UHF Downconverter

[reverend]

A couple of folk asked for the circuit of my UHF downconverter. Rather than hijack Sinus' excellent RDS project thread, I've put it here instead.

uhf-converter.jpg

The output is good for around 45 to 75 MHz so you can downconvert the following frequency ranges using off-the-shelf SAW resonators (X1):

418.00 MHz: 463.00 - 493.00 MHz
423.22 MHz: 468.22 - 498.22 MHz
433.92 MHz: 478.92 - 508.92 MHz

There are other wacky SAW resonator frequencies kicking around (such as 315 and 403.05 MHz) which could be used for other frequency ranges but they're not as widely available.

[sinus trouble]

Excellent work Rev! And thank you for sharing!

I was expecting some kind of IC based prescaler? But looking at the circuit, That is far from the truth!

Correct me if im wrong? Q1 is based around a Local oscillator which feeds Q2

The UHF is introduced and boosted via Q3 and finally mixed with the Local signal in Q2

I am guessing L4 and L5 are mutually coupled?

[reverend]

Q1 is an oscillator using an SAW. For Wideband FM, these are more than stable enough to act as a local oscillator.

Q3 is a pre-amp and yes, L4/L5 are mutually coupled by dint of the fact that they are next to each other (on the PCB). The gain of the pre-amp doesn't need to be that high, it's the fact that it's low noise that is most important. R10 reduces the gain and makes sure it's stable.

Q2 is the mixer.

Hope that helps!

[radium98]

thank you for sharing!