diode mixer. The 2nd order harmonics is -63dbc.
At the beginning of this year, I constructed 5-gang varactor tuned FM valve tuner with a pair of E88CC double triodes and a Mini-circuit ring diode ring mixer like the Mrantz 10B schematic attached. However, I struggled with the non-linearity and RF/LO tracking of the varactors and put the project on the back burner. The output port of the mixer is 50ohm, any impedance mismatch will cause reflection of power back to mixer creating more intermod junks.
While the use of ring diodes are common in QRP homebrewers, they are rarely used in commercial broadcast FM tuners. The notable ones are the mega expensive big boys Marantz 10B and Kenwood K917.
I have been toying with the idea of building 7-gang, air variable capacitor tuning, FM tuner using a double balanced mixer with a pair of dual gate mosfets. However, it requires very high drive power which calls for a buffer amplifier for the oscillator and impedance matching baluns or LC matching network. It starts to get a bit challenging to implement in a homebrew project.