Interesting 10GHz receiver design

I came across this page https://www.radiowave.co.uk/007.jpg and found interesting looking microstrip design. At the top of the image we can see LMX2325 that maybe used as prescaler (coupling to Fin pin), and two mixing diodes. But this thing does not look like balanced mixer. Why using two diode pair in such configuration? I can't understeand where RF comes from. Bottom of the image is oscillator (maybe) with quarterwave finger with vias to GND and similar quarterwave finger filter to provide LO to mixer. This receiver uses nice substrate, so i am almost sure it is microstrip parallel feedback. Although other stubs pretty confusing, especially gold-colored screw (bottom of the image). If anyone can explain where RF coming from in this mixer it would be very nice.

is there any electronics on the backside? that brass screw comining up thru the floor attached to that coupled line filter looks like it might be an LO signal coming up to send into the mixer. If that were the case, then the lower left corner would be a lower frequency VCO, and it and the LO hit the mixer. the output of the mixer hits the Fin of the divider chip.

looks like a 2.5GHz Synth,, hmmm
LMX2325 Fractional N synthesizer/mixer/PLL
Ceramic IF input filter
Tuned L IF filter
SA614AD FM Demod. with RSSI
Directional couplers on stripline
Ceramic Substrate
and interesting stripline

Terminator3 said:

I came across this page https://www.radiowave.co.uk/007.jpg and found interesting looking microstrip design. At the top of the image we can see LMX2325 that maybe used as prescaler (coupling to Fin pin), and two mixing diodes. But this thing does not look like balanced mixer. Why using two diode pair in such configuration? I can't understeand where RF comes from. Bottom of the image is oscillator (maybe) with quarterwave finger with vias to GND and similar quarterwave finger filter to provide LO to mixer. This receiver uses nice substrate, so i am almost sure it is microstrip parallel feedback. Although other stubs pretty confusing, especially gold-colored screw (bottom of the image). If anyone can explain where RF coming from in this mixer it would be very nice.

Click to expand...

Maybe upper left-hand circuit is balanced oscillator (dual FETs or bipolars) and lower circuit is single ended mixer as part of a motion detector circuit. The output would be low frequency Doppler.

Thank for your replies, it gave me more ideas and now it is more clear to me.

edf said:

Maybe upper left-hand circuit is balanced oscillator (dual FETs or bipolars) and lower circuit is single ended mixer as part of a motion detector circuit. The output would be low frequency Doppler.

Click to expand...

I think you are right! Upper-left part of circuit is not mixer! SOT-23 packaged devices with marking X1s must be BFT93 (Siemens PNP Silicon RF Transistor, datasheet: https://www.s-manuals.com/pdf/datasheet/b/f/bft93_siemens.pdf). There are two stubs cutted with knife, that proves more that it is microstrip oscillator, and some tuning was needed after fabrication. Not sure is it classical push-push oscillator. A i understand there must be at least 180deg line or common resonator providing 180deg shift, and power combiner at 2*F0. But relative sizes of quarterwave finger in filter and line connecting transistors are almost the same. So this line does not look like 180deg at f0. 391=390 Ohm resistor seems realistic for biasing.

Then if bottom left part is not an oscillator, then 122 = 1200 Ohm resistor seems realistic for DC return for diode mixer.